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IAEA-TECDOC-426
TROUBLESHOOTING
NUCLEAR INSTRUMENTS
A TECHNICAL DOCUMENT ISSUED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1987
TROUBLESHOOTING IN NUCLEAR INSTRUMENTS
IAEA, VIENNA, 1987
IAEA-TECDOC-426
Printed by the IAEA in Vienna
June 1987
PLEASE BE AWARE THAT
ALL OF THE MISSING PAGES IN THIS DOCUMENT
WERE ORIGINALLY BLANK
The IAEA does not normally maintain stocks of reports in this series.
However, microfiche copies of these reports can be obtained from
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P.O. Box 100
A-1400 Vienna, Austria
Orders should be accompanied by prepayment of Austrian Schillings 100,in the form of a cheque or in the form of IAEA microfiche service coupons
which may be ordered separately from the INIS Clearinghouse.
FOREWORD
The servicing and repair of nuclear instruments is a difficult
task.. The commercial manufacturers of nuclear equipment can assure
reliable service only in the most advanced countries that have many
instruments installed.
In developing countries, as a rule, good
service laboratories organized by the manufacturers do not exist.
The nuclear research laboratories
must rely upon their own
resources to keep the instruments in operation.
The International Atomic Energy Agency is trying to assist the
developing countries by providing different types of service in
this field. This includes help in the establishment of suitable
electronics laboratories, advice and assistance in the topic of
preventive maintenance, and training.
Obviously, a
necessary
p r e r e q u i s i t e for staff responsible
for servicing of nuclear
instruments,
is
the
understanding of
nuclear
electronics.
In
interregional, regional, and national courses, the IAEA is training
nuclear electronics staff, both in basic and in advanced aspects.
The present book is devoted to such persons who have either
received
IAEA training, or have studied nuclear electronics by
themselves at home.
In preparing a book on troubleshooting of nuclear instruments,
one is faced with a number of problems:
(i)
The technical level of the book must be properly defined; it
should not be too elementary, but should avoid the most
advanced aspects.
The present publication is meant for
young electronics engineers who will specialize in nuclear
electronics, for senior technicians, or for the scientists
(physicists, chemists) who are forced to maintain and repair
their instruments themselves.
(ii)
Nuclear instrumentation
is
facing a period of rapid
development.
New instruments are appearing on the market
each week. It would be impossible to analyze all the
electronic circuits in these modern instruments. Therefore,
the book must mainly focus on some general features and use
some specific circuits to illustrate the troubleshooting and
repair procedures that will hopefully be applicable to many
different types of instruments.
(iii)
The present effort to prepare a set of recommendations and
tips on troubleshooting cannot replace a good service
manual. However, good service manuals are an exception; as
a rule, service manuals are not available, or are bad.
Therefore, it is believed that the book will give valuable
orientation for troubleshooting to the persons who are
facing a malfunctioning instrument, and have no proper
service manuals available.
The book is the product of several scientists and engineers
who are closely associated with nuclear instrumentation, and with
the IAEA activities in the field. Everybody contributed to all
c h a p t e r s , but the responsibility to
d i s t r i b u t e d in the following manner:
prepare
Preamplifiers, Amplifiers
Sealers, Timers, R a t e m e t e r s
M u l t i c h a n n e l Analyzers
Dedicated Instruments
Tools, Instruments, Accessories,
Components, Skills
Interfaces
Power Supplies
Preventive Maintenance
T r o u b l e s h o o t i n g in Systems
Radiation Detectors
Overal1 editing
F
J
J
P
the basic text was
M a n f r e d i (Italy)
Sousa Lopes (Portugal)
Pahor (Yugosalia)
Ambro (Hungary)
0. M u t z (IAEA)
S. Hollenthoner (IAEA)
H. Kaufmann (IAEA)
P. Vuister (IAEA),
J. Sousa Lopes (Portugal)
P. V u i s t e r (IAEA)
J. Dolnicar (IAEA)
K.D. Mueller (FRG)
The
organization of the m e e t i n g where the f i r s t draft of the
p u b l i c a t i o n was p r e p a r e d , and
the subsequent
improvement of the
t e x t s were
in the hands of Mr. L. Kofi (Ghana) and Miss L.
Kingston (IAEA).
S t u d y i n g the book, it can be noted
that
there are different
a p p r o a c h e s and d i f f e r e n t styles used in i n d i v i d u a l sections.
This
is in part
the consequence of
the fact
that
each c h a p t e r was
d r a f t e d by a d i f f e r e n t person, but it also r e f l e c t s the o b s e r v a t i o n
t h a t various p a r t s of nuclear electronics require d i f f e r e n t ways of
presentation.
For this reason, no p a r t i c u l a r e f f o r t was taken to
present all the c h a p t e r s of the book in a uniform style.
All the persons who
contributed
to the first e d i t i o n of the
t r o u b l e s h o o t i n g manual are well aware that the book needs f u r t h e r
i m p r o v e m e n t . In the jargon of the electronics e x p e r t s , we r e q u e s t
the users of the book for "a f a s t , positive feedback" that will
enable us to improve the text
and make
it more readable and
u n d e r s t a n d a b l e for the engineers, scientists and technicians to
whom it is i n t e n d e d .
The
troubleshooting and repair of instruments is
illustrated
by some real examples. The circuit diagrams and service manuals of
ORTEC, C A N B E R R A and NARDEAUX instruments were selected
for this
purpose. Obviously the choice of these i n s t r u m e n t s was made only
for
training purposes and has
no relevance to the Agency's
preferences for particular brands of nuclear instruments.
EDITORIAL NOTE
In preparing this material for the press, staff of the International Atomic Energy Agency
have mounted and paginated the original manuscripts and given some attention to presentation.
The views expressed do not necessarily reflect those of the governments of the Member States
or organizations under whose auspices the manuscripts were produced.
The use in this book of particular designations of countries or territories does not imply any
judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of
their authorities and institutions or of the delimitation of their boundaries.
The mention of specific companies or of their products or brand names does not imply any
endorsement or recommendation on the part of the IAEA.
CONTENTS
CHAPTER 1.
INTRODUCTION ........................................................................
9
CHAPTER 2.
ORGANIZATION OF THE LABORATORY ......................................
13
CHAPTER 3.
TOOLS, INSTRUMENTS, ACCESSORIES, COMPONENTS, SKILLS .... 21
CHAPTER 4.
TROUBLESHOOTING IN SYSTEMS ..............................................
69
CHAPTER 5.
POWER SUPPLIES .....................................................................
79
CHAPTER 6.
PREAMPLIFIERS, AMPLIFIERS ................................................... 105
CHAPTER 7.
DISCRIMINATORS, SINGLE CHANNEL ANALYZERS,
TIMING CIRCUITS .....................................................................
145
CHAPTERS.
SCALERS, TIMERS, RATEMETERS ..............................................
155
CHAPTER 9.
MULTICHANNEL ANALYZERS ...................................................
173
CHAPTER 10. INTERFACES ............................................................................
211
CHAPTER 11. DEDICATED INSTRUMENTS ....................................................... 241
CHAPTER 12. RADIATION DETECTORS ........................................................... 279
Chapter 1
INTRODUCTION
-1l
Chapter 1
INTRODUCTION
Nuclear
i n s t r u m e n t a t i o n can be found
in many
different
i n s t i t u t i o n s ; there are nuclear research centres, u n i v e r s i t i e s , and
h o s p i t a l s w i t h their nuclear m e d i c i n e diagnostic or therapy units.
In i n d u s t r y , r a d i a t i o n and
associated
nuclear
instruments are
applied
in p r o d u c t s and in process control. As a rule, nuclear
i n s t r u m e n t s are rather s o p h i s t i c a t e d and d e l i c a t e instruments. If
they
develop
f a u l t s , as any instruments sooner or later do, they
are not easy to repair.
Considerable specialized knowledge,
extensive e x p e r i e n c e , and s u i t a b l e e q u i p m e n t is needed for their
repair and servicing.
For a price, the
manufacturers
of
nuclear instruments offer
the required service for their instruments. If
sufficient
funds
are a v a i l a b l e , a service
contract
can
be e s t a b l i s h e d , and the
m a n u f a c t u r e r , or his service laboratory, will take care of
the
installed i n s t r u m e n t a t i o n . This is sound p r a c t i c e for a laboratory
in an a d v a n c e d
country where the m a n u f a c t u r e r ' s service person is
available on call, and where the rather stiff prices of the service
contracts
can
be a c c o m m o d a t e d
in
the l a b o r a t o r y
budget.
In
developing
countries, both
commodities
(available
service and
s u f f i c i e n t hard currency) are an exception rather than a rule. The
management
of
developing
laboratories must
find an a l t e r n a t i v e
solution.
Creating
a
maintenance and
service
laboratory
in
an
i n s t i t u t i o n in a developing country is not easy.
Following
reasonable a d v i c e , like some given in the present p u b l i c a t i o n , can
h e l p in the s e l e c t i o n of
suitable
testing i n s t r u m e n t s , tools and
components.
However, the training of the staff who must have all
the required skills, knowledge and experience, is a long and
d i f f i c u l t procedure.
It
is very d i f f i c u l t to teach troubleshooting and repair of
In
f a c t , the
a c q u i s i t i o n of
the necessary
repair of electronic
and
electromechanical
instruments is a t y p i c a l case where on-the-job training is most
effective.
As a rule, one should not take the methods that
the
m a n u f a c t u r e r s a p p l y to train their field engineers; their training
is l i m i t e d to a certain line of p r o d u c t s , or even a single
i n s t r u m e n t , and for this they have developed efficient approaches
to convey the required skills in a short time. The staff of a
service laboratory in a developing country cannot be trained in
this manner on all possible types of e q u i p m e n t , which there might
be only one
installed in the country.
Their approach to the
servicing and repair must be more general, and is more demanding.
any
instrument.
experience
for
In
the
nuclear
field,
the
staff
instrumentation maintenance
and
servicing
additional problems.
responsible
for
face a number of
(i)
Nuclear
electronics is not taught regularly
at
the
universities or technical schools in developing countries.
(ii)
Literature on nuclear instrumentation, and particularly on
servicing of such devices, is scarce or non-existing.
(9)
Chapter 1
-2-
(iii)
Manufacturers' information on their products and servicing
is generally either b a d , incomplete or not available.
(iv)
There are very few experts who
of nuclear instruments.
(v)
A talented person who received proper training in servicing
of nuclear instruments can easily find a well-paid job in
private enterprise, and would thus be lost for the nuclear
laboratories.
are familiar with the repair
Accordingly, it is not easy to create a team and a laboratory
for nuclear instrumentation service. In an average laboratory, the
maintenance and servicing abilities are hardly transferred to the
junior
staff.
It takes much time to acquire the necessary
for instrumentation repair,
experience within a group.
and
it
is
difficult
skills
to share the
In many developing countries, there is no alternative to the
decision to create and support a maintenance and repair service.
Depending on the size of the country, and on the amount of nuclear
i n s t r u m e n t a t i o n in the c o u n t r y , it might be sufficient to establish
one
central laboratory, or it might be necessary to plan for
more
of them, s t r a t e g i c a l l y located
in
d i f f e r e n t p a r t s of the country.
This
l a b o r a t o r y , or l a b o r a t o r i e s , should be suitably s t a f f e d
and
e q u i p p e d , and should maintain:
- a
documentation
library
with
data
books,
copies of all
manuals, catalogues of equipment supplies, and a
of electronics textbooks;
selection
- a stock of components and spare parts;
- if possible, a stock of spare instruments;
- maintenance kits.
in
a
It is obviously not easy to create and staff such a laboratory
developing country. The present p u b l i c a t i o n should provide
some help in this d i f f i c u l t task.
(10)
Chapter 2
ORGANIZATION OF THE LABORATORY
-12
ORGANIZATION OF THE LABORATORY
2. 1
GENERAL REMARKS
Chapter 2
The contents of this chapter are limited to some general
observations and conclusions on the organization and operation of
an electronics laboratory that serves nuclear intrumentat ion and
related equipment. The same recommendation applies as well for all
donated instruments. It should be noted that there are several
IAEA publications dealing with the topic (IAEA TECDOC-309, Nuclear
Electronics Laboratory Manual, and TECDOC-363, Selected Topics in
Nuclear Electronics).
The optimal set-up of a service laboratory depends, to a large
extent, on the social, economic and policy situation in the
country. As an example, consider the specific regulations in a
country, referring to the financing and control of material support
to the laboratories.
Therefore, it is d i f f i c u l t to present a
detailed assessment of the best approach for establishing and
operating such a laboratory. Below, only some points that seem to
apply to all countries and situations are summarized.
2.2
PHYSICAL E N V I R O N M E N T
A summary of the main items of recommended facilities required
for a service laboratory is given here. Only a reminder on the
essential features to be considered when a laboratory is created
are
presented.
Further details are given in the following
chapters.
1.
The
laboratory should have a minimal useful area of 10-12
square meters for
each
employee.
If
Computer Aided
Engineering (CAE) facilities are envisaged, anair-conditioned
computer room and a room for work stations should be made
available.
2.
For countries in tropical regions, the laboratory should be
air-conditioned, At least a part of the laboratory should be
equipped to have a dry area, i.e a room with reduced humidity.
3.
An adequate storage room, for special
has to be kept for emergency return
available.
4.
The electronics laboratory should have a good, reliable mains
supply, properly arranged electrical power distribution, a n d ,
preferably, a good dedicated grounding system.
5.
The
laboratory should have good overall
precise work on electronics instruments
places should have additional lights.
6.
Depending on the size of the laboratory, and its activities,
add it ional room space with reduced h u m i d i t y
for storage of
electronic
instruments and components, as well as the proper
packing material that
s h i p m e n t , should be
illumination.
For
individual working
(13)
Chapter 2
-2-
laboratories for
arranged.
production
of
printed
c i r c u i t s , should be
2.3
ADMINISTRATION OF A NUCLEAR INSTRUMENTATION LABORATORY
2.3.1
Staff
The p r o p e r
staffing of
the maintenance and
service of a
nuclear e l e c t r o n i c s laboratory is obviously the most i m p o r t a n t part
of good m a n a g e m e n t .
At
the
start
of
such a laboratory, it is
advisable
to make a complete inventory of all the
instruments
in
the i n s t i t u t e or i n s t i t u t i o n s that are e x p e c t e d to be served by the
laboratory.
According
to the volume and
complexity
of
the
i n s t r u m e n t a t i o n , the r e q u i r e d
skills should be d e t e r m i n e d , and on
this
basis, the persons w i t h
s u i t a b l e professional profiles
recruited.
It is highly a d v i s a b l e
to recruit
for
the
laboratory such
staff
m e m b e r s who
can cover a wide spectrum of
instruments.
N e v e r t h e l e s s , it will be necessary, with
the increasing demands
placed
on
the work of the l a b o r a t o r y , to specialize some of
the
s t a f f m e m b e r s in p a r t i c u l a r a s p e c t s of nuclear i n s t r u m e n t a t i o n .
Some of
the topics of such expertise are: radiation detectors,
analog e l e c t r o n i c s , d i g i t a l e l e c t r o n i c s , i n t e r f a c i n g , c o m p u t e r s a n d
electromechanical
apparatus.
Note:
the
recent
trends
in
e l e c t r o n i c s tend to over-emphasize d i g i t a l e l e c t r o n i c s and c o m p u t e r
software d e v e l o p m e n t . P a r t i c u l a r l y in the nuclear f i e l d , there
is
a c r i t i c a l need for persons with knowledge and experience in high
quality
analog
e l e c t r o n i c s , and this should
not be
neglected.
F u r t h e r m o r e , it should be kept in mind t h a t many i n s t r u m e n t s in the
nuclear
l a b o r a t o r i e s are
not
strictly
nuclear.
Electronic
b a l a n c e s , sample c h a n g e r s , o p t i c a l s p e c t r o m e t e r s , d i f f r a c t orne t ers,
and
ph-meters are research tools that also need maintenance and
service, and the e l e c t r o n i c s
laboratory should be in a position to
offer i t.
The management of
the laboratory should
design a s u i t a b l e
scheme to evaluate and promote the activities of the staff. S p e c i a l
attention should be given to the fact t h a t continuous training of
the s t a f f is required to keep up w i t h
the e x t r e m e l y fast progress
in the world of electronics and c o m p u t e r science.
The
IAEA
is
developing a
maintenance;
necessarily
computer-based management
scheme
such a system can be developed
computer-supported,
and
can
for p r e v e n t i v e
locally,
not
also include repair and
servicing aspects.
2.4
INSTRUMENTATION AND ELECTRONICS COMPONENTS
Chapter 3 provides detailed information on the type and amount
of testing instruments and tools required for normal operation of a
service laboratory, at d i f f e r e n t levels.
It should be emphasized that the testing instruments in a
laboratory should be regularly checked and calibrated for proper
operation; such controls should include connectors and cables.
(14)
-3-
Chapter 2
F u r t h e r m o r e , a c e r t i f i e d recalibrat ion of
instruments used as
m e a s u r e m e n t s t a n d a r d , at a national i n s t i t u t i o n , should be planned.
The acquisition of any instrument, p r e f e r a b l y w i t h two sets of
operator
and service manuals even at a d d i t i o n a l expense, should
be
accompanied
w i t h the provision to order some spare parts and
components. One set of operator and service manuals should stay at
the location of each instrument while the other set should be kept
in the t e c h n i c a l library of the service laboratory.
A good
rule can
be t h a t
1 1/2 % of
the value of
the
i n s t r u m e n t s should be invested in spare p a r t s , at the time when the
purchase
is m a d e . In the following years, each
instrument will
need b e t w e e n 1 and 3 % for
replacement
p a r t s , d e p e n d i n g on the
c o m p l e x i t y and design of the i n s t r u m e n t .
Each
laboratory
should
have
c o m p o n e n t s . A list is presented in
considered
to be a minimal set of
stock in the l a b o r a t o r y and need to
a basic
supply
of e l e c t r o n i c
C h a p t e r 3, Section 3.4; this is
components t h a t have to be
on
be u p d a t e d regularly.
It
is
considered
absolutely
mandatory
that
a nuclear
e l e c t r o n i c s laboratory have access to some local and foreign p e t t y
cash, for
rapid
and
u n b u r e a u c r a t i c a c q u i s i t i o n of p a r t s and
c o m p o n e n t s t h a t are or are not available on the home market.
This
is e s s e n t i a l for r a p i d
turn-around of r e p a i r s ; it is not t o l e r a b l e
t h a t the s t a f f m u s t wait m o n t h s for the a p p r o p r i a t e a p p r o v a l to buy
a minor
electronic
p a r t , and
the e x p e r i m e n t e r
cannot
use his
ins t rumen t.
It
should be pointed out here that there is a tendency more
and more often in modern e l e c t r o n i c
i n s t r u m e n t a t i o n to make use of
h y b r i d analog, c u s t o m i z e d d i g i t a l c i r c u i t s , EPROMs, PALs and
other
programmable c h i p s , to achieve higher packing d e n s i t i e s , b e t t e r
overall performance, and to reduce production cost. It will not be
p o s s i b l e , even in a very w e l l - e q u i p p e d
l a b o r a t o r y , to have all
these
s p e c i a l spare parts available in a stockroom.
Fortunately,
the f a i l u r e r a t e of such components is low. These spare p a r t s have
to be ordered
from the manufacturer of
the
instrument or
its
r e p r e s e n t a t i v e and
are
not
available
from
the s e m i c o n d u c t o r
manufacturer.
Even so, one has to realize the fact that the on-site service
of highly sophisticated nuclear instrumentation may often have to
limit itself to the board level just by swapping boards. Board test
e q u i p m e n t , which is required to enable repair on the chip level,
will only be affordable to the instrument manufacturer in some
strategically located service centers worldwide.
One may complain about this situation, but it also offers a
considerable advantage by reducing the number of boards in a
t y p i c a l nuclear instrument, and consequently, the failure rate.
Many manufacturers are now able to use a functionally p a r t i t i o n e d
approach to break down a design to the board level, which allows
them to p r o v i d e diagnostic routines and facilities to the customer
for easy fault location to the board level.
(15)
Chapter 2
-it-
Special emphasis should therefore be given to this fact when
ordering e q u i p m e n t if such diagnostic facilities are available. In
such a case, r e t u r n of an instrument to the m a n u f a c t u r e r may never
be necessary and the p o s s i b i l i t y of shipping damages when returning
a board to the m a n u f a c t u r e r largely vanishes.
Of
specific
importance
for a service
laboratory
is the
a v a i l a b i l i t y of
s u i t a b l e extension boards and cables. If they
cannot be p u r c h a s e d , they must be p r o d u c e d in the laboratory.
A n o t h e r essential a c t i v i t y of a properly organized l a b o r a t o r y ,
is the good o r g a n i z a t i o n of
the t e c h n i c a l library.
This should
include:
- Originals of all the operator manuals
- Originals of all available service manuals, circuit digrams,
p a r t s l i s t s , and t r o u b l e s h o o t i n g i n f o r m a t i o n ;
- A set of d a t a books on electronic c o m p o n e n t s ; as a m i n i m u m ,
it
is recommended
to have a
set of
U.A.T.A. Books
(D.A.T.A., Inc. P.O. Box 26875, San Diego, California 92126,
USA),
possibly
c o m p l e m e n t e d w i t h some p u b l i c a t i o n s of
i n d i v i d u a l p r o d u c e r s of electronic c o m p o n e n t s ;
- a p p l i c a t i o n notes referring
instrument manufacturers;
to
nuclear
electronics
from
- catalogues.
2 .5
ORGANIZATION OF THE WORK IN AN ELECTRONICS LABORATORY
F r e q u e n t l y , a nuclear electronics l a b o r a t o r y combines
its
a c t i v i t i e s in service w i t h some d e v e l o p m e n t work in order to give
its s t a f f a chance to keep up with the r a p i d d e v e l o p m e n t in nuclear
electronics. In such cases, the basic rule should be:
| The repair and s e r v i c i n g of i n s t r u m e n t s has p r i o r i t y to any
|
| o t h e r a c t i v i t y of the l a b o r a t o r y .
|
I_______________________________________________________________I
The electronics
laboratory
should be involved in a research
instrument
from the moment of its delivery. A staff member should
assist in the u n p a c k i n g , i n s t a l l a t i o n , and initial testing of every
newly acquired instrument. For those instruments that cannot be
r e p a i r e d l o c a l l y , and where
there
is a p o s s i b i l i t y for them to be
sent for r e p a i r , special s h i p m e n t m a t e r i a l should be stored.
(16)
-5-
Chapter 2
In l a b o r a t o r i e s and i n s t i t u t e s in d e v e l o p i n g c o u n t r i e s , we
f r e q u e n t l y observe the "wooden box e f f e c t " :
the delivered
e q u i p m e n t is k e p t in b o x e s , s o m e t i m e s for years. This is
not t o l e r a b l e , and the electronics laboratory can contribute
to the action: upon d e l i v e r y , each i n s t r u m e n t
should be
i m m e d i a t e l y u n p a c k e d , i n s p e c t e d for p o s s i b l e damage (and all
claims should be s u b m i t t e d as soon as possible, otherwise the
w a r r a n t y m i g h t be lost), installed and t e s t e d . A d o c u m e n t
should be p r e p a r e d s p e c i f y i n g the measured p r o p e r t i e s of the
i n s t r u m e n t ; l a t e r , this w i l l p e r m i t a comparison on the
instrument's p e r f o r m a n c e .
For
every
i n s t r u m e n t , a logbook and repair list
should
be
o p e n e d , at the time of its arrival.
All s u b s e q u e n t a c t i o n s , be it
for p r e v e n t i v e maintenance or repair of the i n s t r u m e n t , should
be
registered
in
this book. The head of the workshop should
set
a
good example on the u t i l i z a t i o n of the logbooks, otherwise it will
not
be used by the personnel.
It is highly recommended to start
weekly repair case discussions and
to use the logbook during this
session.
It is a d v i s a b l e
for the repair of
an
instrument
to be
organized
in
the following way: a copy of
the c i r c u i t
diagram
should
be m a d e , and the
values
obtained
in measuring
the
q u a n t i t i e s , such as DC
v o l t a g e and
signal s h a p e , should
be
registered. Such an
"updated" diagram should
be stored in the
logbook.
If no diagrams are a v a i l a b l e , a careful record should be
maintained
of
the m e a s u r e d
values at
selected
points of the
circuit.
The staff of nuclear electronics laboratories should
learn
from
their mistakes.
The
staff
member of
the e l e c t r o n i c s
laboratory who
is e x p e c t e d to r e p a i r a faulty
instrument
should
make inquiries on how the fault
has developed. This might require
an a d e q u a t e a p p r o a c h , some c a r e f u l , not i n q u i s i t o r y i n v e s t i g a t i o n ,
and should p r e f e r a b l y be made by a senior person. The findings
should be recorded in the logbook.
P r e v e n t i v e maintenance is d e s c r i b e d in Chapter 13.
(17)
Chapter 3
TOOLS, INSTRUMENTS, ACCESSORIES,
COMPONENTS, SKILLS
-13
Chapter 3
TOOLS, INSTRUMENTS. ACCESSORIES, COMPONENTS. SKILLS
Tools
are needed for proper m a i n t e n a n c e of instruments.
The
number of tools, however, is very large, even if only a c e r t a i n
category of service work, let us say t r o u b l e s h o o t i n g , of nuclear
instruments has
to be performed.
The
following compilation of
tools shall give an idea of what is needed at a work bench or
in
the
laboratory.
The
discussions on
the a p p r o p r i a t e tools and
instruments
will
distinguish
between d i f f e r e n t
levels
of
troubleshooting. A c c o r d i n g l y , the following list of tools will be
d i v i d e d into t h r e e groups.
Group A:
necessary as a minimum to solve simpler troubleshooting
and maintenance tasks, e.g. cleaning, replacing of simple
components, yes or no tests.
Group B
for
advanced
repair
work,
replacement or repair of
m u l t i p l e step switches,
e.g.
in
addition
to
A,
more complex components like
hybrid circuits
connectors
moving coil i n s t r u m e n t s , e t c .
Group C:
for s o p h i s t i c a t e d
repair
work,
e . g - calibration,
v e r i f i c a t i o n of m a n u f a c t u r e r s p é c i f i c a t i o n , m o d i f i c a t i o n
and new developments (or prototypes).
In the following list, you will also find a Group F: s p e c i a l l y
for field service.
This group will strongly overlap the other
groups of tools but is meant for troubleshooting and maintenance
away from the work bench. For
this purpose, pre-packed tool bags
are
a v a i l a b l e but m o s t l y they do not fully
satisfy
the needs.
Either some items are missing or some of them will never be used.
Therefore, it seems b e t t e r to put tools used at the work bench into
a bag for field service.
(21)
Chapter 3
-2-
3. 1
LIST OF TOOLS (MATERIALS)
3.1.1
3.1.2
3.1.3
3.1.4
3.1.5
Room
Work Benches
Chairs
Bench lights
Trolley table
ABC
ABC
ABC
ABC
ABC
3.1.6
3.1.7
3.1.8
3.1.y
3.1.10
Shelf
Cupboard
Storage cabinet
Storage boxes
Cabinet
ABC
ABC
ABC
ABC
ABC
3.1.11
3.1.12
3.1.13
3.1.14
3.1.15
Screwdrivers
Allen keys
Pliers
Diagonal cutting nippers
Jewellers snips
ABC
BC
ABC
ABC
ABC
F
F
F
F
F
3.1.16
3.1.17
Knives
Spanners
ABC
ABC
F
F
3.1.18
3.1.19
3.1.20
Soldering units
Tin (different types)
Desoldering units
ABC
ABC
ABC
F
F
F
3.1.21
3.1.22
3.1.23
3.1.24
3.1.25
Desoldering tapes (different types)
Calipers
Steel ruler
Measuring tapes
Punches
ABC
BC
BC
ABC
ABC
F
F
F
3.1.26
3.1.27
Hammer
Drills
ABC
ABC
F
F
3.1.28
3.1.29
3.1.30
Trepanning cutters
Correcut drills
Drills for printed circuit boards
BC
C
C
F
3.1.31
3.1.32
3.1.33
3.1.34
3.1.35
Hand drilling machines
El. drilling machine (with stand)
Stand-alone drilling machine
Dental drills
Dental drilling machine
ABC
BC
C
C
C
F
3.1.36
3.1.37
3.1.38
3.1.39
3.1.40
Thread taps
Die nuts (threading dies)
Files
Vices
Clamps
C
C
ABC
BC
C
F
3.1.41
3.1.42
3.1.43
3.1.44
3.1.45
Tweezers
Inspection mirrors
Magnifier mirrors, lenses
Brushes (cleaning)
Sprays (liquids for different purposes)
ABC
ABC
BC
ABC
ABC
F
F
F
F
F
(22)
F
-33.1
Chapter 3
List of tools (materials) (cont'd.)
3.1.46
3.1.47
3.1.48
3.1.49
3.1.50
Tapes
Saws
Saws (special)
Tool bags
Tool kits
ABC
ABC
C
3.1.51
3.1.52
3.1.53
3.1.54
3.1.55
Storage boxes
Cable crimping tools
Wrapping tools
Blower and vacuum cleaner
Screw punches
BC
BC
C
ABC
C
3.1.56
3.1.57
Tools for surface mounted devices
Glue (instant action type like
Loctite or similar)
Winding machine (small size with
turn counter)
C (F)
ABC F
3.1.58
Detailed descriptions of
presented in the following pages.
F
F
F
F
the most important
F
F
F
tools
are
(23)
Chapter 3
3.1.1
-4-
Room
For c o n s t r u c t i o n , the following should be c o n s i d e r e d :
a.
Size - big enough not only for the work bench but also
to allow space
for
tools, test
i n s t r u m e n t s , spare
p a r t s , c o m p o n e n t s , d o c u m e n t a t i o n , faulty instruments
(up
to
rack-size
and
detector
with
crystal
assemblies), repaired i n s t r u m e n t s trolleys . . . . . .
A
s e p a r a t e storeroom, d i r t y room . . . . . . . . . . . .
s e p a r a t e clean room, d o c u m e n t a t i o n room
B
. . . . . . .
C
b.
L i g h t i n g system - p r e f e r a b l y d a y l i g h t w i t h blinds (for
screenwork) and daylight fluorescent lamp
c.
Floor - surface w i t h o u t cracks or chinks
d.
Ma t er ia1 - sealed
and
p a i n t e d concrete, PVC, stone
(terrazzo) . . . . . . . . . . . . . . . . . . . . . .
A
PVC antistatic . . . . . . . . . . . . . . . . . . . .
B,C
e.
(24)
C oIpu r - green, grey, uni
Chapter 3
-53.1.2
Work Benches
work bench with a d d i t i o n a l shelf of
beech material . . . . . . . . . .
ABC
solid construction (permissible
load
100 kg m i n ) , non-inflammable
surface,
preferably
with drawers and
additional
shelves
. . . . . . . . . . . . . . . .
with a n t i s t a t i c bench mats
3.1.3
B,C
Chairs
two for each working place with and
without
backplate;
at
least
one w i t h
adjustable h e i g h t a n d rolls
. . . . . .
3.1.4
Bench Lights
adjustable in all d i r e c t i o n s , arm length
. . . . . . . . . . . . . . .
min 80 cm
fluorescent lights and lenses
halogen lamp (spot l i g h t )
3.1.5
A.B.C
A
. . . . .
. . . . . . .
B
C
Trolley Table
rain, size 6Ü x 70 cm, min. load capacity
60 kg, should have the same height as
t h e working table
. . . . . . . . . . .
A,B,C
(25)
Chapter 3
3.1.6
-6Shelf
p r e f e r a b l y a m o d u l a r system w i t h pillars
a d e q u a t e to the room h e i g h t ; p r e c a u t i o n :
shelf
m u s t be anchored to the wall at a
reasonable h e i g h t
. . . . . . . . . . .
3.1.7
A.B.C
Cupboard
s o l i d , p r e f e r a b l y m e t a l , lockable w i t h
drawers a n d a d j u s t a b l e shelves . . . . . A.B.C
3.1.8
Storage C a b i n e t
me t al
f rame
with plastic
drawers,
drawers w i t h an edge
to avoid i n a d v e r t a n t w i t h d rawal
3.1.9
Storage Boxes
different
sizes, p l a s t i c ,
mainly for spare parts . .
3.1.10
stockable ,
A.B.C
Cabinet
with
drawers,
shelves
suitable
for
files, documents and manuals, lockable .
(26)
A.B.C
A.B.C
Chapter 3
-73.1.11
SCREWDRIVERS
Hints for selection:
a.
The shank should be made of a special alloy (chromium, e t c . )
better than nickel-plated.
Layer shanks are not so handy but
more u n i v e r s a l .
For
b i g g e r sizes, its p r o f i l e
should
be
hexagonal to give a d d i t i o n a l t o r q u e w i t h a spanner.
b.
The tip should be specially
hardened;
the two planes of the
b l a d e have to be parallel in the slot of the screw.
c.
The
handle
should be of special p l a s t i c ,
preferably
hammer
hit-proof.
Hits on the end of
the handle are only allowed if
the
shank goes
fully through the handle
(precaution: no
isolation! ) .
length (mm)
60-100 (shank),
100-120 (shank),
0.4x2.5
0.6x4
ABC F
BC (F)
125 (shank),
170 (shank),
0.8x5.5
1 .6x8
ABC F
BC (F)
170 (shank)
200 (shank)
1.6x10
2 x!2
BC
F
BC (F)
1
x 6
BC
F
PZD 0
PZD 1
PZD 3
BC
ABC
ABC
F
F
F
C
F
C
F
25 (shank),
00
bit profile (mm)
60 (shank),
80 (shank),
150 (shank),
100 (shank),
100 (shank),
0.9x
5
PZD 1-2
o o
(27)
Chapter 3
-8-
w a t c h m a k e r s s c r e w d r i v e r s , set of
0.25x0.8 up to 0.6x3.8 mm
set of 2 P h i l i p s
and 3 Hex
0,1
mm
1.5-2.5 mm
BC
C (F)
8554
(mains t e s t e r screwdriver
ABC
F
split b l a d e h o l d i n g screwdriver
200 mm long, 4 mm blade
BC
If s p e c i a l heads (other than e.g. Pozidrive) are n e e d e d , then
use
d i f f e r e n t b i t s in a h o l d e r ; a spring collar or a m a g n e t
holds
t ne bit in a hexagon socket.
100 mm long, 1/4 inch socket
0286
E x a m p l e of d i f f e r e n t sizes of P o z i d r i v e bits:
"Wer» No 655 H'1. Sirewurivers lor Pozidnv/Supadnv slots C form Extra hard
quality HRC 64-65 Specially intended for tightening sheet metal screws
Ortg. No. 855/1 H=V."0
Ret .. . .. .. .. . .. .........
Pozidnv slots
5014
0052
No
0
25
10
525
Length
Std
mm
pack
Price each
SEK
Orijj. No.855/1H=V."0
Rfl. ..... .......................
Pozidnv slots
Length
Std
Price each
10
020)
2-25
25
10
525
525
0300
0409
0102
1
25
5014 0284
3-25
No
25
mm
10
pack
525
SEK
3-32
4
32
10
1200
32
0250
2-50
50
10
875
oo
5014-5015
Bits for Pozidnv slots
10
1450
Fitting in universal holders
i BOO/I ! 855/1
' 840/1 O
(28)
867/1
-
different
types of bits
C (F)
-y3.1.12
Chapter 3
Allen Keys
keys are removeable from the holder
metric set 1.5-6mm, hex
inch set 1/16-1/4, hex
for m u l t i t u r n d i a l s of precision
p o t s , additional special sizes
should be ordered
3.1.13
BC (F)
BC (F)
Pliers
flat-nose plier, length
170 mm
snipe-nose p l i e r , l e n g t h
120 mm
half-round-nose p l i e r ,
length
140 mm
combination p l i e r , length
water p u m p p l i e r , l e n g t h
wire strip plier, length
ABC
C (F)
ABC (F)
160 mm
130-170 mm
130 mm
ABC
F
C (F)
BC
F
(29)
-11-
SKILLS ;
Chapter 3
General i n s t r u c t i o n s on soldering
Soldering
(and desoldering) is one of
the main
tasks
in
electronic t r o u b l e s h o o t i n g work.
To
solve these tasks p r o p e r l y ,
one has to consider several points:
1.
The p h y s i c a l dimensions of the soldering p o i n t .
The range
is from tiny hybrid
c i r c u i t s , w a t c h e s , d o u b l e or
multi-layer
boards (e.g. pocket c a l c u l a t o r ) up to big area
soldering e.g. for shield grounding.
2.
M a t e r i a l on which soldering must be done.
N o r m a l l y , there will be a p r i n t e d
c i r c u i t board covered w i t h
special p r o t e c t i o n varnish or t i n , silver, g o l d , e t c . ,
but
also on s t a n d - o f f s , leads, p l a t e s , c a b l e s , e t c .
Let us as sume
t h e r e are
big
a d d i t i o n to the
c o n d i t i o n s have
all m a t e r i a l is t i n - s o l d e r a b l e ; n e v e r t h e l e s s ,
d i f f e r e n c e s , e.g. for
iron and
gold.
In
d i f f e r e n t heat transfer also d i f f e r e n t surface
to be
considered.
IT
IS E S S E N T I A L THAT FOR
PERIOD
THE TWO
MATERIALS
REMAIN
CLEAN
(NO
THE
SOLDERING
O R G A N I C MATERIAL OR CORROSION).
3.
M a t e r i a l used as solder.
The material normally
called
"tin" is
in r e a l i t y an alloy
which
is mostly composed of other elements like Pb, Cd
Ag,
Bi . Cu , and Sb .
TABLE 3 . 1 ; Some common types of solder and their c o m p o s i t i o n
Standard
Alloy
Mel t ing
Point C
Specif icat ions
(A)=QQ=S-5 71E, (G)=DIN1707
(B)=BS 219
TLC
145
(B)T, (G)
L-SnPbCdl8
LMP
179
(B)62S
L-SnPbAGl ,8
SN63
60/40
SAVBIT 6
183
188
190
(A)SN63
L-Sn63PbBi03
L-Sn60Pb
50/5U
212
215
( B ) F , ( G ) L-Sn50Pb
(G)
L-Sn50PbCu
221
234
243
255
275
301
(A)Sn96 , (B)96S , (G)L-SnAg5
(B)G,(G)
L-PbSn40
L-SnSb5
(B)95A,(G)
L-Pb70Sn
(B)J(G)
(B)V(G)
L-Pb80Sn
(B)5S
SAVBIT 1
96S
40/60
95A
30/70
2U/80
HMP
(B)DP,(G)
(G)
L-Sn60PbCu2
Used for
solder of galv.
gold
solder of galv.
silver
printed boards
electronic
stops disintegration of cu
electronic
stops disintegration of Cu
w i t h o u t lead
common e l e c t r i c
w i t h o u t lead
E -Mo t or s
lamps
high t e m p , solder
(31)
Chapter 3
-12-
U i t f e r e n t amount
of
these elements result
in alloys w i t h
different
characteristics
like m e l t i n g
point,
conductivity,
a g r e s s i v i t y t o o t h e r m a t e r i a l s (copper), m e c h a n i c a l s t r e n g t h , e t c .
l
370315-
"N
260 -
3
Liquid
%^
V
l
\ paste
^^^
v ,
D
This d i a g r a m shows a
solder wire composition of t i n - l e a d .
^^^*
^*^ ^*~ - ^ p a s t e
There is a pasteregion above and
below the 63% tin
concentration.
5 11,9-
i—3
01
Q.
E
solide
g?
.
*J
O)
()
10
20
30
1,0
50
Ti'nn Sn - Pb [%}
Ok
Zll
F l u x is n e c e s s a r y to
Normally
copper .
the
flux
60 "l 70
63
80
90
100
Eufechc point
allow
ought to be
soldering
or
to make it easier,
inagressive, e s p e c i a l l y against
D u r i n g t h e soldering p r o c e d u r e , some i m p o r t a n t c o n s i d e r a t i o n s
must
be f o l l o w e d . A f t e r switch-on of mains, the s o l d e r i n g
iron
s h o u l d heat up fast and
should
remain at a constant t e m p e r a t u r e .
During the s o l d e r i n g p e r i o d , it should warm up fast the m a t e r i a l of
the s o l d e r i n g p o i n t to a
t e m p e r a t u r e close to the l i q u i d region of
the solder. A d e q u a t e solder has to be a p p l i e d . During the m e l t i n g
ti-ne, the h e a t c a p a c i t y of the solder tip has to be s u f f i c i e n t not
to d r o p the t e m p e r a t u r e beyond the l i q u i d region of the solder.
Why not take a soldering iron with a temperature far above the
m e l t i n g region? The followibg observations speak against such a
procedure.
(32)
-13-
Chapter 3
1.
Sensitive components, cables, connectors, printed circuits,
etc.
will be destroyed if they are exposed too long to higher
temperatures .
2.
The t e m p e r a t u r e of the solder should drop into the solid state
very
fast because movement of the parts in
liquid or
paste
c o n d i t i o n will cause a bad contact (cold solder point).
Considering
selected.
the previous, a d e q u a t e soldering tools have to be
(33)
Chapter 3
3.1.18
-14Soldering irons and bits
Wattage 8 W
Heating up time ca 90 s
Bit temperature 290 °C
Weight without lead 26 g
with rubber rest
Voltages 6 V, 110 V, 130 V, 220 V, 240 V
Multitip 230 8 Wdtt
BC
Order nos.
230 LN/8 W Iron with nickel-plated copper bit
230 LD/8 W Iron with ERSADUR long-life bit
Wattage 15 W
Heating up time ca 60 s
Bit temperature 350 °C
Weight without lead 28 g
with rubber rest
Multitip 230'15 Wall
Voltages 6 V, 12 V, 24 V 42 V, 48 V 110 V,
130V, 220V, 240V
BC
Order nos.
230 LN/15 W Iron with nickel-plated copper bit
230 LD/15 Wlron with ERSADUR long-life bit
Wattage 25 W
Heating up time ca 60 s
Multitip 230/25 Walt
Bit temperature 450 °C
Weight without lead 34 g
9a
with rubber rest
Voltages 6 V 12 V 24 V, 42 V, 48 V, 110 V,
130V, 220V, 240V
Order nos.
230 LN/25 Wlron with nickel-plated copper bit
230 LD/25 Wlron with ERSADUR long-life bit
ABC
F
BC
ABC
F
F
Bits (surface like "Ersadur" type is recommended for all)
Multitip 230/8 W
vernickelt
bits / pannes
nickel-plated
Multitip 230/1 5 W
vernickelt
ERSADUR
nickelée
nickel-plated
Multitip 230/25 W
vernickelt
ERSADUR
nickel-plated
nickelée
ERSADUR
nickelée
«t^a
132LN
132LD
162LN
162LD
172LN
172LD
—G5B
132BN
132BD
162BN
162BD
172BN
172BD
—G=a
132KN
132KD
162KN
162 KO
172KN
172 KD
^'
132SN
132 SD
162SN
162 SD
172SN
172 SD
'
Cordless industrial soldering iron:
The
cordless i n d u s t r i a l soldering
iron is powered by long-life nickel
cadmium b a t t e r i e s , w h i c h are easily
r e p l a c e a b l e , giving tip performance
equivalent
to up to 50 w a t t s w i t h
over 370°C (700°F) tip t e m p e r a t u r e .
C (F)
Soldering
irons w i t h b u t h a n gas
firing w i t h
heat
regulation
and
different
t i p s are
s u i t a b l e , e s p e c i a l l y for h i g h e r heat t r a n s f e r
( g r o u n d i n g ) in the field service.
F
(34)
-153.1.18
Chapter 3
Soldering iron (cont'd.)
030 KK/40 W
Wattage 30 W or 40 W
Heating up time approx. 2 mm
Bit temperature 380 °C, 420 °C
Weight without lead: 95 g
with plastic rest
Voltages 6 V, 12 V. 24 V, 42 V, 48 V, 110.
030 KD/30 W Iron with ERSADUR long-life bit
125.
030 KK/30 W Iron with copper bit
120 V,
135V, 220V, 225.. 235V. 240 ..250V
030 KD/40 W
ABC
__
Lotspitzen / bits / pannes
/==
'
'
"
'
———
F
c^«
Kupfer / copper / cuivre
032 KK
032 JK
032 BK
032 CK
032 NK
ERSADUR
032 KD
032 JO
032 BD
032 CO
032 NO
Wattage SOW
Heating up time- 3 mm
Bit temperature. 410 °C
Weight without lead. 220 g
Voltages:
12V,24V,42V,48V, 110. .120V,
125 ..135 V, 220 V, 225...235 V, 240...250 V
Order nos
080 JK Iron with copper bit
080 JD Iron with ERSADUR long-life bit
ERSA 80
ERS A 50
ERSAOUR
Kupfer
copper
cuivre
ERSADUR
052 JK
052 JO
082 JK
082 JD
052 KK
ACO vn
082 KK
082 KD
052 NK
052 NO
062 FK
Kupfer
bits / pannes
copper
cuivre
y—
,
__
cr^^
ABC
052 DO
/————
^BB™
———————
ABC
052 BO
052 CK
052 CD
Holders
ABC
(35)
Chapter 3
3.1.18
-16Soldering (desoldering station)
Lotspitzen / bits / pannes
ERSADUR
'
602 CD
J
^
'
602 ED
'
~«a
602 BD
This
electronic
temperature
regulated
soldering
and
desoldering
station is especially designed
for
industrial use,
l a b o r a t o r i e s and repair works.
The modular system p e r m i t s a large
field
of
applications.
The
single elements can
be used
in
i n d i v i d u a l combinations.
Because of
a b u i l t - i n v a c u u m pump the efficient
station
is
i n d e p e n d e n t of an air pressure connection.
In a d d i t i o n , the basic
station contains a power supply of 220V/24V which
f i t s all
24V
soldering irons and desoldering irons up to 80W rating.
An e l e c t r o n i c control unit p e r m i t s c o n t i n u o u s v a r i a t i o n of the
soldering bit t e m p e r a t u r e between 150 and 400 degrees C.
By means
of a t h e r m o c o u p l e sensor located
in the soldering iron next to the
(36)
Chapter 3
-17-
the a c t u a l t e m p e r a t u r e and
bit, the electronic
control measures
an
compares
it
to the nominal t e m p e r a t u r e s e t t i n g . By means of
full-wave logic and Triac, the
i n t e g r a t e d zero v o l t a g e swit ch w i t h
t e m p e r a t u r e is elee t ronical1 y c o n t r o l l e d and the operational state
Depending on
the
temperature
is i n d i c a t e d by
a
pilot
1 ight.
unit
difference
or
heat
r e q u i r e m e n t , t h e sensitive e l e c t r o n i c
input
in
a c c o r d a n c e with
the
controls
the h e a t i n g ener gy
load).
The
soldering
bit
and
operational
state
(idling or
d e s o l d e r i n g bit is c o n n e c t e d to the l e v e l - p o t e n t i a l t e r m i n a l via an
integrated high Ohm resistor
The small s o l d e r i n g
iron
is very
efficient
thanks
to its
ceramic
heating
element, which
has a
pronounced
positive
temperature
coefficient
(PTC)
and 80 Watts rating
(at 350
d e g r e e s C).
The
desoldering
system
consists of
the iron on w h i c h a
d e s o l d e r i n g h e a d is m o u n t e d . The t r a n s p a r e n t solder c h a m b e r can be
e m p t i e d simply
and q u i c k l y .
The
necessary
vacuum
i m p u l s e for
d e s o l d e r i n g is r e l e a s e d by a foot s w i t c h .
The
following
picture
gives an overview
of
an
extendable
s o l d e r - d e s o l de r s t a t i o n of one of the l e a d i n g m a n u f a c t u r e r s .
Netzspannung
Grundgerat
station
No. 003
f Lötkolben
2 4 V btsSOW
unregulated iron
24V
up 10 30 W
l im Griff geregeller
( Lotkotben 24 V bis 8Û W
1
regulated in the handle
1
2 -i V up to 80 W
elektronische Regetemheit
elektronische Regetemheit
electrons control unit
No. TCS-flOO
mit Digitalanzeige
electronic control unit
with digital d'spiay
No. TCS-D 800
ungeregelter Lolkolben
im Griff geregelter
24 V bis 80 W
Lotkolben 24 V, bis 80 W
unregulated iron
regulated in the handle
24 V up to 80 W
24 V. up to 80 W
eteklronische Regeteinheit
electronic control unit
No TCS800
elektronische Regeteinheil
mrt Ognalanze.ge
electronic control unit
with digital display
No. TCSO800
(37)
Chapter 3
3.1.19
As
-18Tin
shown on page 11, Table 3.1, one should also select for
a
s p e c i a l purpose the a d e q u a t e solder.
In real l i f e , however, one to three t y p e s of tin alloys
be enougn.
60/40
Savbit b
Savbit 1
w i t h low m e l t i n g region
p r o t e c t s copper
p r o t e c t s copper, h i g h e r m e l t i n g region
will
BC
ABC
BC
F
Tin
of
multi-con
type
with
non-corrosive flux like rosin should
be s e l e c t e d . The q u a n t i t y is normally
100g,
250g, 500g per unit. A handy
dimension gives the 250g spool.
The d i a m e t e r of the solder wire should be about Imra; only for
solder
tiny
circui t-work, a small spool of a b o u t 0.3mm d i a m e t e r
wire is r e c o m m e n d e d to have on stock.
C (F)
(38)
Chapter 3
-193.1.20
üesoldering Tools
BC (F)
The d e s o l d e r i n g bit can be inserted in soldering irons or
soldering stations. In any case the applied t e m p e r a t u r e should not
be
too high
and not too long, otherwise the PC
board will
be
destroyed.
This is d a n g e r o u s especially on m u l É i - l a y e r
boards; in this
case it is b e t t e r to cut the leads of the 1C and remove them one by
one .
Tin suction devices
6750/SP
one-hand operation with
exchangeable t e f l o n tip
length 210, 190 mm
6750/XS
ABC
F
ABC
F
Desoldering t a p e s
l e n g t h 1.6m
w i d t h 1.5, 2, 2.5
Desoldering
tapes are used for absorbing excess solder
on
c i r c u i t b o a r d s , A min ima1
quantity
of
mild and n e u t r a l f l u x is
included.
The
desoldering
tape
is h i g h l y - a b s o r b a n t .
Chapter 3
3.2
-20-
LIST OF ACCESSORIES
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
Strips with banana plugs (0.5, 1, 1.5m)
Alligator clips
Test clips (for strips)
Test clip for IC-tests
Cable reeling units
ABC F
ABC F
ABC F
BC F
ABC F
3.2.6
3.2.7
3.2.8
3.2.9
3.2.10
Oscilloscope probes (1:10, 1:1)
Oscilloscope current probe
Oscilloscope HV probe
HV probe for m u l t i m e t e r
Shielded black box
ABC
BC
BC
ABC
BC
3 .2 . 11
3 .2 . 12
3 .2 . 13
3 .2 . 14
3 .2 . 15
BNC
BNC
BNC
BNC
BNC
3 .2 .16
3 .2 .17
3 .2 .18
3 .2 .19
3 .2 .20
MHV
MHV
SHV
SHV
T-type
1-type female
T-type
I-type female
SHV-MHV-adaptor
ABC
ABC
ABC
ABC
ABC
F
F
F
F
F
3.2.21
3.2.22
BNC 50-Ohm cables (0.3, 0.5, 1, 2, 4m length)
SHV - 5kV - cables (2, 4m)
ABC
ABC
F
F
(40)
5u-0hm
50-Ohm
T-type
I-type
I-type
terminator (male) plug
attenuator (1,3,6,10,10 10,20
dB)
male
female
F
F
BC (F)
BC (F)
ABC
F
ABC
F
ABC
F
Chapter 3
-21-
3.2.6
Oscilloscope Probes
Modular s w i t c h a b l e p r o b e
w i t h xl and xlO a t t é n u a t i o n
AH. 10:1 B a n d w i d t h : DC-250 MHZ
rise time:
1,4 nsec
input resistance:
10 Mohm
AH. 1:1
B a n d w i d t h : DC-10 MHZ
rise time:
35 nsec
input resistance:
1 Mohm
Modular probes allow easy repair of broken p a r t s ;
any m o d u l e can be s i m p l y r e p l a c e d .
3.2.7
ABC
Oscilloscope Current Probe
Sensitivity
Accuracy :
ImV/mA
±
3%
Bandwidth :
I k H z to
rise t irae:
8 nsec
40 MHz
1I10B
I max. dc :
0.5A
I max. ac :
15A
(41)
Chapter 3
3.2.10
-22Shielded Black Box
For compact packaging of
m a t c h i n g networks.
Features shielded housing of
die cast a l u m i n u m .
68 "-22 35
Includes cover and four self
t r a p p i n g sc rews.
BC
3.2.11
BNC 50-Ohm Terminator Plug
1%, 0.5 w a t t s , + 100 C max
g o l d - p l a t e d center contact
BC (F)
3.2.12
BNC 50-Ohm A t t e n u a t o r
Impedance :
f-range :
Accuracy :
Max Power :
(42)
50-Ohm
DC-1 GHz
^0.2 dB
1 Watt
-233.3
Chapter 3
INSTRUMENTS
S e l e c t i o n of i n s t r u m e n t s for t r o u b l e s h o o t i n g and
development
may be t r o u b l e s o m e because nowadays a large amount of d i f f e r e n t
t y p e s , even for similar p u r p o s e s , are available on the m a r k e t .
Even e x p e r t s are only familiar w i t h a few t y p e s of i n s t r u m e n t s
and
these
are mostly
i n s t r u m e n t s t h e y are
dealing w i t h .
The
p r o b l e m is to order the best for a c e r t a i n purpose. To order the
i n s t r u m e n t s at the lowest possible cost is not necessarily the best
solution.
Some a d d i t i o n a l f a c t o r s have to be c o n s i d e r e d :
1.
for w h i c h
purpose
the i n s t r u m e n t s are
n e e d e d , i.e. w h i c h
field of t r o u b l e s h o o t i n g ( d e v e l o p m e n t ) should be c o v e r e d ?
2.
W i l l the i n s t r u m e n t be u p g r a d e d for a d d i t i o n a l purposes in the
future ?
3.
Is the m a n u f a c t u r e r r e p r e s e n t e d on the local m a r k e t ?
4.
Is
the i n s t r u m e n t s u p p l i e d w i t h all t e c h n i c a l information and
service m a n u a l s ?
5.
How is
the s i t u a t i o n
in
servicing
the i n s t r u m e n t
service s t a t i o n , s h i p m e n t , c u s t o m d i f f i c u l t i e s , etc.)?
(local
6.
Can
the
instrument
be o r d e r e d t o g e t h e r
a c c e s s o r i e s , o p t i o n s , service k i t s , etc.?
parts,
7.
Who is the user of
knowledge)?
8.
Where will the i n s t r u m e n t be used (environmental situation)?
the
instrument
(level
with
spare
of e x p e r i e n c e and
These e i g h t
points may
be e x t e n d e d
according
to special
s i t u a t i o n s . The i n s t r u m e n t s listed in the following pages are used
in d i f f e r e n t nuclear e l e c t r o n i c s l a b o r a t o r i e s
that have received
assistance from the IAEA. Considerable experience of experts is
i n c o r p o r a t e d into t h i s selection. N e v e r t h e l e s s , only point 1 and 2,
mentioned
a b o v e , will be covered. For a final ordering
decision,
all o t h e r p o i n t s should be considered.
The following listed
instruments are to be understood as
examples only. This list
also should
be u p g r a d e d periodically o l d - f a s h i o n e d e q u i p m e n t should be replaced.
(43)
Chapters
-24-
List of i n s t r u m e n t s
For analog t r o u b l e s h o o t i n g and d e v e l o p m e n t :
3.3.1
AVO-meter
ABC
F
3.3.2
Digital multimeter
3.3.2a
h a n d h e l d , 4 1/2 digit
3.3.2b
b e n c h m o d e l , 4 1/2 d i g i t
3.3.2c
b e n c h m o d e l , 6 1/2 d i g i t
ABC
BC
C
F
3.3.3
3.3.4
3.3.5
Capaci ty - I n d u e t i v i t y meter
ABC
F
Transistor tester
Insulation tester
ABC
BC
F
3.3.6
Oscilloscopes
3.3.6a
p o r t a b l e (up to 20 M H z )
3.3.6b
b e n c h t y p e (up to 40 MHz)
3.3.6e
bench t y p e (up to 100 M H z )
3.3.6d
storage type
A
A
F
3.3.7
Pulse g e n e r a t o r s
3 . 3 . 8a
d o u b l e pulse g e n e r a t o r
3.3.8b
precision pulse generator
3.3.8c
s l i d i n g pulse generator
BC
C
BC
C
C
3.3.8
DC-power s u p p l i e s
BC
3.3.9
3.3.10
3.3.11
3.3.12
T r a n s i s t o r c u r v e tracer
D C - c u r r e n t m e t e r (current p r o b e )
Noise (RMS) m e t e r
C o m p l e t e e q u i p m e n t for
-spec t roscopy
C
BC
BC
C
F
For d i g i t a l t r o u b l e s h o o t i n g and d e v e l o p m e n t :
3.3.13
3.3.14
3.3.15
Logic t e s t e r probe
ABC
Signal injector
BC
D i g i t a l c i r c u i t t e s t e r (troubleshooter k i t )
BC
3.3.16
3.3.17
3.3.18
3.3.19
3.3.20
Break-outbox
IC-tester
Logic analyzer
Prom p r o g r a m m e r
In-circuit emulator (with t e r m i n a l or PC)
BCF
BC
C
C
C
3.3.21
3.3.22
D e v e l o p m e n t system (with p r i n t e r )
E x p e r i m e n t a l c o m p u t e r board (single board
c ompu ter)
C
(44)
C
F
F
Chapter 3
-25AVO-Meter
3.3.1
Analog i n s t r u m e n t w i t h fast overload p r o t e c t i o n
i n t e r n a l b a t t e r y (1.5V) for
r e s i s t a n c e measurements
s i m i l a r types for higher
c u r r e n t ranges are also
available
Specifications :
Vdc
Ü. IV
5ÜUUV
+_ 1.5% ( 10k
- 500M )
Adc
lOuA
1A( IDA)
+_ 1.5% ( 10k
- 0.24 )
Vac
10V
1000V
+_ 2.5% (200k
-
R
l
5M
20M )
+ 1.5% (with internal 1.5V b a t t e r y )
These kind of instruments have been used for many
years and
might seem old-fashioned nowadays, but nevertheless they are often
needed where
t r e n d s are
to be analyzed
or m e a s u r e m e n t s u n d e r
floating conditions are to be taken.
ABC
F
(45)
Chapter 3
3.3.2a
-26üigital Multimeter
4 1/2 d i g i t , h a n d h e l d , b a t t e r y powered
This type is general purpose, useful for bench or field service
Some s p e c i f i c a t i o n s :
0.2V
to 1000V dc (> 10M )
0.2V
to
0.2k
< u.05%
750V ac (10M, l O O p F )
to 200k
to 300k
0.2-3%
0.07%
(auto)
0.2mA to 2A dc
0.2mA to 2A ac
< 2%
< 0.3%
1-2%
Some useful a d d i t i o n a l f e a t u r e s of a d i g i t a l m u l t i m e t e r :
Diode test:
the voltage drop
across on a diode can be measured up
to 2V with a 1mA dc test current.
Frequency measurement:
Suitable
also
for:
from 12Hz to 200kHz.
R M S , dB (relative to a selected voltage),
conductance, relative values (offset), e t c .
Big d i s p l a y , self-test, d i f f e r e n t additional display indicators
Extensions are available:
H.V. probes (5kV, 40kV)
current probes
HF-probes
current shunt, etc.
ABC
(46)
Chapter 3
-27-
3.3.2b
Digital Multimeter
4
1/2 d i g i t ,
(battery operated)
for
laboratory
and
advanced
field
service
Suitable for long-time measurements
Features:
•4% Digit LCD Display
•Fast Autoranging
• Bench or Portable
• Digital Calibration
• 100 Point Data Logger
•10/iV/10mfi/10nA Sensitivity
•0.03% Basic DCV Accuracy
•TRMS AC
•dBm/Relative Functions
•Min/Max Reading Hold
•Safety Input Jacks
•10A Capability
•100kHz Specified AC Bandwidth
Options:
•Model 1758 Rechargeable Battery Pack
•Model 1753 IEEE-488 Interface
A remarkable feature of the instrument shown above is the Data
Logger.
This is specially suitable for longterm measurements, as
for example the line fluctuations, instabilities, superimposed slow
variations on de-lines.
100 data + Hi and Lo can be stored
in 6
different speeds from 3 reading/sec to 1 reading/h.
Options:
b a t t e r y pack, supply for 6h
IEEE-488 interface for remote control
BC (F)
(47)
-28-
Chapter 3
3.3.3
C a p a c i t y and Indue t i v i t y M e t e r
d i g i t a l , d i r e c t r e a d i n g (3 1/2 d i g i t s )
Capac ity:
1 pF
10 pF
100 pF
2 nF
20 nF
100 nF
1 uF
10 uF
100 uF
1 uF
20 uF
200 uF
Indue t ivity :
1 uH 2 mH
10 uH - 20 mH
100 uH - 200 mH
1 mH 10 mH -
2 H
20 H
The instrument is battery powered (9V)
accuracy :
< 0.5%
lead compensation possible
ABC
(48)
-293.3.4
Chapter 3
P o r t a b l e Transistor Tester
AB(C ) F
This low-cost t y p e of transistor tester uses d i g i t a l high c u r r e n t ,
low d u t y
cycle
pulse-testing
t e c h n i q u e to test semiconductors
even w i t h resistive and c a p a c i t i v e shunt i m p e d a n c e s .
Fast GO/NO-GO in-circuit t r a n s i s t o r testing.
Fast and t h o r o u g h GOOD/BAD o u t - o f - c i r c u i t testing.
Tests FETs and SCRs in-circuit or o u t - o f - c i r c u i t .
Any
test
c l i p to any component lead gives p o s i t i v e e m i t t e r - b a s e c o l l e c t o r i d e n t i f i c a t i o n on LO d r i v e - p o s i t i v e base i d e n t i f i c a tion in HI d r i v e .
L i g h t - E m i t t i n g Diodes i n d i c a t e NPN-OK or PNP-OK.
Power r e q u i r e m e n t s :
6VDC from four "AA" cells.
S t a n d b y c u r r e n t , 4 m A ; average t e s t i n g c u r r e n t , 12mA.
(49)
Chapter 3
3.3.6
-30Ose illoscope
DC-100 MHz, Dual-trace, signal delay, delayed trigger
5mV (ltnV)/div, 3.5ns
An
economic
development work.
instrument
for
advanced
troubleshooting
and
Chl, Ch2 used for
vertical analysis (Y)
Ch3, Ch4 used for (X)
and ex t. t r igger
(t ime r e f e r e n c e )
Specifications (abbreviated):
VERTICAL AXIS (Ch 1, Ch 2 identical)
HORIZONTAL AXIS (Ch 2)
sensitivity: 5mV/div to 5V/div(Xl mode)
ImV/div to lV/div(X5 mode)
modes :
accuracy : +_ 2%
attenuator: 5mV/div to 5V/div
input: IM 22pF
f-response
DC: DC to 100MHz (-3 dB)
AC: 5Hz to 100MHz (-3 dB)
operating modes: C h l , Ch2, DUAL, ADD
X-Y mode is switch
selectable (HORIZ
DISPLAY)
SWEEP
modes
A, ALT, A-INT-B,
B DLY'D, DUAL, X-Y,
HOLDOFF
QUAD, ALT, CHOP
VERTICAL AXIS (Ch3, Ch4)
sensitivity: O . l V / d i v , IV/div +_ 2%
attenuator : 1/1 , 1/10
input coupling mode: DC only
delay method: continuous delay
trigger delay
delay t ime
TRIGGERING
A modes: AUTO, NORM, SINGLE, FIX
source: V MODE, CHl, CH2, (EXT) CH3
1/1 and 1/10, LINE
coupling: AC, LFREJ, HFREJ, DC, VIDEO
B modes
0.2 to 10 t imes
the sweep time
from 200ns to
0.5s, continuously adjustable
STARTS AFTER DELAY,
TRIGGERABLE AFTER
DELAY
INTENSITY MODULATION INPUT
VERTICAL AXIS, GATE OUTPUT (A and B)
BC (F)
(50)
-313.3.8a
Chapter 3
Double Pulse Generator
The simultaneous positive and negative o u t p u t s deliver 2 w a t t s into
50 ohms.
Pulse a m p l i t u d e , w i d t h , delay and r e p e t i t i o n rate are continuously
variable.
Other c a p a b i l i t i e s include single or double pulse operation,
external
triggering
synchronous or
asynchronous
gating,
reference
trigger o u t p u t s , sine wave triggering, and manual
single pulse operation.
Specifications:
repetition rate
10 Hz - 10 MHz
external trigger
_+ 0.25V, 20nsec min, 50 Hz - 10 MHz
sin. IVrms manual cycle
synchronous, asynchronous gating possibilities
advanced trigger
+ 1.7 V min, 15ns
reference trigger
+ 2 V rain, 15ns
pulse mode - single:
- double
one output pulse at the end of the delay
period
two identical pulses per cycle first
after the reference trigger, second
after the selected delay
pulse delay
40 ns - 10 ms
pulse width
40 ns - 10 ms
pulse height
0.5 - 10V (50 )
ABC
(51)
Chapter 3
3.3.9
-32DC-Power Supply
A large number of different types
are
available on
the
market,
covering
the range
from r a t h e r
simple ones to high p e r f o r m a n c e
power supplies.
Typical
specifications
e l e c t r o n i c s l a b o r a t o r y are:
for
an
application
in a
nuclear
3 independent outputs
0-5.5
V/7A
Ü-3Ü
V/1A
0-30(65) V/1.2
(0.6)A
O u t p u t v o l t a g e a d j u s t a b l e with precision pot.
±
0.3%
O u t p u t current a d j u s t a b l e , short circuit proof
Load e f f e c t (0-100%)
< 5 (< 8) mV
Source e f f e c t (^ 10%)
< 2 mV
Noise
< 5 mVpp
Over-voltage p r o t e c t i o n
ABC
(52)
-333.3.14
Chapter 3
Digital Logic Probe
C o m p a t i b l e w i t h DTL, TTL, CMOS.
v o l t a g e s 4.5V to 30V dc.
Min pulse width lOnsec.
supply
ind i c a t i o n s :
HI
LO
red
green
open-circuit :
yellow
pulse-memory :
red
mode :
DELUXE MOLDED PLASTIC CARRYING CASE
pulse/raeraory
level :
TTL/CMOS
frequency range:
GROUND LEAD
dc - 50 MHz
input impedance:
> 10 M
IC-CLIP LEAD
General specifications:
LOGIC THRESHOLD
TTL Logic "1"HI: 2.2V +0._3V(@5VDC)
Logic "0"LO: 0.7V +0.3V(@5VDC)
CMOS Logic "1"HI: 70% VDD^IOZ VDD
Logic "0"LO: 30% VDD+_10% VDD
Power: 4.5V to 30V DC, 50mA max. @5V DC
Input Overload Protection: _+ 50V DC/AC continuous
Max.
+_ 120V DC/AC for 10 seconds
Power Input Protection:
+_ 50V DC/AC continuous
Max.
_+ 100V DC/AC peak for 10 seconds
A u d i b l e Warning:
Built-in buzzer emits alarm when an input signal
exceeds the VDD of the circuit being tested or when
a voltage higher than 30V
DC is applied to power
input or when power lead is connected reversely or
with AC line.
ABC
F
(53)
-34-
Chapter 3
3.3.16
D i g i t a l C i r c u i t Tester
A complete
multi-family
kit.
Stimulus-response c a p a b i l i t y , i n circuit
fault
finding, dynamic
and
static
testing, m u l t i - p i n
testing.
To a c c o m p l i s h t r o u b l e s h o o t i n g at
the node and gate
level, b o t h
stimulus
(Pulser) and
response
(Probe, T r a c e r , C l i p and Comparator) i n s t r u m e n t s
are
needed.
Moreover,
instruments
with both
voltage
and
current
troubles h o o t i n g c a p a b i l i t y help
isolate
electrical
faults
where
the
precise physical location is hard
to i d e n t i f y .
FAULT
HÏSWNK
STIMULUS
Shorted Node'
Pulser!
Current Tracer
Stuck Data Bus
Pulser'
Current Tracer
TIST METHOD
• Pulse shorted node
• Follow current pulses to
short
• Pulse bus hnt(s)
• Trace current to device
holding the bus n a
stuck condfton
• Pulse and probe test point
simultaneously
Signal Un« Short
to Vcc or Ground
Pulser
Prob«.
Current Tracer
• Short to Vcc or Ground
cannot be overridden by
pulsing
• Pulse test pomt and
toBow current pulses to
the short
• Remove power from circuit
under test
• Disconnect electrolytic
Supply to Ground
Pulser
Current Tracer
Short
bypass capacitors
• Pulse across Vcc and ground
using accessory connectors
provided
Internally
Open 1C
Pulser'
Solder Bndgi
Puter'
Probe
Current Trxer
• Trace current to lauft
• Pulse device nput(s)
• Probe output tor response
• Pulse suspect kne(s)
• Tract current pulses to
the fault
• light goes out when solder
bridge passed
• Circuit clock de-activated
SequentuHogK
Fault m Counter
orSNrtfitgrsler
Puiser
Op
• Use Pulser to enter desired
number ol pûtes
• Place Of on counter or
sMt register and verity
device truth table
(54)
The t a b l e shows a series of
t y p i c a l node and g a t e
faults
and the c o m b i n a t i o n of tools
used
to
troubleshoot
the
c i r c u i t . As w i t h
all sophist i c a t e d m e a s u r i n g instruments,
operator skill
and circuit
knowledge are key factors once
the various clues, or "bits"
of
i n f o r m a t i o n , are
obtained
using the 1C Troubleshooter s.
BC (F)
Chapter 3
-353.3.17
Break-Out Box
A tester for V24, RS-232 interfaces
The instrument is inserted into the RS-232 link.
LEDs show the status of each line.
With UlP-switches each line can be interrupted and arranged, also a
low or high level can be applied.
A delay of two adjustable times is possible for each signal.
Such economic instruments are recommended
of serial interfaces.
for fast troubleshooting
ABC
(55)
Chapter 3
3.3. 20
-36Prom Programmer
suitable
for programming
Proms of the type 2716 to
27512
serial and parallel interfaces
512
k-bit
memory
RAM
internal
The c a p a b i l i t i e s of such an instrument are:
p r o g r a m m i n g (4 programming algorithms)
e d i t i n g (e.g. e d i t i n g the buffer RAM)
CPU communication (operation from a host c o m p u t e r )
e m u l a t i o n (transfer d a t a from b u f f e r RAM to optional emulator
module )
i n t e r f a c e to an external device (RS-232, Centronics)
3 a d a p t o r s for
copying d i f f e r e n t
types of E-Proms
2 adaptors mainly
used for m u l t i p l e copying of E-Proms
(gang-programming)
Some Prom-programmers on the
Bi-polar devices and PALs.
market
are also able to program
But normally, PAL-circuits are protected and
therefore no
copying is possible (spare PAL circuits have to be ordered from the
manufacturer).
(56)
-37-
Chapter 3
The Prom Programmer
described
requires
for E-Proms a
personality
module for the individual Prom families to
be
p r o g r a m m e d . Especially with new devices, it takes some time until
such cards become available from the manufacturer and
sometimes
even a m o d i f i c a t i o n to the basic instrument is necessary to provide
the required features.
New Prom Programmers which are now available allow programming
of the i n d i v i d u a l parameters for burning a device into their memory
and are
therefore more
flexible.
Personality
cards for these
i n s t r u m e n t s are not necessary.
(57)
Chapter 3
3.4
-38-
RECOMMENDED ELECTRONIC COMPONENTS
Below
is a list of spare parts and electronic components that
are considered to represent
an
optimal store
for a medium large
electronics service laboratory. The last column in the catalogue
n u m b e r refers to a large mail house in the FRG (Fa. Buerklin, P.O.
Box 200440, 8000 M u n i c h , FRG).
The value of these components
is
a b o u t US$ 4.000.-. By adjusting the number of ordered components,
a less or more expensive stock can be acquired.
Item
Quant i t y
Description
Cat.
No
Resistors
1
2
3
4
5
50
50
50
50
100
6
7
8
9
10
M e t a l resistor
1ROO
3R32
5R90
6R81
10RO
31E710
31E760
31E778
31E781
30E100
100
100
100
100
100
33R2
51R1
75RO
100R
475R
30E150
30E168
30E184
30E196
30E261
11
12
13
14
15
200
100
200
200
200
1KOO
3K48
10KO
100K
1MOO
30E292
30E345
30E389
30E485
30E582
16
17
18
19
20
50
50
1
5
5
3M90
10MO
Service Sortiment, CCR-011R
Resistor
5W.OR33
OR47
30E645
30E695
27E230
62E112
62E116
21
22
23
24
25
5
5
3
3
3
26
27
28
3
20
20
OR68
1RO
Resistor network
10W.MR1
MR2
MR3
MR4
Varistors
275VAC.1W
150VAC,1W
62E118
62K124
37E900
37E905
37E910
37E919
82E2255
82E2235
Capacitors, coils
29
30
(58)
50
50
Tantal capacitor
6.8uF,16V
6.8uF,35V
26D479
26D600
Chapter 3
-39lOuF, 16V
10uF,35V
22uF, 16V
22uF,35V
47uF, 16V
26D480
26D605
27D524
27D568
27D528
31
32
33
34
35
50
50
50
50
50
36
37
38
39
40
50
1
50
50
50
41
42
43
44
45
50
50
50
50
50
46
47
48
49
50
50
50
20
20
20
10nF,40V
1 OOnF,50V
Ceramics, high vol. lOpF, 3kV
100pF,3kV
22pF,3kV
59D250
53D661
61D300
61D324
61U308
51
20
61D352
52
53
54
55
56
20
20
20
20
20
10nF,3kV
10uF,6kV
Styroflex capacitor 470pF , 160V
lOOOpF, 160V
lOOOOpF, 160V
HF-coil
22uH
lOOuH
47uF,35V
Service S o r t i m e n t , CCC-001
1000uF,40V
Electrolytic cap.
2200uF,40V
4700uF ,40V
Ceramics cap.
10000uF,25V
10pF,63V
1 OOpF,63V
470pF,63V
lnF,63V
27D572
28D810
11D315
11D320
11D325
11D274
59D162
59D174
59D182
59D200
49D304
49D308
49D314
74D316
74D324
Potent iome t ers
57
58
59
60
10
10
10
10
61
62
63
64
65
66
5
3
3
3
3
3
Potent iome t er
IK
4K7
22K
100K
lOturn p o t e n t i o m e t e r , IK
2K
5K
10K
100K
Dial
68E223
68E225
68E227
68E229
67E853
67E854
67E855
67E856
67E860
20H712
Transistors and linear integrated circuits
67
68
69
70
10
10
10
10
JFET, 2N4416
2N4861
2N3819
2N3823
27S7700
27S8300
27S4700
27S4850
(59)
Chapter 3
-40-
100
100
20
10
10
Bipolar Transistor, 2N3904
2N3906
Transistor, BFY90
Comparator, LM710
LM711
27S5200
27S5300
17S4000
41S5300
41S5350
10
10
10
10
10
LM311D
Operational amplifier, LM741CD
LM748CD
LM318D
LM324D
41S2290
41S5380
41S5405
41S2575
41S3175
10
10
10
10
20
LF356N
LF357N
TL071CP
CA3140E
Voltage regulators, LM723CD
40S9000
40S9060
49S5500
40S2950
41S5370
10
10
10
10
10
UA7805CKC
UA7812CKC
UA7824CKC
UA7905CKC
UA7912CKC
50S1450
50S1600
50S1750
50S1800
50S1950
10
20
20
10
10
UA7924CKC
LM317T
LM337T
LM340K12
Darlington Transistor, BD651
50S2100
41S2550
41S3600
41S3750
14S6150
10
5
5
5
5
Power
Power
Power
Power
BD652
Transistor, 2N3055S
switching Tr. MJE3055
HV Transistor, BU208
switching MOS FET BUZ80
14S6200
27S2500
24S4900
19S2950
19S6800
5
20
20
20
20
Power Transistor, TI.P33F
npn Transistor 2N2219A
pnp Transistor 2N2905A
npn Transistor, 2N2222A
pnp Transistor, 2N2907A
25S2875
27S1100
27S1950
27S1250
27S2150
10
10
5
npn Transistor, BD139-16
pnp Transistor, BD140-16
Thyristor, 2N4441
13S5300
13S5500
27S7800
Diodes , zener diodes
109
110
111
112
113
114
115
(60)
50
100
20
5
3
5
10
Diode, 1N4007
1N4148
1N5408
Bridge rectifier
3A.600V
10A.500V
Switching diode, MR854
Zener diode
ZPD 5 . 1
26S8100
26S8150
26S8876
55A658
57A190
24S4910
25S8000
Chapter 3
-41116
117
118
119
120
10
10
10
10
10
ZPD
ZPD
ZPD
ZPD
ZPD
5.6
6.8
8.2
10
12
121
122
123
10
10
10
ZPD 15
ZPD 24
ZPD 33
25S8050
25S8150
25S8250
25S8350
25S8450
25S8550
25S88UO
25S8950
Fuses
124
125
126
127
128
1
1
20
20
20
129
130
131
132
20
20
20
20
Servicesortiment, fast
slow
Precise fuse
s low,
0. 1A
0.4A
1.6A
46G240
46G244
46G268
46G274
46G280
4A
0.4A
1.6A
4A
46G284
46G319
46G325
46G329
Soldering supplies
133
134
135
5
5
5
FLUITIN soldering wire, 100g, 0.7 5mm 11L402
Desoldering wire, 1 . 3mm
10L752
2 . 5mm
10L756
Connectors and cables
136
137
138
139
140
5
5
20
3
3
141
142
143
144
145
BNC extension connector
BNC, T
BNC, cable plug
NIM connector, male
NIM connector, female
78F270
78F290
78F200
55F7251
55F7253
50
50
100
100
6
Centering pin
Centering recept ors
Contact pins
Contact receptor s
Protecting cover
55F760
55F766
55F7725
5 5F77 8
55F798
146
147
148
149
150
3
3
3
3
300
Connector ,
Pins
55F400
55F401
55F410
55K412
55F408
151
152
153
154
155
300
25m
10m
20m
10m
Pins
Cable RG58C/U
RG59B/U
Cable , 5pol
Flat cable , 50po 1
55F418
96F730
96F746
94F306
94F439
9pol ,
25pol ,
9pol ,
25pol ,
male
male
female
female
(61)
Chapter 3
-42-
ü i g i t a l i n t e g r a t e d circuits
156
157
158
159
160
50
5Ü
50
50
50
SN74LSOON
161
162
163
164
165
166
20
20
20
20
10
10
SN74LS138N
SN74LS123N
SN74LS191N
SN74LS193N
SN74LS374N
SN74LS245N
43S5900
LED,
67S4350
67S4450
67S4500
SN74LS02N
SN74LS10N
SN74LS74AN
SN74LS90N
43S2750
43S2900
43S3250
43S4600
43S4950
43S5550
43S7150
43S7250
43S9900
43S7800
LEDs
167
160
169
170
20
20
20
20
3 m m , red
3mm , green
5mm , red
5mm, green
67S4600
Swi t ehe s
171
172
173
5
5
5
S w i t c h , Ipol, E-E
2pol, E-E
2pol, E-A-E
10G700
10G740
10G750
Miscellaneous
174
175
176
177
178
179
180
(62)
1 set
1 set
1
1 pak .
2
2
10
Screws
16H695
16H954
80B533
12H592
46G628
Nuts
Heat conducting paste
Wire wrap pins
Fuse h o l d e r ,
5x2 Omm
6.3x31. 7mm
46G642
IC-breakable sockets
16B110
-43-
SKILLS :
Chapter 3
R e a d i n g and U n d e r s t a n d i n g of Circuit Diagrams
Diagrams are the main aids to repair. For fault location the
most essential information is t h a t concerning functional s t r u c t u r e ,
i.e.
how the components are connected to perform their required
f u n c t i o n . C i r c u i t diagrams are
o f t e n c r i t i c i z e d on the basis of
bad p r e s e n t a t i o n ; due to t h i s , national s t a n d a r d s were d e v e l o p e d to
specify the r e q u i r e m e n t s for an e f f i c i e n t diagram.
A c c o r d i n g to B r i t i s h S t a n d a r d :
"Diagrams should be drawn so
that
the main sequence of c a u s e - t o - e f f e c t goes from left to r i g h t ,
and/or from top to b o t t o m . The input should always be on the l e f t ,
and
t lie o u t p u t
on
the r i g h t . When this is
i m p r a c t i c a l , the
d i r e c t i o n of operation should be shown by an arrow. Components
a s s o c i a t e d w i t h each operational stage should be grouped together."
U n f o r t u n a t e l y , really good c i r c u i t diagrams are still rare.
It is o f t e n said that the after-sales policy of the manufacturer is
mirrored in the diagrams.
Some companies prefer
to give repair
services and
they d i s t r i b u t e almost useless circuit diagrams to
scare away n o n - f a c t o r y approaches to maintenance.
The
s t a n d a r d i z a t i o n of the graphic symbols for components is
much more e f f i c i e n t ; however, some companies still use their own
local s t a n d a r d s in 'the nuclear field. The same applies to cable
connection notations and markings. There is no general rule; each
company
has its own g r a p h i c symbols in m u l t i - s h e e t diagrams for
i n t e r c o n n e c t i o n s , test p o i n t s , e t c .
During repair work, correct reading of the graphic symbols is
essential. This should be t e s t e d very carefully; even a single
m i s t a k e in identifying
the symbols indicates further
learning
requirements.
The
next
step
is to test ones' colour code
reading
c a p a b i l i t i e s . For
e f f i c i e n t repair work, one should acquire a
faultless reading c a p a b i l i t y with a rate of ten items per minute.
Colour b l i n d n e s s i n h i b i t s the c o r r e c t
i d e n t i f i c a t i o n of resistor
values. This can be r a t h e r dangerous in repair work and it should
be avoided by employing personnel without this deficiency.
It
is very important to be able to locate certain components
on the c i r c u i t board
from the circuit diagram and vice versa.
Skills can be quickly developed by training. An a c c e p t a b l e level
is d e m o n s t r a t e d by being able to c o r r e c t l y locate ten components,
b o t h ways, in ten m i n u t e s .
The c a p a b i l i t y to locate components running at high voltage is
a needed skill in nuclear instrument repairs. The repair personnel
should be able to correctly mark with red pencil all high voltage
lines and c o m p o n e n t s on the diagram within 30 m i n u t e s , and
they
should be able to locate the same in the i n s t r u m e n t as well.
In many systems, interlocks, fuses, and thermo-switches might
inhibit
o p e r a t i o n due to present or past hazard situations. Often
(63)
Chapter 3
-44-
they are connected to logic c i r c u i t s , timers, e t c . The technic ians
should be able to correctly identify such circuits w i t h i n ten
minutes
Circuits can
be
inoperative
because
of
faulty switch
functions.
It is important to be able to identify which circuit
points should be connected
together
to secure operation and which
lines should be c u t .
Such decisions should be c o r r e c t l y made
w i t h i n five minutes.
Signal propagation determination is very important. This can
be a c o m p l i c a t e d task. The general rule is to find each active
components inputs and o u t p u t s . This requires f a m i l i a r i t y with the
1C pin configurations: location of the power inputs and
signal
lines. It is not i m p o r t a n t to memorize these pins, however. In two
minutes time one should be able to list the e x p e c t e d voltage levels
and signals on a 14-pin 1C, after looking on its diagram and the
s p e c i f i c a t i o n s in the catalogue.
A
nuclear electronics service man should be
able
to
discriminate between filtering c i r c u i t s around the power lines and
frequency characteristics d e t e r m i n i n g components round amplifiers.
They should be able to correctly group such components at a rate
of four items per m i n u t e .
In some circuits the feedback loops are rather elaborated.
However, they should be correctly identified as the time limit is
one hour.
The real test of "understanding"
a circuit is
electronician can prepare a correct functional diagram
circuit.
if
of
the
the
The general rules for producing functional diagrams are:
the main signal flow must be from left to right;
the signal flow must be emphasized; this can be done by
showing
flow paths in a straight line, avoiding crossover
of flow paths;
arrows may be used to
avoid ambiguity;
indicate
direction of flow, only to
main inputs must be on the left and outputs on the right;
the source of all inputs and the destination of all outputs
must be shown;
all plugs, sockets, controls, test points and terminals
useful in troubleshooting must be shown and referenced;
normal state measurements for all test points must be
available ;
symbols must be according to local standards.
It is a good practice to mark points where stages can be
isolated from each other, with indication of possible test signals
and expected correct outputs.
(64)
-45-
Chapter 3
In
the case of missing drawings and
documentation,
functional diagram preparation is recommended as an efficient
in repair work vs. the t r a d i t i o n a l d r a f t s m a n a p p r o a c h .
the
aid
(65)
Chapter 4
TROUBLESHOOTING IN SYSTEMS
-1-
Chapter 4
TROUBLESHOOTING IN SYSTEMS
4. 1
INTRODUCTION
A Nal detector is used in many fields of application
of
nuclear techniques, e.g. in nuclear medicine, radioimmunoassay,
agronomy, radiation protection and uranium prospection. Therefore,
a Nal detector system was chosen as an example of troubleshooting
in a d e t e c t i o n system. Such a system consists of a detector,
usually surrounded by
a
radiation
absorbing shield and a
collimator, a p r e a m p l i f i e r , an amplifier, a single channel analyzer
(SCA) and a sealer-timer or a ratemeter, or, instead of the last
three, a multichannel analyzer (MCA). Furthermore, the system
contains a high voltage supply
(HV)
and several low voltage DC
supplies (LV). The units composing the system and their functions
are given in Fig. 4.1. For other detectors, a similar set of
electronic units is used with characteristics adapted to the
particular d e t e c t o r . In the following t e x t , examples are taken
from medical applications.
The reader should be able to find
equivalent ones in his own field of application.
Fig.
4.1: A typical Nal system
Most of the instrument users, applying nuclear techniques,
work
in fixed geometry, and with fixed HV, amplifier
and
single-channel-analyzer settings.
Most of their measurements are
relative measurements: a comparison is made with radioactive
standards. Thus when the user or operator says, "My instrument
does not behave as it should", one expects that
they have not
changed their instrumental settings.
However, checking those
settings is the first part of troubleshooting, since a clear
distinction must be
made
between
user/operator errors and
instrument failure.
Many
of
the
questions which must
be
asked
during
troubleshooting can only be answered correctly when instrument
settings and previous test results are available in a logbook.
(69)
Chapter 4
4.2
-2-
INSTRUMENTS AND SKILLS
The following instruments and
tools are required for a system
check (for d e t a i l e d information, see Chapter 3):
- a m u l t i m e t e r (MM);
- an oscilloscope (0)
0-15/20 MHz, with dual beam or dual
trace, sensitive trigger properties (it must be possible to
trigger both channels at the same time or alternatively with
the signal on one channel) and with signal delay lines to
enable visualization of the front edge of pulses;
- a
set of screwdrivers, Alien keys, plyers, flat keys;
- (a) pulse generator(s) (PG) with which nuclear pulses can be
simulated, both for analog pulses and for digital pulses;
and
- a radioactive test source, with known a c t i v i t y ,
the same as used during acceptance testing.
preferably
In order to do troubleshooting one should know: (1) the usual
composition of a Nal detection system and the functions as given in
Fig. 4.1; (2) the forms of
the signals at the output of the PMT,
the p r e a m p l i f i e r , amplifier and single channel analyzer; (3) why
one does not observe a signal at the input of a charge sensitive
amplifier; (4) that the rise time of the pulse at the output of the
PMT depends on the half life of the scintillation process in the
d e t e c t o r ; and (5) that the pulse heights and shapes at the o u t p u t
of the pulse amplifier are so chosen as to fit best to the SCA (or
MCA) and to guarantee optimum s p e c t r u m resolution.
Each of the following paragraphs gives a possible indication
of failures as may be given by instrument users/operators and a
list of possible origins or reasons for
these failures a n d , when
not obvious, actions to be taken. There may be among the operators
very unexperienced ones. This may lead to some reasons which seem
ridiculous; nevertheless, they are based on experience in the
field.
4.3
THE INSTRUMENT DOES NOT WORK
To be checked:
1.
Is the instrument switched on?
2.
Is there power on the AC outlet?
3.
Are the connections in the
cable all right?
4.
Are the fuses on the instrument all right (AC and DC)?
5.
Are the DC power supplies all right?
(70)
AC
plug
all right?
Is the power
-36.
A.A
Chapter A
Are the signal lamps faulty?
THE INSTRUMENT DOES NOT COUNT
To be checked:
1.
Is the test sealer in test function?
2.
Is the KV switched on and connected to the detector?
3.
Is the input polarity of the amplifier correctly set?
A.
Has the threshold of the SCA been set to maximum or the window
to 0?
5.
Are the DC fuses all right?
6.
Are the DC supplies correct? Check voltages of HV and LV.
7.
Are all signal cables alright? Check the presence of signals
at the output or preamplifier, the input and output of the
amplifier, the input and output of the SCA and at the input of
the sealer. Check whether the plugs are correctly mounted,
e.g.
the pin in a male BNC may be moving backwards when
connected to the socket.
8.
Are the cables connected to correct inputs and outputs?
A. 5
THE COUNT RATE IS LOWER THAN USUAL
Possible reasons:
1.
There is no radioactive source in the measuring position, or a
wrong source, a wrong patient or sample, a patient wrongly
injected.
2.
The instrument is set incorrectly for the radioisotope used.
3.
There is more absorbing medium between source and detector
than usual: different source (test-tube) shapes, or the test
tube has
thicker walls or is made of material other than
what is usual.
A.
The source-detector geometry has changed,
not fully reach the measuring position.
5.
A wrong colliraator is used (particularly for a scanner).
6.
The AC frequency is too high (in case the timer uses
frequency as time base). Check with wristwatch.
7.
The HV setting is incorrect.
or the sample did
the
AC
(71)
Chapter 4
-4-
8.
The settings of the amplifier have been changed.
9.
There is a change in the SCA setting.
10.
There is a change in the sealer
scale factor or input polarity.
11.
The
12.
The preamplifier or amplifier is faulty, check pulse shapes.
13.
There is a change in pulse height or shape.
14.
The SCA is faulty:
incorrect.
15.
The sealer has an intermittent failure or is faulty.
16.
The time clock is defective.
17.
A broken resistor in the dynode chain, a bad phot omul t i p 1 i e r ,
a bad optical coupling between scintillator and PM, a shorted
capacitor in the dynode chain.
4.6
THE COUNT-RATE IS HIGHER THAN USUAL
LV
threshold
s e t t i n g , in the
power supplies are faulty; the HV supply
output
pulse
height
is
faulty.
See section 4.7
or
pulse duration
Check with wristwatch.
Possible reasons:
1.
There is an extra background source: detector directed to
source preparation or store rooms, injected patients near to
detector, radioactive contamination, a test source is not
placed
in its
lead
container,
people in neighbouring
laboratories are using radioactive sources, e t c .
2.
The patient is injected with the wrong
patient wrong source.
3.
The patient was previously injected with another radioisotope.
4.
The instrument is set incorrectly for the radioisotope used.
5.
There is less absorbing medium between the source and the
detector than usual; a different source (test tube) shape,
the test tube has a thinner wall, or is made of a different
material than usual.
6.
The source-detector geometry has changed.
7.
A wrong collimator is used.
8.
The lead shield or collimator has been removed or changed
position.
9.
There is a change in amplifier, SCA or HV setting.
(72)
radioisotope ,
wrong
in
-5-
Chapter 4
10.
The AC frequency is too low
(in
case the timer uses the AC
frequency as time base). Check with wristwatch.
11.
The
sealer
has a wrong setting of the scale
factor and/or
input p o l a r i t y , or the setting of input discriminator has been
changed.
12.
For m e d i c a l laboratories: the activity meter (dose calibrator)
is f a u l t y .
13.
There are parasitic pulses, spurious counts or
section 4.7
14.
The o u t p u t pulses of the amplifier have overshoots. Overshoots
may be counted too! Pay
attention to signal reflections in
long cables. Overshoots may cause ghost peaks on MCA systems
w i t h a fast ADC. Check signal shapes. See section 4.7
15.
The earthing has changed or ground connections have loosened.
16.
The DC power supplies are not correct (LV, HV).
17.
The a m p l i f i e r or preamplifier is faulty.
18.
There is a change in pulse height or shape.
19.
The SCA is faulty.
20.
The sealer is defective.
21.
The timer is defective.
22.
There is & light leak in the PMT and/or the canning of the
d e t e c t o r resulting in extra pulses in the low energy part of
the spectra and peak widening.
4.7
noise.
See
Check pulse shapes.
See section 4.7
THE PULSE HEIGHT IS LOWER OR HIGHER THAN USUAL
Possible reasons are:
A.
W i t h o u t change in pulse shape;
1.
Incorrect radioactive source.
2.
Incorrect HV setting.
3.
Incorrect
amplifier
settings:
amplification
factor,
pulse
polarity.
4.
The detector assembly has been interchanged.
5.
The amplification factor of the preamplifier has been changed.
6.
A channel shift or wrong setting of the SCA
comes up only when no oscilloscope is used).
(this
question
(73)
Chapter 4
-6-
7.
The d e t e c t o r is broken or yellow-brown (this cannot be checked
for an integral line assembly).
8.
A bad o p t i c a l joint
between
the d e t e c t o r and the PMT (this
cannot be checked for an integral line assembly).
9.
A HV s u p p l y failure.
B.
W i t h change
in pulse shape (rise time, fall time, overundershoots) :
10.
Incorrect s e t t i n g of
the amp 1 if ier .
11.
The
pulse height and/or
shape
p r e a m p l i f i e r has changed.
A faulty pulse-shaping amplifier.
12.
differentiation
at
and
and integration time of
the
output
of
the
13.
The ground connection to the d e t e c t o r
canning is loosened or
i n t e r r u p t e d , or in g e n e r a l , a change in earthing.
14.
The
PMT
is faulty or has changed its c h a r a c t e r i s t i c s .
The
d y n o d e s may have
changed
position
or
shape.
E s p e c i a l l y in
portable
instruments,
rectilinear
scanners
and
other
i n s t r u m e n t s of which
the d e t e c t o r
heads are moving or are
exposed to m e c h a n i c a l shock.
15.
Bad c o n t a c t s in or f a u l t y
HV v o l t a g e d i v i d e r , or bad c o n t a c t s
between PMT and base. A t t e n t i o n should be paid to c a p a c i t o r s
at the dynodes near the anode.
16.
The u - m e t a l s h i e l d has changed its p r o p e r t i e s or is dislocated
(in
f a c t , such a deficiency mainly influences the h e i g h t
of
the pulse).
17.
A very low AC voltage.
18.
A failure in one of the LV supplies.
19.
F a u l t y p l u g - c a b l e connections.
4.8
SPURIOUS COUNTS
1.
Take
all radioactive
sources
background is higher than usual.
2.
Check whether background
of (parasitic) pulses.
3.
If this is the case, observe pulses on oscilloscope or sealer
and try to detect relations between pulse trains and the
switching
of
equipment
(incubators,
deep
freezers,
r e f r i g e r a t o r s , floor polishers, workmen with drills and other
machines) in own or neighbouring laboratories.
(74)
away, and
If yes:
check
whether
pulses appear as i n t e r m i t t a n t trains
-7-
Chapter 4
4.
Check whether pulses are coincident with the
fluorescent lamps.
5.
Or with starting or running
parking.
6.
Unproperly filtered AC.
flickering
of
motorbikes and cars on an outside
Other origins of spurious counts may be:
7.
E l e c t r o s t a t i c discharges, especially in very dry laboratories.
8.
HV sparking due to dust or humidity or parts starting to fail.
9.
N e a r b y switching thyristors or triacs.
10.
When there are nearby radio or TV receivers, check whether the
instrument
is properly grounded. Try different points, and
check connections and coaxial cables. •
11.
E l e c t r i c a l discharge
humid ity.
12.
Loose cable c o n t a c t s .
ics in PM tube.
4.9
on
surface
of
components, due to high
Cable-movement-caused noise, raicrophon-
THE SPREAD IN THE NUMBER OF COUNTS IS LARGER THAN MAY BE
EXPECTED ACCORDING TO STATISTICAL FLUCTUATIONS WHEN
M E A S U R E M E N T S OF THE SAME SOURCE IN FIXED GEOMETRICAL
POSITIONS ARE DONE EACH DAY
Possible causes may be:
1.
An irreproducible source positioning.
2.
The use of different source holders or test tubes.
3.
Changes in the absorbing medium:
4.
The timer uses (the f l u c t u a t i n g ) AC frequency as time base.
5.
The setting of the HV is irreproducible or the HV is unstable.
6.
I r r e p r o d u c i b l e settings of the amplifier and/or SCA.
7.
Large temperature
ins t rumen t .
8.
Spurious counts.
9.
Large AC fluctuations.
10.
Unstable LV power supplies.
11.
Bad earthing.
dust or dirt in factories.
changes, or too short warming-up times
of
(75)
Chapter 4
4.10
-8-
LOSS OF SPECTRUM RESOLUTION
Possible origins are:
1.
Disturbances in the HV supply:
2.
Detector colouration or a broken crystal.
3.
A bad optical joint.
4.
Unstable LV power supplies.
5.
An unstable preamplifier or main amplifier.
6.
Light leaks into the PMT or detector.
7.
The deterioration of the photo-cathode of the PMT.
8.
An oscillating preamplifier.
9.
Bad earth connections.
10.
Mechanical
scanners ) .
(76)
vibrations
of
ripple and fluctuations.
dynodes
in
PMT
(rectilinear
Chapter 5
POWER SUPPLIES
-15
POWER SUPPLIES
5.l
GENERAL REMARKS
Chapter 5
Before starting
to discuss t r o u b l e s h o o t i n g for
specific
power
s u p p l i e s , some general c o m m e n t s , and some h i n t s for t r o u b l e s h o o t i n g ,
are given.
The
discussion on some
specific power
supplies
is
p r e s e n t e d in Sections 5.3 through 5.7.
There are two
main types of power supplies:
The linear r e g u l a t e d supplies with power t r a n s f o r m e r ,
r e c t i f i e r s , c a p a c i t o r f i l t e r i n g , pass t r a n s i s t o r , c u r r e n t
sense resistor for current limitation either for constant
c u r r e n t or w i t h f o l d b a c k c h a r a c t e r i s t i c s and the error
a m p l i f i e r including t h e voltage r e f e r e n c e source.
The s w i t c h e d
mode power
supply with
direct
ac
line
r e c t i f y i n g , filtering, fast s w i t c h i n g power
transistors,
ferrit-core
transformer
for
power
t r a n s f e r , fast
switching rectifier
diodes, filters with
chokes and
c a p a c i t o r s c u r r e n t sense
resistor for c u r r e n t l i m i t a t i o n
e i t h e r for constant c u r r e n t or f o l d b a c k c h a r a c t e r i s t i c ,
r e g u l a t o r w i t h pulse w i d t h m o d u l a t i o n
including voltage
r e f e r e n c e source and in the f e e d b a c k loop e i t h e r an
opto-coupler or a pulse transformer.
The first s t e p in t r o u b l e s h o o t i n g
is a visual c h e c k , looking
for burned c o m p o n e n t s . If the i n s t r u m e n t smells, or if smoke
is
coming o u t , t h e r e is obviously something wrong inside. Try to find
out
in which part of the circuit the fault appears. Look for cold
soldering p o i n t s . Check the fuses.
Transformers are f r e q u e n t l y
damaged by an overload; fuses of the wrong value were inserted, and
did not p r o t e c t the c i r c u i t s .
The
rectifier
reasons are:
a)
b)
diodes
can get d a m a g e d ;
the
most
frequent
r e p e t i t i v e peak c u r r e n t , and
reversed bias voltage.
O c c a s i o n a l l y , we m i g h t
find
that
the designer made a wrong
selection
of
the diodes;
the assumed
s p e c i f i c a t i o n s do not
correspond with
the ratings.
Typical examples are
with
the
frequency of the mains (50:60Hz) or voltage (117:220V).
In older e q u i p m e n t , capacitors may become defective due to
h e a t ; they are used at the limits of their voltage s p e c i f i c a t i o n s ;
this
is especially true for t a n t a l i u m drop-form c a p a c i t o r s .
Pass
or power transistors are mainly d e s t r o y e d by o v e r v o l t a g e , excessive
(79)
Chapter 5
-2-
current
or
overheating.
Defects
in regulators are
caused
by
reversed b i a s and overvoltage
(voltage
spikes which are exceeding
the m a x i m u m rating). Sense resistors may be destroyed by excessive
output current.
Dust and
dirt
obstructs
cooling
of c o m p o n e n t s ,
which m i g h t lead to o v e r h e a t i n g and breakdown.
ATTENTION : N E V E R t r u s t fully the c i r c u i t diagram. Compare it
w i t h the c i r c u i t r y and wiring.
ATTENTION : NO normal fuse can p r o t e c t a s e m i c o n d u c t o r ; it is
always the other way around.
NOTE : A f t e r r e p a i r , a re-adjustment of the i n s t r u m e n t has to
be made.
If e l e c t r o n i c components were replaced
by
similar ones (and not exactly
identical), a quality
control check after repair has to be performed
and
recorded in a log
book.
This is very i m p o r t a n t for
further troubleshooting.
Different
power
supplies are being
used
in
nuclear
i n s t r u m e n t s , and in auxiliary equipment
that is being a p p l i e d in
nuclear l a b o r a t o r i e s . Typical types of power supplies are:
i)
about 400W,
NIM power supply , usua lly
24 (+6,-6);
volt ages : +12,
+24 , -12
ATTENTION :
with the followin
Some of the NIM power supplies do not have +/-6V.
There are NIM modules that require these voltages
Some NIM power supplies are delivered in the form of a plug-in
m o d u l e ; these usually have current specifications much less
than a normal behind-the-crate supply.
Furthermore, some of
the module connectors in the crate are not supplied with the
ma ins power, so a plug-in module, in such a case, would not
work .
I ATTENTION;
(80)
The plug-in NIM power supplies, with their limited
power capacity, are easily overloaded. With three
modules inserted into the crate, the supply might
collapse; DO NOT plug such a module into a NIMcrate that has a power supply in the back.
-3-
Chapter 5
(ii)
Special supplies for individual instruments, such as MCA
pulser ;
(iii)
Switched mode power supplies, used mainly in computers; they
are rather noisy and usually not suitable for powering
analog circuits;
(iv)
High voltage power supplies; there are several different
types, depending on the requirements of voltage, current,
and stability.
NOTE£:
5.2
or
HV-Supplies are designed as DC/DC converters and very
seldom as switched mode power supplies.
TOOLS, INSTRUMENTS. COMPONENTS
Below is a list of the most essential tools, instruments, and
electronic components needed
in repair and servicing of power
supplies in nuclear instruments.
TOOLS:
Pliers, c u t t e r , tweezers, solder iron, desoldering tape
INSTRUMENTS:
V o l t m e t e r (digital), ammeter (5A), variac (220V, 3A, 50Hz)
Power supplies, either a NIM crate with power supply, or
two DC supplies 0-30V, 1A
Oscilloscope 30MHz
Variable power resistor 100 ohm with up to 3A load capacity;
such a power resistor can be easily made using a power
transistor on a heat sink controlled by a potentiometer
at its base.
COMPONENTS:
-
Rectifier diodes (1A, 3A, Usp 700-1000V)
-
Rectifier bridges (5A, 25A Usp 200-500V)
Transistors:
Thyristor:
2N2219
2N2905
2N3904
2N3906
2N3055
BD651
MJE 371
MJE 321
2N4441
(81)
Chapter 5
-4-
Set of z e n e r diodes, from 5.1V up to 24V
-
Switching diodes
O p e r a t i o n a l amplifier LM356 or LM741 or almost any other
i n t e r n a l l y compensated amplifier
E l e c t r o l y t i c c a p a c i t o r s , 1OOOuF/35V(63V), 4700uF/35V,
10000uF/25V
T a n t a l u m foil e l e c t r o l y t i c c a p a c i t o r s , 10uF/35V
5.3
BEHINu-THË-CRATË NIM POWER SUPPLY
A very common power supply
is
the one b e h i n d the NIM c r a t e .
Such a supply is very c o m p a c t ; it is d i f f i c u l t to reach
all
its
components for servicing.
As an e x a m p l e , a N I M - c r a t e power s u p p l y , Canberra Model 7021,
is d e s c r i b e d below (see Figs. 5.1 and 5.2).
5.3.1
General Circuit Description
The
voltages of the secondary w i n d i n g of a power
transformer
are r e c t i f i e d by a b r i d g e where
the center tap t e c h n i q u e is used.
T h e r e f o r e , one
rectifier
b r i d g e can generate a p o s i t i v e and
a
negative voltage.
After
r e c t i f i c a t i o n , t h e r e is the classical
capacitor
filter.
In a d d i t i o n to six
r e g u l a t e d v o l t a g e s , two
u n r e g u l a t e d v o l t a g e s are g e n e r a t e d by the v o l t a g e d o u b l e r t e c h n i q u e
(D203, C207, D205, C209 for a p o s i t i v e v o l t a g e ; D204, C208, D206,
C21Ü for a n e g a t i v e v o l t a g e ) , to s u p p l y
the m o n o l i t h i c v o l t a g e
r e g u l a t o r 1C for the +24V line, or to bias the driver t r a n s i s t o r of
the -24V line. In order not to exceed
the m a x i m u m r a t i n g s of the
+ 2 4 V r e g u l a t o r , the p o s i t i v e v o l t a g e is l i m i t e d by a zener d i o d e .
An a d d i t i o n a l + 5 V v o l t a g e is g e n e r a t e d by 1C 201. All supplies use
the common r e g u l a t o r 723 w i t h b u i l t - i n v o l t a g e reference source and
the p o s s i b i l i t y of c u r r e n t l i m i t a t i o n .
5.3.1.1
Positive output voltages
The unregulated DC voltage passes through a current sensing
resistor
to the collector of a npn-power
transistor.
This
t r a n s i s t o r is d r i v e n by an e m i t t e r follower t r a n s i s t o r ; its base is
controlled
by
the v o l t a g e r e g u l a t o r . The DC
o u t p u t voltage
is
compared
with
the internal
t e m p e r a t u r e compensated
reference
voltage.
Any d e v i a t i o n from the nominal o u t p u t v o l t a g e causes an
a m p l i f i e d error signal to the base of the driver t r a n s i s t o r . Each
of
these
r e g u l a t o r s is biased from the next
higher u n r e g u l a t e d
s u p p l y line.
The internal
reference source is about 6.9V;
t h e r e f o r e , for the 6V line, the v o l t a g e a d j u s t m e n t must be done at
the n o n - i n v e r t i n g
input
instead
of
the
inverting
input. The
c a p a c i t o r s in t h i s c i r c u i t are used for frequency compensation. The
v o l t a g e drop across the c u r r e n t sensing resistor drives the current
(82)
- 5-
Chapter 5
»IV >
Fig.
5.1:
NIM-crate power supply, Canberra Model 7021
(83)
Chapter 5
Fig.
(84)
-6-
5.2: NIM-crate power supply, Canberra Model 7021
-7-
Chapter 5
limiting
transistor
that is biased by a constant current
source.
This constant current source
is controlled
by a fraction of the
output
voltage,
to
achieve
foldback
current
limiting
characteristics.
5.3.1.2
Negative output voltages
The
unregulated DC voltage is fed to the emitter
of a
npn-power transistor; its base is driven by a pnp-transistor. An
integrated
v o l t a g e regulator gives the a m p l i f i e d error signal
to
the
base of
the driver
transistor.
For
optimum operational
conditions of the error amplifier, a zener diode is used for level
s h i f t i n g . The -6V line r e g u l a t o r gets its power at +V from the +5V
line and its -V from the -6V sense line. The two other regulators
are powered at +V from
common and
at -V from the corresponding
sense line. The 6V r e g u l a t o r ' s non-inverting input is connected to
common and the i n v e r t i n g i n p u t
is w i r e d to a resistor chain which
is connected between the reference-voltage and the sense line. For
the -12 and -24
v o l t a g e s , the reference-voltage
is
fed
to the
inverting
input
and the non-inverting input of 1C2 and
IC3,
and
g e t s its signal from a fraction developed
between common and the
c o r r e s p o n d i n g sense line. The negative regulated o u t p u t
voltage
passes t h r o u g h a current sense resistor at the collector of the
output
power
transistor.
The
voltage drop
is added
to the
auxiliary +5V v o l t a g e and a fraction of
the o u t p u t voltage. The
v o l t a g e of the m a t r i x point d r i v e s a current l i m i t e r transistor in
case of excessive load current.
5.3.2
Ma intenance
For r e a d j u s t m e n t of
output
voltages and current, remove the
upper
cover of the power supply. The corresponding p o t e n t i o m e t e r s
for voltage and current adjustments are marked with U and I at the
top of the regulation board. Maximum current shall not be adjusted
above 120% of nominal value. To verify exact voltage measurements,
a separate bin connector and probe cable must be used for load
and
measurement, to avoid any voltage drop error.
For removal of the chassis from the b i n , the two screws on the
left and r i g h t side of chassis behind the bin have to be removed.
5.3.3
Troubleshoot ing
When dealing w i t h a nonfunctional NIM power supply, the first
step is to measure all output voltages. If one or some output
voltages are missing, measure
the corresponding unregulated DC
voltage.
If all unregulated voltages are there, check if there is
an excessive load current by measuring
the voltage drop across the
current
sense r e s i s t o r , corresponding to the missing o u t p u t
voltage.
If until now
e v e r y t h i n g seems normal, measure
the
voltages of the following pins at the corresponding regulator 1C.
Compare these values with the theoretical ones.
(85)
Chapter 5
-8-
P i n 7 , 8 (+V depends on supply)
Pin 5 (common for p o s i t i v e voltages or negative supply line)
Pin 4 (must always be 6.9V higher than pin 5)
No d i s c r e p a n c y
found,
check
that
the current
limiting
t r a n s i s t o r is fully cut off (any one of the transistors Tl, T2 , T3 ,
T5, T7, T 9 ) . In normal c o n d i t i o n s the base must always be more
p o s i t i v e than the e m i t t e r . If a fault is r e v e a l e d , measure the DC
v o l t a g e s in the c i r c u i t
around
this p o i n t . Here you can already
o b t a i n the i n f o r m a t i o n of w h a t could be the cause of the fault. If
e v e r y t h i n g is n o r m a l , measure pin 6 of r e g u l a t o r 1C.
The f o l l o w i n g considerations are valid for the p o s i t i v e supply
lines: the o u t p u t v o l t a g e of the error amplifier should be 1.2-1.4V
h i g h e r t h a n the c o r r e s p o n d i n g o u t p u t v o l t a g e line. If this voltage
is m u c h h i g h e r , one
of
the following
t r a n s i s t o r s is d e f e c t i v e
because
they
are used as v o l t a g e follower.
In t h i s w a y ,
a
d e f e c t i v e c o m p o n e n t can be easily
d e t e c t e d . If the IC-output does
not
show the m e n t i o n e d v o l t a g e v a l u e , and the voltage between pin
1U and pin 1 is not h i g h e r than 0.4V, check the i n p u t v o l t a g e s and
compare t h e m w i t h the output voltages: if there is any discrepancy,
r e p l a c e the 1C.
If the v o l t a g e d r o p is higher, the fault might be
e i t h e r a d e f e c t i v e s h u n t resistor (R46, R60, R74 d e p e n d i n g on which
line the f a u l t
occurs) or
at
the c u r r e n t
limiting circuitry.
Measure
the v o l t a g e s around the c o n s t a n t c u r r e n t source t r a n s i s t o r
(base, e m i t t e r , c o l l e c t o r
of
transistor
T4,
T6, T8 d e p e n d i n g on
w h i c h line t lie f a u l t occurs).
For the n e g a t i v e s u p p l y lines, p r o c e e d
voltage
of
as follows: the o u t p u t
the error a m p l i f i e r , measured a f t e r the
zener
diode,
s h o u l d be about Ü.5V n e g a t i v e c o m p a r e d
to the c o r r e s p o n d i n g o u t p u t
voltage.
If
this voltage
is more n e g a t i v e , one
of the next
following
t r a n s i s t o r s are
defective.
If the o u t p u t voltage
m e a s u r e d a f t e r zener diode does not have the nominal voltage value,
check the input voltages of
the 1C and compare
them with the
o u t p u t . If any d i s c r e p a n c y is found, replace the 1C.
At the node
of the zener d i o d e and the c o l l e c t o r of the l i m i t i n g transistor, a
voltage
is d e v e l o p e d by the current sum over a given resistor.
T h e r e f o r e , you m u s t check t h a t either this voltage is g e n e r a t e d by
the c u r r e n t l i m i t a t i o n or by the v o l t a g e r e g u l a t i o n . Due
to the
condition that
no excessive current
load should be there, the
v o l t a g e regulation should be d o m i n a n t . If this is not
the case,
the fault m u s t be in the c u r r e n t limiting c i r c u i t . For the current
limiting
t r a n s i s t o r the following rule is v a l i d : the base must be
more p o s i t i v e
than the e m i t t e r . If it is n o t , you must find out
how to d e t e c t the d e f e c t i v e component.
NOTE ;
(86)
If any component was changed, a readjustment has to be
made .
i
VO
i
"
r—f"
r—("• i—f" i—f" r—f-
n
3o>
oo
rc
1-1
Ln
Chapter 5
5.4
-10PLUG-IN NIM POWER SUPPLY
As already mentioned, there are NIM-plug-in power supply
modules available to supply a NIM bin.
Such a supply module is
described below, and shown in Fig. 5.3.
This module has some l i m i t a t i o n s :
- it is available only for + 1 2 V / 1 A , +24V/0.5A, - 1 2 / l A ,
-24V/0.5A;
- its maximum power capability is 48W at 50C.
DO NOT plug in sue h a module in a NIM
crate that
already has a powe r supply in t he back .
1 NOTE:
|
1
5.4.1
Circuit Description
The power transformer provides four separate low-voltage ac
sources.
These
sources
are
full-wave-rectified, capacitorf i l t e r e d , and regulated by electronic series regulator circuits,
which have the capability of r e g u l a t i o n and current l i m i t a t i o n w i t h
foldback
characteristics.
Each supply voltage has
its
own
reference source and the p o s s i b i l i t y to adjust the output v o l t a g e
by a p o t e n t i o m e t e r .
All four supplies use the same active components; however, the
voltage d i v i d e r s
are
tuned
to
the
requirements
of
the o u t p u t
voltage. Two d i f f e r e n t preregulators are used: one consists of the
voltage doubler (Dl, D2, Cl, C2, na d zener diode D4) for the +12V
and
+ 24V
-24V
supply.
supply.
The second uses a 10V zener d i o d e for -12V
The reference voltages of
and
6.95V -are derived from a reference
diode (Ul, U3, U5, U7).
The regulators are of the type LAS1000 or equivalent.
As a
pass transistor a npn-Darlington transistor is used (type TIP100 or
equivalent) .
A reverse-current protection is also built in by the reversed
biased diode from the type 1N5819 or equivalent.
Following the above observations,
troubleshoot the instrument.
Nevertheless,
useful.
the
following
it
additional
should
be
comments
easy
to
might
be
There are two shock-hazard locations to watch for: the wiring
side of the input line cord connector block, and the two thermal
switches (SI for thermal cutout, S2 for thermal warning) mounted
against the heatsink. These locations are exposed to the ac-line
voltage .
(88)
-11-
Chapter 5
In Table 5.1, the t y p i c a l de-voltages (measured with
respect
to TP1 or ground.), are
g i v e n , w h i c h should
be very h e l p f u l for
troubleshooting.
The
pin assignments for the NIM
connector are
presented in Table 5.2.
5 .5
MCA POWER SUPPLY
From the c i r c u i t
diagram
(Fig. 5.4), it
is d i f f i c u l t to
realize
t h a t the -»-24V line is the d o m i n a n t
supply v o l t a g e .
From
t h i s v o l t a g e all others are
derived
by using the 24V e i t h e r as
r e f e r e n c e v o l t a g e or by s u p p l y i n g it to the o p e r a t i o n a l a m p l i f i e r s
w h i c h are used
to g e n e r a t e the v a r i o u s v o l t a g e s .
This 24V
line
is
c o n t r o l l e d by a o v e r h e a t i n g
sensor
switch and a p p e a r s only if the
t e m p e r a t u r e of the h e a t s i n k at the b a c k - p a n e l is below the c r i t i c a l
value.
In the 24V line r e g u l a t i o n c i r c u i t , there is a zener d i o d e . It
is
used to p r o v i d e the r e g u l a t e d 24V as the r e f e r e n c e v o l t a g e
for
the o p e r a t i o n a l a m p l i f i e r . This c i r c u i t
r e d u c e s the influence of
the
line v a r i a t i o n s to the o u t p u t v o l t a g e . At all
other
supply
v o l t a g e s , the r e g u l a t i o n
is
implemented
in a c l a s s i c a l way
using
operational
amplifier
and
current
limitation with
foldback
characteristic.
Only
the 5V supply uses two
parallel power
t r a n s i s t o r s because a h i g h c u r r e n t has to be d e l i v e r e d .
In a d d i t i o n , it is necessary
to
p r o t e c t the d i g i t a l ICs from
overvoltage:
an o v e r v o l t a g e p r o t e c t i o n circuit is i n t r o d u c e d .
After
power
is s w i t c h e d on, 5 LEDs
indicate
the
following
v o l t a g e s : + 5V, + 1 2 V , +24V, - 1 2 V ,
-24V. This is very c o n v e n i e n t
because
it is a l r e a d y an indication as to where and how
to start
with troubleshooting.
If no LED is on, and the main fuse ( F l ü l ) is
not
b l o w n , the fault may
be in the + 2 4 V supply line. Check
the
t e m p e r a t u r e s w i t c h ; if it is
c l o s e d , then measure the reference
v o l t a g e 018. N e x t measure the o u t p u t of the o p e r a t i o n a l a m p l i f i e r
A3, w h i c h s h o u l d
be a p p r o x i m a t e l y
1 5 V . The
base v o l t a g e of Q2
compared
to ground should be a l i t t l e higher than +25V.
If n o t ,
check the v o l t a g e across Q3.
If this v o l t a g e is less t h a n 1 . 5 V , it
indicates
that
there is an overload at this o u t p u t
and
current
l i m i t a t i o n is a c t i v e . This is valid for all o t h e r o u t p u t v o l t a g e s ,
except
for the +5V. There, it is not easy to d i s t i n g u i s h b e t w e e n
an overload s i t u a t i o n and the presence of the a c t i v a t e d o v e r v o l t a g e
protection.
Due to the overvoltage protection, the thyristor Q19
is f i r e d and shorts the o u t p u t
to g r o u n d . As long as the current
l i m i t a t i o n works p r o p e r l y , there is only one way
to reset
the
c u r r e n t l i m i t a t i o n : by switching
the i n s t r u m e n t off and on again,
after
removing
the load. Only in this way
is
it possible to
determine whether current
l i m i t a t i o n or
o v e r v o l t a g e causes the
action.
If
you
suspect
that the o v e r v o l t a g e p r o t e c t i o n has
become
a c t i v e , you should switch off the i n s t r u m e n t s , and connect it via a
VARIAC to the m a i n s , as shown in Fig. 5.5. Increase slowly the
voltage
to
the i n s t r u m e n t .
In
this way
it
is
possible
to
d i s t i n g u i s h if there was a bad adjustment of the overvoltage
p r o t e c t i o n or if a transient
occurred
during the s w i t c h i n g on of
the ins t rumen t.
(89)
Chapter 5
TABLE
5.1:
-12-
Typical üC-Voltages
wit hou t load )
Nod«
Volt*»»
Nod«
U2 pin 8
3
5
7
2
1
1-36.5
+ 2.5
0
0
+ 2.5
+24.0
+23.9
T25.3
U6 pin 8
3
5
7
2
1
US pin 8
7
-36.5
* 2.5
0
0
2
1
+ 2.5
+12.0
2
1
10
6
10
6
U4 pin 8
3
5
10
6
TABLE 5 . 2 :
Pin
(measured with respect
10
6
3
5
7
+11.8
-M3.5
assignment,
for
VolUffi
+ 9.5
- 9.5
-12.0
-12.0
- 9.5
0
- 0.2
+ 1.4
+ 9.5
-21.5
-24.0
-24.0
-21.5
0
- 0.2
+ 1.3
NIM modules
BIN/MODULE CONNECTOR PIN ASSIGNMENTS
FOR AEC STANDARD NUCLEAR INSTRUMENT
MODULES PER TID-20893 (Rev 4)
(adopted by DOE)
Pin
1
2
3
4
5
6
7
8
9
•10
•11
12
13
14
15
•16
•17
18
19
20
21
22
(90)
Function
-3 volts
-3 volts
Spare Bus
Reserved Bus
Coaxial
Coaxial
Coaxial
200 volts dc
Spare
-6 volts
-6 volts
Reserved Bus
Spare
Spare
Reserved
-12 volts
-12 volts
Spare Bus
Reserved Bus
Spare
Spare
Reserved
Pin
23
24
25
26
27
'28
'29
30
31
32
'33
"34
"35
"36
"37
38
39
40
'41
'42
G
Function
Reserved
Reserved
Reserved
Spare
Spare
*24 volts
-24 volts
Spare Bus
Spare
Spare
117 volts ac (Hot)
Power Return Ground
Reset (Sealer)
Gate
Reset (Auxiliary)
Coaxial
Coaxial
Coaxial
117 volts ac (Neut )
High Quality Ground
Ground Guide Pm
to
TP1
-13-
Fig.
5.4:
Chapter 5
Low V o l t a g e Power Supply of a MCA
(91)
Chapter 5
-14-
variac
ac-line
MCA
line input
Fig 5.5:
MCA
Using a variac to i d e n t i f y a fault
The
arrangement as shown in Fig 5.5 can be used
also
for
t e s t i n g in the case when the fuse (F101) is blown i m m e d i a t e l y when
the
instrument
is switched on.
This i n d i c a t e s t h a t
there
is a
short c i r c u i t somewhere in the power s u p p l y . Before you s t a r t to
look. into the c i r c u i t in d e t a i l , remove the whole load
(take out
all b o a r d s , disconnect
the display
unit).
Slowly increase the
output voltage
of
the v a r i a c and observe the c u r r e n t w i t h an
a m m e t e r (see Fig 5.5). When the a m m e t e r
i n d i c a t e s roughly 60-80%
of
the nominal
fuse c u r r e n t , stop increasing the v o l t a g e .
Now
measure the voltage at the o u p u t
of
the variac. Here you will
already
have some i n d i c a t i o n as to where the short
circuit
might
appear.
(i)
Less
than 4% of the ac line voltage i n d i c a t e s
d e f e c t ive
line f i l t e r network, or a f a u l t y transformer.
(ii)
A b o u t 4% of line voltage can i n d i c a t e a bad transformer or a
bad
rectifier.
S e p a r a t i o n of
t r a n s f o r m e r and r e c t i f i e r
diodes or
b r i d g e s has to be m a d e to d e f i n e
the d e f e c t i v e
p a r t of the s u p p l y .
(iii)
6-8% of line v o l t a g e i n d i c a t e s a s h o r t c i r c u i t either in the
rectifier
p a r t or
a
defective
filter
c a p a c i t o r ; by
s e p a r a t i o n of
these two p a r t s of the power s u p p l y , it is
p o s s i b l e t o d e t e c t t h e d e f e c t i v e component.
(iv)
8-10% of the line v o l t a g e
the pas s-1ransis t or , and
working p r o p e r l y , t h e r e f o r e
i n d i c a t e s a short c i r c u i t a f t e r
the c u r r e n t
limitation
is
not
all
o u t p u t v o l t a g e s have to be
m e a s u r e d t o locate t h e d e f e c t i v e c o m p o n e n t .
The s e m i c o n d u c t o r s must be checked in the c o n v e n t i o n a l way. In
measuring base-emitter voltage
(around 0.7V), c o l l e c t o r - e m i t t e r
v o l t a g e m u s t not be less t h a n 0.7V otherwise the transistor would
o p e r a t e in the s a t u r a t i o n r e g i o n and would no longer be linear.
If
these v o l t a g e s are c o m p l e t e l y d i f f e r e n t , as previously m e n t i o n e d ,
remove
the t r a n s i s t o r s and
check
it again.
To
check
the
o p e r a t i o n a l a m p l i f i e r , its two input v o l t a g e s should have the same
value. If t h e y do n o t , compare them w i t h the o u t p u t .
If t h e r e is
a discrepancy, remove this operational amplifier.
(92)
-15-
Chapter 5
After
replacing
the d e f e c t i v e c o m p o n e n t s , check
the power
supply again. Increase
the o u t p u t
voltage of the v a r i a c to the
nominal mains v o l t a g e , but also measure the ac current used by the
power supply. This current should not be more than 20% of the fuse
current
since
there
is no load. Readjust
all
supply
voltages
according to the
circuit
diagram
or
service manual.
A f t e r this
p r o c e d u r e , the o u t p u t voltage of the +5V line follows when the
o u t p u t v o l t a g e of
the +24V
line
is
increased.
Simultaneously
measure both
o u t p u t v o l t a g e s . When the +5V line
reaches around
5.6V,
the o v e r v o l t a g e
p r o t e c t i o n should be activated. In this
case, decrease the o u t p u t v o l t a g e of the +24V line to the nominal
value, switch off
the
instrument
to
reset
the memorized
o v e r v o l t a g e , and switch the instrument on again. Then measure the
o u t p u t voltages to be sure that everything is p r o p e r l y set. If the
overvoltage p r o t e c t i o n did not t r i g g e r , try to detect the d e f e c t i v e
components (D13, Q20, Q190, or the passive components around them).
The
c o m p o n e n t s 1C A5, Q 1 5 , and Q16 are there for power reset,
which is necessary for the microprocessor reset. 1C A6 generates a
fixed clock f r e q u e n c y .
The current limitation can now be checked. For this p u r p o s e ,
an ammeter
in series with a variable power resistor has
to be
connected to the o u t p u t , and simultaneously
the o u t p u t v o l t a g e has
to be measured. Increase the current by changing
the resistor.
When exceeding 100% of
the nominal current, the o u t p u t v o l t a g e
should
decrease.
After
removing the load, the nominal o u t p u t
voltage should come back again.
A f t e r c o m p l e t i n g this p r o c e d u r e , you can i n s t a l l all removed
boards and the display unit. Make sure the MCA is now working. If
not see Chapter 9, MULTICHANNEL ANALYZER.
5.6
DETECTOR BIAS SUPPLIES
In this section, bias supplies for^detec t ors are described.
can d i s t i n g u i s h between several types of such supplies.
(i)
High
voltage supplies which are designed for
d e t e c t o r s must be very stable in
scintillation
respect to o u t p u t v o l t a g e ,
and relatively powerful (up to l-2mA at a b o u t 2000V);
and r i p p l e is not so c r i t i c a l .
(ii)
We
noise
High v o l t a g e s u p p l i e s w h i c h are designed
for solid s t a t e
d e t e c t o r s should
have
low noise and r i p p l e , but are
not
r e q u i r e d to be highly stable with r e s p e c t to o u t p u t v o l t a g e .
The current r e q u i r e d for d e t e c t o r s such as Si(Li), p u r e Ge ,
or Ge(Li), is in the order of
less than lOOnA. Surface
barrier d e t e c t o r s sometimes require a higher current of
to 2uA (at relatively low voltage of about 100V).
up
(93)
Chapter 5
-16-
For troubleshooting, a high-voltage instrument is
recommended.
This is
NOTE:
- either a digital v o l t m e t e r with lOMohm input resistance using a HV-probe with built-in v o l t a g e
divider 1000:1 or 100:1, or
- s t a t i c voltmeter
5.6.1
Example 1; ORTEC Model 459
Fig.
5.6 shows a
solid state d e t e c t o r .
This
HV-supply
circuit
diagram
of a HV-bias supply for a
can deliver up to lOOuA and
the
circuit
is
simple to understand.
NOTE ;
NEVER measure with a digital voltmeter in the HV-path
unless
you use
a HV-probe with a voltage
divider
1000:1 (such a high voltage probe is available from
Fluke) or
100:1.
Through such a probe the input
resistance of a d i g i t a l voltmeter
is
10000 MOhms.
A n o t h e r possibility is to use a s t a t i c v o l t m e t e r , but
pay a t t e n t i o n to HV isolation problems.
A HV-bias supply is usually
built
around
a DC/DC c o n v e r t e r .
An oscillator, push-pull c i r c u i t r y , a transformer and a regulation
c i r c u i t are necessary on the primary
s i d e ; on the secondary s i d e ,
the AC
is r e c t i f i e d and f i l t e r e d or a v o l t a g e m u l t i p l i e r may
be
used .
Before you start
troubleshooting, make sure t h a t the working
area where the HV supply will be tested is free of all conducting
material, such as wire ends, e t c . , to prevent HV-shock hazards.
5.6.1.1
The
loop .
Troubleshooting
specific HV-bias supply described here
has no feedback
Be sure that all required input supply voltages are provided.
Using an oscilloscope, check that the oscillator (Ql, Q2) is
working.
Frequency can be changed wit h potent iome ter R42.
If n o t ,
check Ql and Q2.
The next step is to inspect the operation by
measuring with the voltmeter the emit ter-vol tage of transistor Q7
Increasing the
while changing the front panel potent iometer R23
setting of the potentiometer should induce the voltage change of
If
the emitter-voltage of transistor Q7 (from -24V up to -t-lOV)
(94)
-17-
Fig.
5.6:
Chapter S
HV-bias supply (ORTEC Model 459)
(95)
Chapter 5
-18-
this is not o b s e r v e d , remove the load by unsoldering capacitor C13.
Check again.
If there is no voltage at the o u t p u t , check the current
l i m i t i n g c i r c u i t r y which consists of zener diode D3 and transistor
Q9;
it works like a shutdown device by driving the transistor Q10
into saturation.
When e v e r y t h i n g seems normal, check w i t h the
oscilloscope
the waveform of the signal which appears at collector
of transistors Q5 , Q6.
If there is no rectangular pulse visible
(the
amplitude
of
the pulse should be d e p e n d e n t
on
the
e m i t t e r - v o l t a g e of transistor Q7 ) check
transistors Q3 to Q6 or
measure with an ohmmeter
if the transformer winding has
an
i n t e r r u p t i o n . When the t r a n s f o r m e r has
a shorted w i n d i n g , this
a p p e a r s as an overload, and c u r r e n t l i m i t a t i o n should take place.
In t h i s case, transformer Tl has to be replaced.
A f t e r inserting capacitor C13 and you observe t h a t an overload
appears a g a i n , a d e f e c t i v e component must either be in the
filter
capaci t or' C27 to C29 or in the voltage m u l t i p l i e r .
Remove one
a f t e r a n o t h e r c a p a c i t o r s C27, C28 or C29, and check again. If no
d e f e c t i v e c o m p o n e n t was found, the next step is the inspection of
the voltage multiplier.
When
t r o u b l e s h o o t i n g a v o l t a g e m u l t i p l i e r , work carefully!
Measure the voltage of each m u l t i p l i e r stage e i t h e r w i t h a HV-probe
or with a s t a t i c - v o l t m e t e r . If a stage has the same voltage level
as the previous one, it is d e f e c t i v e
(either
the diode or the
capacitor). Replace the d e f e c t i v e component and check again. When
e v e r y t h i n g seems to be working
properly, you can
start
to
recalibrate the instrument.
Set the front panel dial to 5.00, which should correspond to
5000V, and measure the o u t p u t voltage. W i t h two p o t e n t i o m e t e r s R22
and R 1 5 , the voltage can be adjusted: R22 is responsible for the
s e t t i n g of the 5000V level, R15 is used to adjust the low voltage
region. Check the linearity of
the dial s e t t i n g by measuring the
o u t p u t v o l t a g e ; do this slowly so that the capacitors can follow by
increasing t h e i r charge. W i t h p o t e n t i o m e t e r R42 you can change the
oscillator f r e q u e n c y , which is necessary to minimize the current
consumption of the + 24V and -24V supply line.
Most of the faults in the earlier version of this HV-supply
were the consequence of burning out of
the polarity indication
lamps. N o w a d a y s , these lamps are replaced by LEDs. Also, there
were sometimes problems with the tantalum capacitors due to the
spikes w h i c h occur from switching an inductor on and o f f .
5.6.2
Example 2; Canberra Model 3102
Another
type of HV-bias supply is the rather complicated one
from Canberra, Model 3102. This circuit has a feedback circuitry
to stabilize the HV-bias voltage. Canberra even uses special ICs,
which are not very common on the market.
Fig. 5.7 shows the circuit of the above-mentioned bias supply.
It is controlled and regulated by a feedback loop and the ratings
are up to 2mA at 2000V.
(96)
-19-
Chapter 5
NOTE : This is an example of a case when you cannot trust the
circuit diagram.
There are
some changes in the
c i r c u i t , which are not shown in the circuit d i a g r a m ,
e.g. oscillator swing is from -10V to 0V and due to
the slow rate of the used operational amplifier it has
a trapezoidal waveform. The feedback resistor network
R107, R i l l is connected to -12V and not to ground. The
positive supply of the
operational
amplifier
is
connected to ground and not as it is shown in the
c i r c u i t diagram to +12V.
Therefore, the oscillator
operates without the module being switched on.
Also,
the component number IC104 does not correspond to the
actual lay-out, which is in this case IC101.
There
might be other inconsistencies.
5.6.2.1
Circuit description
ATTENTION:
Circuit operation is described according to the
diagram, and does not include the modifications
later introduced by the manufacturer.
Zener diode CR101 and operational amplifier IC104A generate
the -9V reference voltage. The oscillator is built around IC104B.
The driver stage, built around IC105, is coupled through C108
and
C107 to the power transistors Q101 and Q102 of the push-pull stage.
The d r i v e r stage is controlled via capacitor C104 by the oscillator
and de-controlled via the
amplifier
bias current input by
operational a m p l i f i e r IC102B. This operational amplifier is itself
controlled by the voltage setting operational amplifier IC103A.
The output to the voltage monitor is taken from amplifier IC101.
The over-voltage detection is performed by a summing amplifier (1C
103B, 1/2 of CA3240) and operates in the same way as the shut-down
function of the inhibit gate.
5.6.2.2
NOTE:
Troubleshoot ing
Use the HV-probe to measure the output voltage. Such a
probe should have a built-in 1000:1 or 100:1 voltage
divider. Be careful when measuring high voltage.
(97)
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OJ
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rt
Ul
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c/r/
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Fig. 5.7: Circuit diagram of Canberra Model 3102
221
-21-
Chapter 5
Measure w i t h
the d i g i t a l
voltmeter
the -9V
reference.
Adjustment
can be done by changing the potentiometer R172 (1C
101
pin 1).
Next check if the s e t t i n g c i r c u i t r y works p r o p e r l y . This
is done by v a r y i n g the front panel p o t e n t i o m e t e r or by changing the
v o l t a g e s e l e c t o r switch at the front
p a n e l , while m o n i t o r i n g the
v o l t a g e at
the o u t p u t
of 1C 103a pin 1.
Now
check w i t h
the
oscilloscope to see if the o s c i l l a t o r is working; the signal at the
o u t p u t should have a t r a p e z o i d a l signal w i t h an a m p l i t u d e of
about
UV to -11V and a frequency
of
about
20kHz.
If n o t , measure the
i n p u t s w i t h an oscilloscope and compare them w i t h the o u t p u t . C108
s h o u l d be c h a r g e d and
discharged
(amplitude
a b o u t -6V to - 1 1 V ) .
Due to the not very s h a r p signals at the o u t p u t s of 1C A5 pin 6 and
8, a t r i a n g u l a r signa] s i t t i n g
on a dc level s h o u l d appear. If
not, change the 1C. On the collector of the power transistor Q101
and Q102, you should measure
+ 24V when the i n s t r u m e n t is s w i t c h e d
off.
If
not,
t h e r e m u s t be an i n t e r r u p t i o n
in
the H V - m o d u l e .
A f t e r s w i t c h i n g on
the i n s t r u m e n t , a sinusoidal w a v e f o r m should
a p p e a r , w i t h the a m p l i t u d e d e p e n d i n g on the H V - v o l t a g e s e t t i n g .
If
n o t , the H V - m o d u l e is d e f e c t i v e and must be r e p l a c e d by a new one.
The
H V - m o d u l e consists of
transformer, voltage doubler,
f i l t e r i n g n e t w o r k and f e e d b a c k resistor c h a i n , and is sealed
so
t h a t no h u m i d i t y can reach the components.
If for any reason the
feedback
loop
is
i n t e r r u p t e d , an
over-voltage
detection
is
a c t i v a t e d and s h u t s down the
i n s t r u m e n t . For t e s t p u r p o s e s , it is
necessary to d i s c o n n e c t (or remove) diode CR109.
5. 7
SWITCHED MODE POWER SUPPLIES
Nowadays
the so-called
"SWITCHED MODE" power s u p p l i e s are
frequently
used.
This t y p e of s u p p l y has
some
a d v a n t a g e s ; an
i m p o r t a n t one is t h a t they are
smaller and l i g h t e r c o m p a r e d to the
linear
r e g u l a t e d ones. They are f r e q u e n c y i n d e p e n d e n t
(40-400Hz)
over a wide range, and
the voltage variations range covers from
-1-10%
to -20% of the nominal line voltage. The e f f i c i e n c y of
such
s u p p l i e s is in the order of 75%; this means less h e a t p r o d u c t i o n .
On
the o t h e r h a n d , there are also some d i s a d v a n t a g e s . A
lot of
f i l t e r i n g is absolutely necessary
otherwise t h i s s u p p l y would be
very
noisy. The load regulation is c r i t i c a l . From the theory
of
m a t c h e d o s c i l l a t o r s , it is easy
to u n d e r s t a n d
that the load can
only
change
about
1:10 otherwise the oscillator would not
be
m a t c h e d . This is t r u e , for
example, for c o m p u t e r s where the load
changes are small and load r e g u l a t i o n not c r i t i c a l . The
recovery
time is about 3-5 msec.
For
nuclear a p p l i c a t i o n s
such a long
recovery
time could be c r i t i c a l if a s u p p l y v o l t a g e is used for a
reference purpose.
A m o d u l e for NIM crate is available (EG&G Model 495) to supply
an
a d d i t i o n a l 6V-line, e i t h e r p o s i t i v e or negative s w i t c h a b l e , if
this voltage is missing in the crate. F i g .
5.8 shows the circuit
of
this module. Such a module can be used to supply the d i g i t a l
logic of an ADC which is now produced as a NIM-module.
(99)
8
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L
.
.
.
.
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.
J
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Fig.
J» .1« ,;u
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^
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: r.
5.8: Circuit diagram of the EG&G module 495
-235.7.1
Chapter 5
Circuit Diagram Description
The regulation
is done by a single p u l s e - w i d t h - m o d u l a t i n g
integrated
circuit Ul.
This 1C is powered by
transformer Ï3,
r e c t i f i e d by b r i d g e D6 and f i l t e r i n g c a p a c i t o r C4 . The ac power is
rectified
by
b r i d g e Dl, f i l t e r e d
by
c a p a c i t o r s Cl, C2
and
transformed by T l. This transformer is controlled by power MOS-FET
Ql
which
is controlled by 1C Ul via transformer T2.
(Note: line
s e p a r a t i o n can be achieved only by
transformer or opto-coupler,
depending on the power which has to transferred.) On the secondary
side of the t r a n s f o r m e r , the line is r e c t i f i e d by a fast switching
diode b r i d g e D2 f i l t e r e d by i n d u c t a n c e Ll and Capacitors C5,
C6.
The
pulse-width-modulating
integrated
circuit Ul has, besides
regulation, the capability of current
limitation with
foldback
c h a r a c t e r i s t i c s . The current
is sensed by the resistor R5.
If
overvoltage occurs for any reason, a crowbar circuitry is fired (1C
U2, t h y r i s t o r Q2). Reset is only possible by switching off the NIM
module.
5.7.2
Troubleshooting
ATTENTION ; NEVER operate the power supply with the J u m p e r s Wl
and W2 set for opposite polarities; the internal
circuits
will be damaged.
Be careful
when
measuring:
some
components
are
at ac-line
potential.
| NOTE :
|
For troubleshooting, do not plug the module in a crate
where 6V is already supplied.
If the module is not working, check the NIM crate that proper
ac input voltage (nominal 1 1 7 V )
through bin pin 33 and 41 is
supplied .
Check
necessary
the
ac
input
fuse on
the
rear
panel.
Replace
if
If the output voltage has exceeded the crowbar trigger level,
the front panel LEDs will not light. To restore operation, remove
the ac power input from the module and then turn it on again.
If the instrument still does not work, remove the load and try
again.
When t h i s does not work, remove jumper between transistor
Ql and t r a n s f o r m e r Tl.
Now measure the voltage across C4 (about 16V). If there is no
v o l t a g e , remove
1C Ul and check by measuring
the supply
path
(101)
Chapter 5
-24-
(transformer T3, r e c t i f i e r b r i d g e D6 and c a p a c i t o r C4). If v o l t a g e
is p r e s e n t , the d e f e c t i v e 1C has to be r e p l a c e d .
The next step is to check w i t h
the oscilloscope
if trigger
pulses are coming out from the o u t p u t of the 1C.
If n o t , check the
v o l t a g e d r o p across r e s i s t o r R5.
It should be near zero, o t h e r w i s e
1C Ul is d e f e c t i v e and has to be r e p l a c e d .
ATTENTION : W i t h
line
from
from
the next steps you enter i n t o , this is w h e r e
p o t e n t i a l occurs.
REMOVE the grounding lead
oscilloscope p r o b e to p r e v e n t a short c i r c u i t
a mains phase via oscilloscope earth ground.
As
the n e x t s t e p , check w h e t h e r pulses a p p e a r at the g a t e
of
power MOS-FET Ql.
If n o t ,
check
the p a t h ,
(capacitor
C14,
t r a n s f o r m e r T2, resistors R2 , R9 and c l i p p i n g zener d i o d e s D4 , D5 ) .
If e v e r y t h i n g is OK i n s e r t the jumper w h i c h connects transistor Ql
and t r a n s f o r m e r Tl.
Now check w i t h the oscilloscope at this jumper if pulses occur;
due to c u r r e n t l i m i t a t i o n being a c t i v a t e d , this could be p r e v e n t e d .
T h e r e f o r e , the v o l t a g e drop across r e s i s t o r R5 has to be m o n i t o r e d .
Under n o r m a l c o n d i t i o n s there should be only a small v o l t a g e drop
because
t h e r e is no load. It could be that due to bad
adjustment
of the o u t p u t v o l t a g e , the crowbar
t r i g g e r level has already been
exceeded.
R e m o v e 1C U2, and check again.
If the v o l t a g e drop
is
again too h i g h , check the value of r e s i s t o r R 5 (may be broken or
value change
due
to overload).
If the value of R5
is c o r r e c t ,
remove t h y r i s t o r Q2 and when pulses o c c u r , r e p l a c e t h y r i s t o r . When
the o u t p u t
voltage
comes back, a bad a d j u s t m e n t
of
the o u t p u t
v o l t a g e may be the reason for
the shut-down or a d e f e c t i v e 1C U2.
Measure
the o u t p u t
v o l t a g e of
the m o d u l e and
adjust
with
p o t e n t i o m e t e r R 1 3 . If the a d j u s t m e n t
is not p o s s i b l e , check the
feedback loop.
If t h i s is OK, replace 1C Ul.
If there is no o u t p u t v o l t a g e , connect the oscilloscope p r o b e
to common and look at the diode b r i d g e Dl pin 1 (this diode looks
like a power transistor). Pulses must be there; if n o t , either
t r a n s f o r m e r Tl, c a p a c i t o r C3 or
s w i t c h i n g d i o d e D3 is d e f e c t i v e .
To
find
the d e f e c t i v e c o m p o n e n t , the c o m p o n e n t s m u s t
be
removed
from the c i r c u i t . If pulses are
observed
but s t i l l there is no
output
v o l t a g e , the c o m p o n e n t s D2, LI may be f a u l t y , or one of the
c a p a c i t o r s C5, C6 may be shorted.
After
these checks the o u t p u t
v o l t a g e should be t h e r e . Measure and if necessary readjust
output
voltage.
Insert
1C U2, check again;
if no o u t p u t voltage is
p r e s e n t , replace 1C U2 .
(102)
Chapter 6
PREAMPLIFIERS, AMPLIFIERS
-16
PREAMLIFIERS. AMPLIFIERS
6.l
GENERAL ASPECTS OF EFFECTIVE TROUBLESHOOTING OF
ANALOG INSTRUMENTATION
Chapter 6
Analog
instruments basically
consist of
analog circuits,
either
linear
or non-linear. Some logic functions, however, may
also be p r e s e n t .
Therefore, when you open an analog instrument, you will see
inside discrete c o m p o n e n t s ,
analog
integrated
c i r c u i t s , like
operational amplifiers, c o m p a r a t o r s , analog switches, a n d , more
rarely, multipliers and possibly some digital integrated circuits.
It is
important to point out that even in the most modern
analog instruments, there are some parts that must be mandatorily
implemented in discrete form, for integrated technology is still
unable to provide the level of performance required for some
functions.
So, typically, low-noise sections in preamplifiers and
a m p l i f i e r s are still b u i l t in d i s c r e t e or
hybrid form. The same
applies
to some high-speed a m p l i f y i n g circuits.
The
sections
implemented in discrete form are usually the more important sources
of
failure or malfunctioning
in analog instruments.
These
sections, during the lifetime of
the instrument, are more prone to
suffer partial modifications and incorrect component r e p l a c e m e n t
from the user and are more exposed
to adverse environmental
conditions.
Besides e v i d e n t
failures and easy-to-appreciate kinds of
m a l f u n c t i o n i n g , analog
i n s t r u m e n t s may
present some performance
degradations whose detection may imply a p p l i c a t i o n of s o p h i s t i c a t e d
tests.
The
last point represents the peculiar d i f f e r e n c e in
troubleshooting between analog and digital instruments.
The steps of intervention required to eliminate a given
trouble in analog instrumentation are described in Table 6.1.
Steps a) and b) are carried out in the same way regardless of
the nature of the faulty instruments; test c) has to be tailored to
the actual nature of the faulty
i n s t r u m e n t , though
requiring a
common testing structure which exists in most laboratories; while
d) may r e q u i r e even purposely d e v e l o p e d instrumentation.
The correct way of carrying out steps a) to c) will now be
discussed, while test d) will be described with reference to the
specific instruments that will be considered afterwards.
The analog instrument you will be required to repair or to
bring back to operate within the manufacturer's specification can
be either :
- a desktop instrument w i t h a built-in power supply;
- a rack instrument with a built-in power supply; or
- a module to be fitted into a NIM,
CAMAC, or
EUROCARD crate without built-in power supplies.
(105)
Chapter 6
-2-
TABLE 6.1: What to do in case of trouble?
STEPS TO BE TAKEN
evident failure
(input signal not t r a n s m i t t e d to the
o u t p u t , basic function no longer
implemented and so forth)
or
a) VISUAL INSPECTION
b) ANALYSIS OF dc WORKING
CONDITIONS
obvious type of m a l f u n c t i o n i n g
(signal d i s t o r t i o n , excessive noise
of any nature, reduction in the
dynamic range and so forth)
c) INVESTIGATION OF THE
SIGNAL BEHAVIOUR WITH
ARTIFICIAL PULSES
performance degradation
a), b), c)
(appearance of a small non-linearity,
slight increase of noise, rise-time,
time jitter and so forth)
d) SPECIFIC TESTS
6.1.1
Visual Inspection
Visual inspection aims at discovering whether:
a.
there is any stray jumper shorting two wires;
b.
t h e r e is any resistor, which because of a change in
colour or a deformed shape, suggests that the maximum
ratings have been exceeded;
c.
there is any cold-looking soldering;
d.
there is any wire or component pin disconnected.
NOTE ;
6.1.2
The
instrument under repair should be illuminated by
an intense lamp. Even if you have good eyes, a
magnifying lens can help you to see more details.
Checking dc Conditions
If the visual did not reveal any faults, proceed to the next
step, namely to check dc conditions throughout the instrument.
(106)
-3-
NOTE ;
Chapter 6
The analysis of dc conditions, if done properly, will
help you to detect the fault in most cases.
Several precautions must to be taken during a dc analysis.
Check
the condition of the fuses if the
instrument
to be
r e p a i r e d is supplied
directly
from the mains. Then, remove
the cover and after turning the power ON, check that the
s u p p l y voltages are
present
at the correct
values on the
output
lines of the power supply. If the instrument
to be
repaired is a m o d u l e which receives the supply voltages from a
c r a t e , check with an o h m m e t e r that no short circuit exists
between the supply v o l t a g e
inputs and ground
on the rear
c o n n e c t i o n . If no short c i r c u i t is p r e s e n t , connect it to the
c r a t e supply through a flat cable or through an extension and
t u r n power ON. Check on the m o d u l e under repair w h e t h e r
the
correct supply voltages appear at the relevant points.
If
a short-circuit
is
detected
from either
resistance
measurement
or
supply v o l t a g e measurement
at
the relevant
points in the instrument,
repeat the visual inspection
following the power supply wire where the short circuit
exists. If the visual inspection remains unsuccessful, remove
one at a time the filtering c a p a c i t o r s connected between the
power supply and ground and repeatedly check whether the short
circuit still exists or not. Most likely, the short circuit
is due to one of these f i l t e r i n g capacitors.
Once the possible short
c i r c u i t s are removed and the correct
dc supplies appear on the relevant lines, measure the dc
levels inside the instrument by using a d i g i t a l voltmeter w i t h
at
least
3 1/2 digit resolution and
10 M or more input
resistance. MAKE SURE THAT ONE INPUT OF THE VOLTMETER IS
SAFELY
GROUNDED;
SOLDER ON THE TIP OF ITS LIVE
PROBE A
10K
RESISTOR. Such a resistor has a decoupling purpose and avoids
the
s i t u a t i o n where the relatively large input capacitance of
the v o l t m e ter connected
between
the e m i t t e r of a high
frequency transistor and ground make it oscillate.
6.1.3
Checking a Faulty Transistor
The following simple rules will help you understand whether or
not a transistor is faulty.
Assume
that the transistor of Fig. 6.1 is expected to be ON.
All transistors in the amplifying part of an analog instrument
should actually be ON.
Determine the currents through Rl and R2 by measuring the
voltage across Rl and R2 and applying Ohm's Law and the VBE voltage
of Tl. If 1C, IE and VBE lies in
the range 600-700 mV for a
silicon transistor or 300-500 mV for a germanium transistor, then
Tl works properly.
(107)
Chapter 6
-4-
If Rl is not present, but there is a resistance in series with
the
base
of Tl, like in Fig. 6.2, then
determine
the
currents
through R3 and R2 and the V(BE) v o l t a g e .
If VBE lies in the same
range as before and I(B) is 10 to 100 times or more lower than
IE,
then again Tl works properly.
Fig.
6.1: Basic transistor
stage
Fig.
6.2: Transistor stage
w i t h base resistor
The transistor has to be replaced if
a.
While VBE falls into the correct range, 1C is much
smaller than IE (Fig. 6.1) or IB is nearly equal to IE
(Fig.
6.2). (These s y m p t o m s are t y p i c a l for a transistor
where the b a s e - t o - c o l l e c t o r junction is OPEN).
b.
VBE
exceeds + 1 V . This
base junction is OPEN.
c.
VBE
is
zero.
happens
when
the
emitter-to-
not within the correct range and VCE
This
happens
when
the
base
has
is almost
been
punched
through or collector and emitter are short-circuited.
Such a short circuit may also be due to an external
unwanted
jumper,
in
which case
VISUAL
INSPECTION
IS
ADVISABLE BEFORE REPLACEMENT.
d.
V(BE)
is zero, yet IE
is
d i f f e r e n t from zero.
Base and
e m i t t e r are s h o r t - c i r c u i t e d , either inside or outside the
device.
VISUAL
REPLACEMENT.
|
|
|
I
NOTE :
INSPECTION
IS
AGAIN
The resistors may also be responsible
operation
have
of
changed
a circuit.
its value
for incorrect
resistor can
actually, a
because
ADVISABLE BEFORE
of
a
previous
incorrect intervention on the circuit, during which
excessive power was dissipated owing to an accidental
short circuit, because of a manufacturing d e f e c t or
because of strongly adverse environmental conditions.
(108)
Chapter 6
-5-
As a limiting case, a resistor can either be open-circuited or
short-circuited. Table 6.2 describes what happens in the circuits
of Figs. 6.1 and 6.2 with Rl or R2 either open-circuited or
short-circuited.
TABLE 6.2; Failures in the circuit of Fig. 6.1 caused by resistors
Rl open-circuited
VCE « 0
VBE in
in normal
normal
range
Tl saturated
R2 open-circuited
1C
« 0
VBE *
0
Tl off
R2 short-circuited
VE
«
VBE *
0.6V
VBC small
and negative
or positive
Rl short-circuited
no voltage
drop across
Rl
-E2
VBE in normal
range
What
happens in the circuit of Fig. 6.2
open-circuited is described in Table 6.3.
if
R3
is
TABLE 6.3: Failures in the circuit of Fig. 6.2 due to resistors
R3 open-circuited
VB *- E2
VE«-E2
IE » 0
The case of RB short-circuited can be investigated
direct RB measurements.
only
by
If the above described tests (visual inspection, transistor
fault diagnosis, and resistor "short-circuit/open-circuit" test
do not show anything abnormal, measure the resistor values after
removing the transistor.
(109)
Chapter 6
-6Consider now
the circuit
of
Fig.
6.3,
which shows
transistor,
T3,
in
the
common-base configuration.
If
you
suspect
that
there
is
something wrong in t h i s c i r c u i t
and that it may
be responsible
for
the wrong b e h a v i o u r of the
network
to
which
it
is
connected,
check
first
the
v o l t a g e between B and ground.
If such a v o l t a g e s u b s t a n t i a l l y
d i f f e r s from the nominal v a l u e
E1R2
- Ë2R1
Rl + R2
Fig.
6.3: Transistor stage with
ac c o u p l i n g
it has to be concluded t h a t the
circuit
is f a u l t y . The
fault
may
actually
depend
on C
behaving as a short
circuit
because of
an
irreversible
damage
occurred
to
it,
on
t r a n s i s t o r T2 or on the voltage
divider Rl. R2.
If you c o n c l u d e that C is s h o r t - c i r c u i t e d , d i s c o n n e c t C on one
side and measure VB again, to have e x p e r i m e n t a l e v i d e n c e that your
h y p o t h e s i s is correct.
There may be some situations
in which C is not a true short
c i r c u i t , but presents a finite resistance across its t e r m i n a l s , in
which case it is d i f f i c u l t to distinguish w h e t h e r the wrong value
of
VB d e p e n d s on C or on the open BC junction of T2.
In such a
case, first d i s c o n n e c t C on
one
side and
check VB again. If VB
remains at
the wrong v a l u e , then turn off power
and
check
the
resistor values.
NOTE :
Some d e f e c t i v e resistors exhibit the correct value
when measured with the ohmmeter, but then present a
d i f f e r e n t value under applied voltage. In a
d i f f i c u l t case, you can consider to replace them
even
if the resistance measurement gives the
correct resistor values.
The fault symptoms are described in Table 6.4.
(110)
a
-7-
TABLE 6.4;
1.
Diagnostics of the fault symptoms of the circuit in
Fig. 6.3.
C behaving as a short circuit, E3 at ground p o t e n t i a l
VB
2.
0
VBE in the normal range
C behaving as a short circuit, E3 = - E2
VB >= - E2
3.
Chapter 6
VE = - E2
T2 with open-circuited BC junction
VB lower than the nominal value by
R1R2 . IE
RI + R2
4.
R2 short-circuited
VB » - E2
VE = - E2
indistinguishable from 2. and 5.
5.
Rl open-circuited
VB = - E2
VE = - E2
indistinguishable from 2. and 4.
6.
Rl short-circuited
VB = + El
7.
VB in normal range
R2 open-circuited
VB much closer to El than predictable according to
nominal values of the components
VBE in the normal range
(111)
Chapter 6
-8Consider now the common-emitter
transistor
connection
of
Fig. 6.4. Besides the faults
depending on the RI, R2 voltage
divider and
on T3
that
fall
under the discussion developed
for the circuit of Fig. 6.2, one
has to consider now the problems
related to a short circuit on C
or R3.
The relevant
symptoms
are :
VE « - E2
VB « - E2 + .7V.
Fig.
6.4: Common-emit ter
transistor stage
If these symptoms appear, disconnect C on one side and measure
VE and VBE again. If the h y p o t h e s i s of C is
conf irmed . in the
sense t h a t w i t h C disconnected
VB «
E1R2 - E2R1
Rl
VE
VB - Ü.7V,
R2
replace C.
If with C disconnected the fault does not disappear,
you
have to suspect that R3 is s h o r t - c i r c u i t e d , therefore replace
it .
ATTENTION:
Some of the shot t-circuit/open-circuit situations
may
not necessarily be due to a
defective
component, but could be simulated by a stray
jumper or an ill-soldered terminal in the layout.
Therefore, prior to proceeding to disconnect a
suspected
component,
visually
inspect
the
concerned part of your circuit.
R e p l a c e m e n t of a d e f e c t i v e transistor cannot always be done
with the same type of device.
This is especially
true if the
instrument under repair is of old design and the components to be
replaced are obsolete. It may sometimes happen that the specimen
used for replacement actually has b e t t e r gain-bandwidth properties,
in which case the new transistor may exhibit high frequency
oscillations.
Sometimes
the frequency
of
these
spurious
oscillations is so high that you cannot d e t e c t it unless a sampling
scope is available. However, there is a very simple trick to judge
whether GHz oscillation is present simply by reading dc levels. The
trick is based upon the fact that the high frequency oscillation,
r e c t i f i e d by the non-linearities
present in active devices,
actually modifies the dc levels in the circuit. Monitor then the
(112)
Chapter 6
-9de levels in the proximity of
the
replaced
transistors
and
see
whether or not they change by adding
a small capacitance between
base, c o l l e c t o r , e m i t t e r of the new transistor and ground. Such a
capacitance will modify the amplitude of the oscillation and along
with
it the dc levels.
To add such a c a p a c i t a n c e , just handle
an
ordinary screwdriver, possibly
long
and t h i n , and touch w i t h its
short
metallic
top
the transistor leads, being careful to avoid
circuits (see Fig. 6.5).
Fig. 6.5: Testing of a transistor stage for oscillation with
a screwdriver
frequency
If no m o d i f i c a t i o n s occur in the dc levels, no high
If, however, the dc levels change when you
touch or simply approach the transistor leads with the screwdr iver
t i p , then the oscillation is p r e s e n t .
oscillation is present.
In this case the newly connected transistor can be neutralized
by adding a 100 ohm resistor in series with the collector lead or
the base lead and as close as possible to the transistor can.
The
previously
analyzed
situations
refer
to
elementary
circuits,
but the m e t h o d s employed can easily be extended to more
c o m p l i c a t e d circuits built up from
these simple ones. The d o t t e d
lines in Figs. 6.1, 6.2, and 6.3 imply connections to other parts.
Sometimes
an
instrument
undergoes
a
catastrophic failure,
owing to which, in the individual elementary circuits like those of
Figs.
6.1
through 6.4, more than one component may be damaged.
There are so many
possible
combinations
of faulty elements that
t r o u b l e s h o o t i n g may require a procedure-describing flowchart.
A
flowchart will be developed here with reference
to a single
transistor circuit of broad v a l i d i t y , the one shown in Fig.
6.6.
To judge whether this
voltages
VB,
VC,
VE
circuit
is
properly
are to be measured and
operating or n o t , the
compared
with
the
nominal values:
(113)
-10-
Chapter 6
VB
«
E 1 R 2 - E2R1
Rl + R2
VC
»
El - VE •*• E2
R4
VE
VB - 0.7V
R3
•-E,
Fig.
6.6: Basic transistor stage
If the measured values differ from the nominal ones by more
than 10%, which accounts for the worst case component tolerances,
the circuit has to be considered f a u l t y . As previously pointed
out,
the fault may be due to either:
1.
2.
3.
4.
5.
d e f e c t i v e voltage d i v i d e r ,
d e f e c t i v e transistor,
d e f e c t i v e e m i t t e r resistor,
d e f e c t i v e collector resistor
d e f e c t s in layout,
or to all the possible
causes of fault.
Referring
combinations
active
of two, t h r e e , four or
to the circuit in Fig. 6.6 immediately
d o u b t about whether the fault
the
or
device
and
five
removes any
depends on the v o l t a g e d i v i d e r or on
associated resistors.
nominal value, the fault has to be
If VB
is
at
its
a t t r i b u t e d to the a c t i v e d e v i c e
and/or to the R3 , R4 resistors.
If, however, VB is not c o r r e c t , the v o l t a g e divider or the (T,
R3 , R4 ) amplifier or b o t h may be defective.
To disentangle
r e s p o n s i b i l i t i e s , remove the t r a n s i s t o r
and check VB again.
If VB
is
near its nominal value, then the fault has to be a t t r i b u t e d
to
(T, R3 , R4). As T was removed, you have a very e f f e c t i v e way of
checking
(R3, R4 ) , Fig. 6.7.
Connect in series w i t h R3 and R4
resistor of accurately known value and such that the current I
flowing across the series connection R3 , R* , R4 :
El + E2
R*
R3 + R4
will be equal
to
the
current
IE
1C
which
would
flow
across
transistor T and resistors R3, R4 if no fault were present. Such a
current is easily evaluated from the table of nominal values (1):
(114)
a
-11IE * 1C ~ VE + E2
R4
Fig. 6.7:
Chap ter 6
VB - 0.7V + E2
R4
Replacement of a transistor by a resistor
Besides, R* must be able to stand a power of at least
5 . (El + E2)2 .
R*
Once R* is c o n n e c t e d , check w h e t h e r or not the v o l t a g e drops
across R3 , R4 c o i n c i d e with the nominal values d e t e r m i n e d by the
knowledge of L and of
the nominal values of R3, R4 . If they
c o i n c i d e , then R3 and R4 are not d e f e c t i v e and you can focus your
suspicion on
the t r a n s i s t o r .
If, however, any
or none of the
v o l t a g e drops across R3, R4 stick to their nominal values,
c o n c e n t r a t e on R3, R4 .
Mind the following:
1.
If the v o l t a g e drops across R3 or R4 or b o t h are zero, but the
one across R* is equal to El + E2, then R3 and R4 b e h a v e as
short
circuits.
The
short c i r c u i t s may
depend
on
the
resistors t h e m s e l v e s or on
some
stray
jumper on the l a y o u t .
Check the latter p o s s i b i l i t y by accurate v i s u a l inspection. If
there is no evidence of s t r a y jumpers, p r o c e e d to replace R3
and R4 and check the voltage drops across the R3, R*, R4
d i v i d e r again. Most likely the f a u l t has been e l i m i n a t e d .
2.
If the voltage drops across R3, R*, R4 are all zero, there
is
a c i r c u i t i n t e r r u p t i o n along it. Again this may depend on R3
or R4 or both being interrupted or on some broken connection
on the layout. Search for
it by accurate visual inspection
and if you
discover any defective connection, repair it.
O t h e r w i s e , again replace R3 and R4 and check the voltage drop
across the R3, R*, K4 divider again.
Either way the fault
should have been eliminated.
Once you have removed the transistor and made sure that no
d e f e c t had to be a t t r i b u t e d to R3 and R4, or if there was a d e f e c t ,
that it has been fixed, proceed to check the transistor.
(115)
-12-
Chapter 6
NOTE:
The most
transistor
reliable
with
procedure
a transistor
would
curve
be
to test the
tracer.
This
instrument will tell you whether
the transistor is
still alive or d e f i n i t e l y gone, but would also enable
you
to d e t e r m i n e w h e t h e r a d e t e r i o r a t i o n
in its
c h a r a c t e r i s t i c s has occurred.
If a curve tracer is not available in your l a b o r a t o r y , you can
construct the very
simple
transistor
tester of Fig.
6.8.
tester
accepts on two different sockets both NPN and
PNP
transistor. The two-position, two-pole switch selects either type.
The t e s t e r is basically a B meter and is surely much more e f f e c t i v e
than a simple ohmmeter
employed to determine whether either
junction is open or not. W i t h the tester of Fig. 6.8 you put the
transistor under test on the relevant socket having previously
correctly positioned the switch, and take note of the panel meter
reading. If such a reading is between 0 and 3V, your transistor
has to be r e p l a c e d .
DIGITAL PANEL
METER
Fig.
6.8: Simple transistor tester
It should be realized that transistor testers are available on
the market
IN-CIRCUIT
(References: PORTABLE
TESTS, BK PRECISION).
TRANSISTOR TESTER MODEL 510 FOR
They usually
can be directly
applied to the transistor to be tested without removing it from the
printed circuit board. If such a fixture is available, then you
may immediately make clear whether the fault in your circuit
depends on the transistor or not. If it does, you simply replace
the transistor and check whether
the
circuit works or not. If it
does not work, then in any case you must disconnect the transistor
and check (R3, R4) and (Ri, R2).
(116)
The
-13-
Chapter 6
It has still to be considered how to proceed if the VB voltage
turns out to be wrong, after the transistor has been disconnected.
The troubleshooting procedure for a voltage divider consists of
checking whether VB is either equal to El or to -E2. In the former
case, Rl is short-circuited or the B-through R2- to -E2 line is
i n t e r r u p t e d . Short circuit and interruption may actually reside on
the p r i n t e d circuit board, in which case accurate visual inspection
may lead to their detection. Otherwise, they have to be attributed
to Rl or R2 or to both. Replace both resistors.
Check VB again and you will
value.
see that now VB is at its nominal
Having the transistor disconnected, check it with a curve
tracer, a se 1f-cons t rue ted tester or with a commercial transistor
fixture and decide whether to replace it or not.
Check R3 and R4 and decide accordingly whether they
kept or must be replaced.
can
be
Finally, reassemble the circuit and check VB, VE, VC again.
The flow-chart summarizing
procedure is given in Fig. 6.9.
the
described
troubleshooting
(117)
Chapter 6
-14-
FIX —OP SMO«TCiROiirS
AND iiJTEPRuPTIO/JS 1(4
rue Lflrour >uo/oR.
CU6(?SM0O77* PtlflOMf
»seo UPOM »csisrot B*
TfST
A fJEtJ SPfc/„fH
o
nre OK OF
He*eeo fou
|w"»TVf ijf K/rfou/fnu.
Fig.
(118)
6.9: Flowchart for t r o u b l e s h o o t i n g a
stage
/
basic
transistor
-15-
Chapter 6
Troubleshooting considerations similar to the previous
can be applied to instruments or parts of them based
operational amplifiers.
ones
upon
An instrument may be designed to use several operational
amplifiers. The search for a fault
in the instrument requires a
fault analysis for the individual operational amplifier circuits,
which can be introduced with a q u i t e general approach by discussing
the connections as shown in Fig 6.10.
Fig.
6.10: Basic operational amplifier configuration
Suppose t h a t the o u t p u t voltage Vo, which should be close to
zero if this circuit is intended to be a linear a m p l i f i e r , is found
instead to be close to the positive saturation voltage (+11 to
+ 14V,
depending on the type of operational amplifier e m p l o y e d , for
a + 1 5 V bias).
Assume also t h a t visual inspection has been
accurately carried out on the circuit and that n o t h i n g has come to
your a t t e n t i o n .
Follow the instructions suggested in the flow chart presented
in Fig. 6.11.
(119)
Chapter 6
-16-
[MEASURE v0[
IresTOKCg.^"*"^*"1"^ S^-vca^ff*,--* -
~^^
.^y
V ilOHWM. <*1U» FUjf
1
,
Fig. 6.11: Flowchart for troubleshooting a basic operational
amplifier configuration
(120)
Chapter 6
-176.1.4
Signal Analysis
Have you finished the dc analysis of the i n d i v i d u a l parts
your i n s t r u m e n t ?
Have you made sure
that
everywhere
c o n d i t i o n s meet the expected values?
If
so,
you
can
quickly
i m p l e m e n t s its basic function.
check
of
in the instrument the dc
whether
your
instrument
For instance:
If the i n s t r u m e n t is a p r e a m p l i f i e r
or an a m p l i f i e r , does a
signal
appear at the o u t p u t when you a p p l y a signal at
the
i n p u t , and are the shape and
the a m p l i t u d e of
the o u t p u t
signal reasonable?
If the instrument is a single channel analyzer, does the logic
o u t p u t signal appear at
the o u t p u t when you apply an input
position ing
signal,
and
slowly
move
the
threshold
p o t e n t i o m e t e r u n t i l t h e input signal falls into the channel?
If the i n s t r u m e n t is
a t i m e - t o - a m p l i t u d e converter, does a
ramp
signal appear at the o u t p u t when you apply a s ignal at
the START input and a delayed one at the STOP input?
d.
If the i n s t r u m e n t is a linear
gate,
is the signal applied at
the
analog input t r a n s m i t t e d almost u n a l t e r e d to the
output
during those
intervals when
it overlaps
in
time with the
g a t i n g command a p p l i e d at the LOGIC INPUT?
Some s i t u a t i o n s are
summarized in F i g . 6.12.
For
each
i n s t r u m e n t there is an a p p r o x i m a t e indication of the c o r r e c t and of
a p o s s i b l e d e f e c t i v e o u t p u t signal. If the signal is a c c e p t a b l e ,
you can go on w i t h a d e t a i l e d
analysis and
calibration
of the
instrument. If the signal is strange, it is necessary to carry on
w i t h a signal analysis of the i n d i v i d u a l stages.
To apply signal analysis, you must send a signal to the input
and
then
follow its path t h r o u g h o u t the i n s t r u m e n t , making sure
t h a t every c i r c u i t performs on
the signal the correct function.
Signal analysis will tell you some very i m p o r t a n t
facts.
1.
The signal is present at the input of a certain circuit and is
no longer present at the output. The circuit under test has
an i n t e r r u p t i o n on the signal path.
2.
The signal appears at the output of an amplifier clipped on
the t o p , although the input was in the correct range.
The
circuit dynamic range is out of the specified value.
3.
The signal passing through a linear amplifier appears at the
output
smeared by an oscillation whose frequency exceeds
100
MHz.
You can almost be sure that in the circuit there is an
ill-neutralized transistor.
(121)
Chapter 6
-18-
E—» n mi ti*f(
j——i, rwM e.tMt*»>.
fVJl U«M •».«
Mrr>.
UM ik*
WVUTUM »•-» «W«
TO IT r «•»«
n
*»mr ««
UNmniM
\
NÏMM
to TW« «*»»e
^S.
CW^Vctlv«
totM**A"*
' \-^^ir£
M
^1*0^
vrut i caff
Fig,
(122)
6.12:
Test set-up for some typical nuclear electronic
instruments w i t h acceptable and d e f e c t i v e o u t p u t
s ignals
-19-
Chapter 6
The
signal passing through
a linear feedback amplifier
presents
at the o u t p u t a d a m p e d oscillatory behaviour.
Most
p r o b a b l y the f e e d b a c k loop is not s u f f i c i e n t l y compensated.
A linear
feedback a m p l i f i e r , whose
function
is
that
of
a m p l i f y i n g and not that of s h a p i n g , slows down excessively the
signal. Your feedback loop is over-compensated.
NOTE :
To carry on signal analysis, you need a rectangular
pulse generator
and a dual
channel oscilloscope
with at least
100
MHz
bandwidth.
It must be
e q u i p p e d with two Ix probes and two lOx probes. Do
not use the Ix probes if
this
is not necessary to
achieve a high s e n s i t i v i t y of your scope. For
an
easier observation on
the scope, use an e x t e r n a l
trigger by sending to the oscilloscope
input
the
advanced trigger signal from the generator. Set on
the generator a 200ns delay. Make sure that no
false contacts exist in the cables, connectors and
probes. Check the ground wires of the probes; make
sure that
they are
safely soldered to their
alligator-clips.
Add in series with each probe a
300 resistor to avoid spurious e f f e c t s due to probe
c a p a c i t a n c e . H a v i n g done t h i s , send the signal from
the generator o u t p u t to the input of the instrument
under test. Use the two channels of the scope to
m o n i t o r , for each part
or
elementary
c i r c u i t in
your i n s t r u m e n t , input and o u t p u t signals.
Ground
the probes to the ground conductors that are
closest to the p o i n t s you are going to monitor.
Some ideas about how to proceed with the signal analysis will
now
be discussed.
They will
refer
to p a r t i c u l a r l y
simple
s i t u a t i o n s where the instrument is made only of a m p l i f y i n g p a r t s ,
all working in the linear range and not requiring any logic
command.
More c o m p l i c a t e d s i t u a t i o n s will be considered afterwards
with s p e c i f i c cases. The analysis referred to in the linear case
is, however, e x t r e m e l y useful in i n t r o d u c i n g the basic principles.
Suppose
that
following
the signal p a t h , you
arrive
at a
circuit i l l u s t r a t e d in Fig. 6.13, w i t h signal shapes as indicated.
(123)
Chapter 6
-20-
J"
Fig.
6.13
AC-coupled operational amplifier with open signal
path
If
the circuit had previously undergone dc analysis and was
found to operate correctly de-wise, then the present f a u l t , that
is, open signal path between input and o u t p u t , may depend on an
open C capacitor or on an interrupted connection in series with C
on the printed-circuit board.
ATTENTION:
Such a fault
cannot be
analys is of DC cond it ions.
detected
from
the
Another frequent source of fault is a shunting capacitor
which , because of a mistake made in a previous repair, is much
larger than it should be. Such a situation is illustrated in Fig.
o t v"appear
a p p e a r to the oouuttppuutt,, as
as it
it is
6.14.
The signal actually does n
not
short-circuited
to ground
by the lOOnF capacitor
connected by
mistake.
The collée tor-to-ground
capacitor
of large value
interrupts the signal path again. In this case DC analysis would
not reveal the fault.
(124)
Chapter 6
-21-
DUTPUT
lOOpF NOMINAL
lOOnF ACTUAL
INPUT
-E
Fig.
6.14:
Transistor stage w i t h d e f e c t i v e o u t p u t signal
Consider
now
the following
two-1ransis t or
operational
a m p l i f i e r (Fig.
6.15)
and look at the o u t p u t signal (a). Suppose
that
the oscillation frequency exceeds 100 MHz.
Such a limit
is
purely e m p i r i c a l , but it h e l p s you
to guess w h e t h e r an oscillation
has
to be a t t r i b u t e d to a component or to the f e e d b a c k loop.
The
transistor
T2
appears to be
unneutralized.
Most
likely
the
oscillation
is due to T2.
To damp it, add a 100 ohm t r a n s i s t o r in
at
T2 , and as close as possible to
series w i t h the c o l l e c t o r lead
its can .
Fig.
6.15:
Two-transistor operational amplifier with signal
oscillator
If the o u t p u t signal looks like that of case (b), with the
frequency of the damped oscillation of 50 MHz or less, most likely
(125)
-22-
Chapter 6
the o s c i l l a t i o n is d e t e r m i n e d by the feedback loop. Add
then a
small c o m p e n s a t i n g c a p a c i t o r , 5pF to 50pF, between the c o l l e c t o r of
Tl and g r o u n d . The d a m p e d o s c i l l a t i o n will d i s a p p e a r .
Look now at the a m p l i f i e r c i r c u i t of Fig. 6.16.
Suppose you
find t h a t its signal
gain
differs
considerably
from the nominal
v a l u e of 10.
Such a d e f e c t may d e p e n d on a change t h a t o c c u r r e d in
one or b o t h r e s i s t o r s because
of
some
kind of a c c i d e n t or on the
fact
t h a t for some reason the o p e r a t i o n a l a m p l i f i e r is
completely
out of s p e c i f i c a t i o n s as far as
its signal gain is concerned.
In
t h i s case the r e s i s t o r s do not draw c u r r e n t in the standing
state.
Turn off p o w e r , d i s c o n n e c t t h e m
from the c i r c u i t and measure their
values
w i t h an o h m m e t e r .
If the measured values are close to the
nominal ones, proceed to replace
the o p e r a t i o n a l amplifier.
The
d e s c r i b e d f a u l t o b v i o u s l y e s c a p e d d c analysis.
NGMINAL
VALUES
Fig. 6.16: O p e r a t i o n a l a m p l i f i e r with a gain of 10
Consider
now
the following a m p l i f i e r , whose nominal
gain
should be 10, but actually is found to be 1 (Fig. 6.17). The fault
may
d e p e n d on Rl o p e n - c i r c u i t e d or R2 s h o r t - c i r c u i t e d .
Open and
s h o r t c i r c u i t may
be r e l a t e d
to faults on
the p r i n t e d circuit
b o a r d . In this case visual i n s p e c t i o n aiming at d e t e c t i n g possible
interruptions, accidentally
disconnected
c o m p o n e n t s , or
stray
jumpers has to be carried out f i r s t . If nothing comes out of
this
i n s p e c t i o n , measure
the resistors
Rl and
R2 and
change
the
d e f e c t i v e one(s). Remember that t h e r e is still the possibility
of
a gain in the operational a m p l i f i e r much below the nominal v a l u e ,
in which case it has to be replaced.
(126)
Chapter 6
-23-
lOnF
ÜUTPUT_]
NOMINAL VALUES
Fig. 6.17:
6.2
AC-connected operational amplifier with a gain
of 10
TROUBLESHOOTING OF SPECIFIC EQUIPMENT
6.2.1
Troubleshooting of a Charge-Sensitive Preamplifier
Existing commercial charge-sensitive preamplifiers fall
four c a t e g o r i e s .
(i)
Resistive feedback preamplifiers:
(ii)
Resistive feedback preamplifiers:
(iii)
O p t i c a l feedback preamplifiers:
into
with input FET
operating at room
temperature
with input FET
cooled
with input FET
operating at room
temperature
(iv)
O p t i c a l feedback preamplifiers:
with input FET
cooled
Each category requires specific troubleshooting procedures. As
an e x a m p l e , a preamplifier with
resistive feedback and input FET
o p e r a t i n g at room t e m p e r a t u r e is analyzed.
6.2.1.1
General considerations
A t y p i c a l block diagram of such a preamplifier (PA) is given
in Fig.
6.18, where the shielding box, the connectors, the
d e t e c t o r b i a s i n g and the test networks are also i n d i c a t e d .
(127)
Chapter 6
-24-
DETECTOR BIAS
AMPLIFIER
DETECTDR
INPUT
POLE-ZERO
ADJUST
NETWORK
OUTPUT
TEST INPUT
Fig.
6.18: C h a r g e - s e n s i t i v e p r e a m p l i f i e r
The basic b l o c k s are the c h a r g e - s e n s i t i v e l o o p , the pole-zero
a d j u s t m e n t n e t w o r k , and the o u t p u t amplifier. The c h a r g e - s e n s i t i v e
loop d e t e r m i n e s , to a major e x t e n t , the noise p e r f o r m a n c e of
the
preamplifier.
The f u n d a m e n t a l c o n c e p t s a b o u t
these blocks can be found in
document
IAEA
TECDOC-363,
SELECTED
TOPICS
IN
NUCLEAR
E L E C T R O N I C S , pages 72 t h r o u g h
77, and
in the d o c u m e n t
IAEA
TECDOC-309, N U C L E A R E L E C T R O N I C S L A B O R A T O R Y M A N U A L , p a g e s 77 t h r o u g h
83.
The d r a w i n g of Fig. 6.16
also shows, i n s i d e the shielding
box,
the d e t e c t o r
b i a s network consisting of
(RICH) low-pass
f i l t e r , and d e c o u p l i n g r e s i s t o r RB , the i s o l a t i n g c a p a c i t o r Ci, and
the
test
network consisting
of
terminating resistor
Ro
and
injection c a p a c i t a n c e Cinj.
the
If you have such a t y p e of p r e a m p l i f i e r to t r o u b l e s h o o t , check
first whether
or
not the o u t p u t lies w i t h i n
its
linear
range,
w i t h o u t even o p e n i n g the PA b o x . For such a purpose:
- c o n n e c t the PA o u t p u t to a scope;
- put the scope on the position of h i g h e s t v e r t i c a l
s e n s i t i v i t y , p o s s i b l y 2 to 5 m V / c m ;
- use ac c o u p l i n g in the scope;
- t r i g g e r the scope on AUTOMATIC;
- turn on power on PA and look at the o u t p u t baseline.
If the o u t p u t
baseline
looks to be smeared by noise (Fig.
6.19a), t h e n your PA is likely to be in a correct dc condition.
(128)
-25-
NOISY LINt:
Chapter 6
NOISE-FREE L I N E
a)
Fig.
b)
6.19: Output baseline of a preamplifier as seen on a
scope
In t h i s case you can go on
by sending a signal to the test
i n p u t , and see w h e t h e r , according to Fig. 6.12, the o u t p u t signal
is a c c e p t a b l e or badly d i s t o r t e d .
In the former case, your PA has
successfully
passed the s i m p l e s t test and most likely it requires
more sophisticated and specific
procedures. If instead the output
signal
looks to be badly d i s t o r t e d or unexisting at a l l , you must
carry on a c o m p l e t e t r o u b l e s h o o t i n g p r o c e d u r e . Open the PA box and
start with dc analysis.
Returning
to
the
o u t p u t baseline
displayed
on
the
o s c i l l o s c o p e , if it is not
smeared at all by noise, then the PA is
saturated
somewhere.
Again,
in
this
case, a
complete
t r o u b l e s h o o t i n g procedure has
analys is.
to
be
carried out s t a r t i n g from dc
To
i l l u s t r a t e the above c o n s i d e r a t i o n on the example of
a
commercial p r o d u c t , let us consider the CANBERRA 2004 p r e a m p l i f i e r ,
with its c i r c u i t r y shown in Fig. 6.20.
The
charge-sensitive
loop consists of JFET Ql of
the
long-tailed
pair Q2,
Q3
n o n - i n v e r t ing
gain
stage, and
of
e m i t t e r - f o l l o w e r Q4, which is the o u t p u t stage. Transistors Q5 and
Q6
are
current
sources, r e s p e c t i v e l y
absorbing
the c o l l e c t o r
current of Q3 and forcing the current into the e m i t t e r of Q4.
The
pole-zero adjustment network consists of potentiometer
RV2, of resistors R8, R 1 8 , R27 and of capacitor C6.
Transistors
Q7,
Q8,
Q9, Q10, and
Qll
constitute
a
discrete-component operational a m p l i f i e r , whose gain can be fixed
to I or to 5 w i t h the jumper connected respectively between A and C
or A and B.
6.2.1.2
DC analysis to the charge-sensitive loop
Take out
the sliding l i d , connect the
earth, and switch power ON.
preamplifier
box
to
(129)
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CANBERRA
Chapter 6
-27-
Check w h e t h e r the voltages at points A, B, C, and D are
to the v a l u e s s p e c i f i e d before:
close
VB = (R7 + R16)V1 - R 6 V 1
R6 + R7 + R16
VC ~ VB
VD < Ü
ranging between -0.1 and -IV
VA = VC -
(note that R3=R6)
R4 x 2V1
R6 + R7 + R16
There are t h r e e possible sources of trouble:
1.
The channel of JFET Ql is OPEN.
2.
The transistor Q2 has got either base-emitter
junction OPEN or e m i t t e r - c o l l e c t o r p a t h OPEN.
3.
The c u r r e n t source Q5 does not p r o v i d e current.
You can easily
distinguish
s i t u a t i o n 1 from 2 and 3
by
m e a s u r i n g the v o l t a g e at point
A.
If such a v o l t a g e is equal to
+ V1, then JFET Ql has an open channel, for it d r i v e s no
current
although the positive v o l t a g e
at Q4
e m i t t e r through the 100 Mfi
resistor forces a forward bias of Ql gate-to-source junction.
If
the FET is s u s p e c t e d , remove it from the c i r c u i t and check
it w i t h the c i r c u i t of Fig. 6.21 a) and b).
Fig.
6.21a:
Measuring IDSS
of a JFET
Fig.
6.21b:
A circuit to measure
the transductance
If the charge-sensitive loop has correct dc conditions,
troubleshooting can continue by checking the o u t p u t amplifier.
However, if these voltages are far from the specified values,
and,
as frequently
happens
in
a
damaged charge-sensitive
p r e a m p l i f i e r , VD is some volts positive, the following failures may
have occurred.
(131)
Chapter 6
-28-
(a)
No drain current is measured in the JFET; it is definitely
damaged and has to be replaced. If, however, current flows in
the JFET, and this current is not smaller than 5mA, it is
worth going to a dynamic transconductance t e s t , according to
the c i r c u i t of Fig. 6.21 b. For detailed
information about
FET t e s t s , read IAEA TECDOC-309, pages 78 through 81.
(b)
The FET e x h i b i t s an open channel with the test of
or
a value of gm which is typically below 10 mA/V;
its replacement.
Fig. 6.21a
proceed to
Actual FET replacement is not a simple operation because most
commercial charge-sensitive preamplifiers employ SELECTED FETs. If
the type of FET is specified, you can
try to purchase some of them
and
then make a selection, choosing, according to the procedure
outlined in IAEA TECDOC-309, the specimen which has the largest gm.
Pay a t t e n t i o n , however, to the following point. If you do not want
to change other components in the circuit, the new specimen should
also be not
too different in IDSS from the previous one. The
current actually flowing in the FET is given by
VI - VA.
Rl
For instance, with the components of Fig, 6.20, VA ~ 4V, the
current in the FET is approximately 28mA. Therefore, your new FET
should not have an IDSS below 30 mA, but should also not exceed
IDSS 40mA.
Things are more complicated when the FET type is not specified
or, if s p e c i f i e d , is not available. The replacement then will
rarely
be
completely
satisfactory
and
the original noise
performances will not be reached. Nevertheless, replace your FET
following the r e c o m m e n d a t i o n s below so you will not keep the PA out
of
use.
In the m e a n t i m e , order a specimen closer to the one which
was originally put in your i n s t r u m e n t .
6.2.1.3
Replacement recommendations
1.
Deduce from the circuit diagram
standing drain current ID.
of
the PA the value of the
2.
If ID is near or below 10 mA, then stick to a FET of the
2N4416 type.
This
type
should
be available in your
laboratory.
Select the one which features the highest gm
value (circuit 21 b) among t h o s e , rather than have IDSS values
between 10 and 12 mA.
3.
If ID
is above
10 mA, typically 20 to
selection among FETs of the 2N4861A type.
30
mA,
make
your
Remember, incidentally, that a very reliable manufacturer of
FETs for low noise is INTERFET, 322 Gold Street, Garland, Texas
75042 USA. You can ask for their catalogue and determine their
equivalents of 2N4416 and 2N4861A.
(132)
-29-
Chapter 6
More complicated
is the selection when the PA
paralleled FETs and one or both have
to be replaced.
employs
two
Although one
FET may have survived, this can be useless, for it may be very
d i f f i c u l t to match it without having components of the same type or
even not knowing the type. Proceed to group out of your b a t c h of
either t y p e , depending on the standing current they worked at, the
pairs of FETs that have IDSS close in value. Then parallel these
pairs and measure the gm of the parallel combination. Next select
the pair with the highest gm.
Once
your selection is made, it would be better to check the
Q4 , Q6 anyway, just to make sure
that your accurately selected FET(s), once put on the circuit,
won't be damaged because of the charge-sensitive loop. Only if Q3,
Q5 , Q4 , Q6 are in good condition, introduce the FET(s) you have
selected. Now the charge-sensitive loop should be in the correct
de cond i t ion.
c o n d i t i o n of transistors Q3, Q5,
Consider now the case in which, being VD positive of some
v o l t s , VA is much lower than VI, which shows that the FET is in a
good state; if so the failure is likely to have occurred in Q2, Q3,
Q4. Apply the already-explained dc analysis to these transistors
one at a time, following the recommendations of section 6.1.3. Once
you make the necessary replacements, check again the dc conditions
of the charge-sensitive loop.
Once this is fixed, you can go ahead by testing the dc
c o n d i t i o n of the output amplifier.
Measure voltages at points E,
F, G, H, L, after removing R8.
Point out that VE must be close to
VF, and that VG must
be near 0V.
Besides, the current flowing
across R19 ( 5 mV) splits in almost equal parts between Q7 and Q8,
that is, about 2.5 mA in each transistor.
2.5 mA will then flow
across R9, thus keeping VH to about 17.7V. VL must be about .7 V
more positive.
If the voltages VE, VF, VG, VH, VL differ
considerably from the stated values, and especially if VG is
p o s i t i v e or negative of some volts, then the output a m p l i f i e r is
d e f e c t i v e . To troubleshoot it, first of all check the output stage
and determine whether or not current flows across R24 and R26. The
presence of current across R24 and R26 tells you that Q10 and Qll
are alive. If it is so, focus your attention on Q7, Q8, Q9.
Disconnect
jumper AC-AB, in which case you open the feedback loop,
and you can check the behaviour of Q7, Q8
in the open-loop
situation.
Disconnect Q9 and make sure that the currents across
Q7, Q8 are not very
different
from each o t h e r , or at least that
none of them are at zero. In this way you check the condition of
Q7 , Q8.
If r e q u i r e d , replace the d e f e c t i v e component. Check then
with the transistor tester Q9, and if necessary, replace it. Then,
again connect Q9 and reintroduce the jumper. The dc condition of
the o u t p u t amplifier should now be correct.
Consequently, the whole preamplifier should now be in order
from the dc standpoint and you can proceed to signal analysis.
(133)
Chapter 6
6.2.2
-30-
Troubleshooting of Spectroscopy A m p l i f i e r s
Modern spectroscopy
amplifiers have reached a high level of
s o p h i s t i c a t i o n , as
d i c t a t e d by
the need
of ensuring a d v a n c e d
p e r f o r m a n c e s in terms of resolution, counting rate c a p a b i l i t i e s and
fast recovery from heavy overload.
To meet the demand arising from high r e s o l u t i o n s p e c t r o s c o p y ,
a considerable
improvement has been i n t r o d u c e d on
the b u i l t - i n
base-line restorers and
pile-up
rejectors
in order
to avoid
spectral
d i s t o r t i o n s t h a t may arise from baseline f l u c t u a t i o n s at
high counting rates and from pulse-on-pulse p i l e - u p .
Restoring a spectroscopy amplifier which has undergone a
failure
to w i t h i n
the factory guaranteed
p e r f o r m a n c e s may
be
o u t s i d e the reach of anybody but
the designer and a few service
engineers.
However, rescuing a f a u l t y s p e c t r o s c o p y a m p l i f i e r and
b r i n g i n g it back to an a c c e p t a b l e working c o n d i t i o n can be done by
somebody who has clearly u n d e r s t o o d the p r e v i o u s
troubleshooting
procedures r e f e r r i n g to elementary linear c i r c u i t s .
Before
proceeding to the r e p a i r , find in the manual the block
diagram of the a m p l i f i e r and
try
to understand the functions it
implements.
Although
the s p e c t r o s c o p y
amplifiers
from
the
d i f f e r e n t m a n u f a c t u r e r s may
differ
considerably
from each o t h e r ,
the d i f f e r e n c e s are usually r e s t r i c t e d to the more a d v a n c e d
parts,
like baseline restorer, p i l e - u p
i n s p e c t o r , d e a d - t i m e and l i v e - t i m e
monitors.
The
basic
functions still
follow a w e l l - e s t a b l i s h e d
p a t t e r n , which is recognizable in the block d i a g r a m of F i g . 6.22.
As
shown in Fig. 6.22, the a m p l i f i e r
consists of an input
s e c t i o n , which generally includes an impedance m a t c h i n g b u f f e r , the
pole-zero a d j u s t m e n t
network and
the
first d i f f e r e n t i a t o r . The
input b u f f e r may not be p r o v i d e d in some s p e c t r o s c o p y a m p l i f i e r s .
The
input
section
is followed by the gain section, w h i c h
usually
consists of
three w i d e b a n d
feedback a m p l i f i e r s , w i t h
provisions for coarse and fine gain settings. A signal w i t h short
r i s e t i m e is usually taken from
the first
a m p l i f i e r of the gain
section and sent to a fast channel. The fast channel shapes the
incoming signal to a narrow w i d t h , a few tens of nanoseconds, and
through a t h r e s h o l d discriminator provides the t r i g g e r i n g
signal
for the PILE-UP REJECTOR.
The fast channel is an auxiliary
signal p a t h which has the
purpose of enabling
the PILE-UP REJECTOR to d e t e c t
couples of
events coming too closely spaced in time. The main signal p a t h
from
the o u t p u t of the GAIN SECTION goes to the shaping section
w h i c h , in most cases, consists of t w o ,
second-order d i f f e r e n t i a tors.
The
signal it provides is unipolar, nearly gaussian in
shape, and its path goes through
the baseline restorer to the
UNIPOLAR
SIGNAL OUTPUT. An a l t e r n a t i v e p a t h , through the second
d i f f e r e n t i a t o r , provides
relevant connector.
(134)
a
bipolar
output,
available
at
the
GAIN SECTION
W;
COARSE GAIN SETTING
FINE GAIN ADJUSTMENT
H09
A
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SECOND
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INTEGRATOR
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RESTORER
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SHAPING SECTION
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DISCR,
PILE-UP
REJECTOR
o
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(l
Chapter 6
-32-
To
t r o u b l e s h o o t a faulty spectroscopy a m p l i f i e r ,
start
a p p l y i n g at the input a lOinV square signal w i t h a 2 K H z s i g n a l and
set on
the r e l e v a n t p a n e l knob a value of the gain around
500.
Connect the o u t p u t of the a m p l i f i e r to an oscilloscope t h r o u g h a 2m
long c a b l e .
In t h i s c o n d i t i o n you should see on
the scope a
sequence of p o s i t i v e and n e g a t i v e g a u s s i a n signals of a b o u t 5V in
amplitude
and
related
in
time to the
LOW-TO-HIGH
and
to
the
HIGH-Tu-LOW
transitions
in the input
square wave.
Use
of a
t w o - c h a n n e l o s c i l l o s c o p e d i s p l a y i n g , in the ALTERNATE p o s i t i o n , the
i n p u t square wave and the o u t p u t gaussian signals, is a p p r o p r i a t e .
If
the signals
are
present
at
the a m p l i f i e r o u t p u t ,
s y n c h r o n i s e the scope on the channel d i s p l a y i n g the o u t p u t signals,
make a single gaussian pulse firmly v i s i b l e on the d i s p l a y , and
p r o c e e d to the following tests.
a.
Switch
the COARSE GAIN knob through all
its a v a i l a b l e
p o s i t i o n s and make sure t h a t
the signal never disappears on
the
scope and t h a t simply its a m p l i t u d e changes according
to
tne knob s e t t i n g . If, for a given p o s i t i o n of the COARSE GAIN
k n o b , the signal d i s a p p e a r s or looks to be b a d l y d e f o r m e d ,
check the c o n d i t i o n of the r e s i s t o r s , and
in t h a t p a r t i c u l a r
position of
the COARSE G A I N k n o b , d e t e r m i n e the gain.
The
r e s i s t i v e network w h i c h d e t e r m i n e s the COARSE GAIN in t h a t
particular
COARSE
even d i s c o n n e c t e d .
GAIN s e t t i n g m i g h t b e b a d l y
soldered,
or
b.
If the COARSE GAIN s e t t i n g reveals no d e f e c t s , keep it fixed
in a g i v e n p o s i t i o n and
r o t a t e the
fine gain p o t e n t i o m e t e r
throughout
its range. Make sure t h a t no
sudden jumps, dc
level
variations,
appear
at
the
preamplifier o u t p u t .
Otherwise,
c o n c e n t r a t e your a t t e n t i o n on
the
fine-gain
p o t e n t i o m e t e r and p o s s i b l y p r o c e e d to change it.
c.
Check w h e t h e r or not all the p o s i t i o n s of the time c o n s t a n t
c o n t r o l l i n g knob p e r f o r m their
f u n c t i o n by leaving the shape
and
a m p l i t u d e unchanged and by s i m p l y m o d i f y i n g the w i d t h
of
the signal.
d.
Check the presence of
the signal
r
r»n it
if- checks
i ^ h p r l r c a),
a *i
H ^ and
a n H c).
r ^ r e*spneoaatf" on
b)
at
the b i p o l a r o u t p u t and
If, i n s t e a d , upon a p p l i c a t i o n of the i n p u t square wave no
signal appears at the o u t p u t , p u l l your a m p l i f i e r out of the NI M
BIN and put it open on the desk by powering it through an e x t e n d i n g
cable. Increase the a m p l i t u d e of
the input square wave to about
l O U m V and follow its p a t h t h r o u g h the gain section by looking w i t h
a Ix p r o b e at the input and
at
the o u t p u t of the subsequent gain
stages.
If the signal is present at the input of a gain stage and
is no longer present at the o u t p u t , stop the signal analysis and
a f t e r s w i t c h i n g off the generator, proceed to dc analysis of
that
p a r t i c u l a r gain stage. Check whether
this is in the linear range
or its o u t p u t is s a t u r a t e d . dc analysis should reveal where the
defect is. Proceed to remove it and only after dc conditions have
returned to satisfactory levels, s w i t c h on the generator again and
carry on signal analysis.
It may easily occur, in a misused
(136)
-33-
Chapter 6
i n s t r u m e n t , t h a t more than one gain or shaping stage is faulty.
Then, p r o c e e d i n g from input towards the o u t p u t , r e p e a t the outlined
p r o c e d u r e for all the d e f e c t i v e stages.
As an example of real
troubleshooting a c t i o n , a specific
a m p l i f i e r , Canberra 2020, w h i c h
is very
frequently used in high
r e s o l u t i o n s p e c t r o s c o p y systems, will be considered.
This a m p l i f i e r is composed of several stages (see Figs. 6.23a,
6.23b, and 6 .23c) :
1.
Input b u f f e r for impedance m a t c h i n g .
2.
First differentiation stage.
3.
First a m p l i f y i n g stage (gain 3-10 adjustable by fine gain
p o t e n t iome t er).
4.
Second a m p l i f y i n g stage
(gain 30
fixed, at the first 3
gain positions amplifier is by-passed).
5.
Third a m p l i f y i n g stage (gain 1-10).
6.
First a c t i v e integrator stage.
7.
Polarity amplifier.
8.
Second a c t i v e integrator stage.
A f t e r this stage, the signal is split along two paths:
a)
Second
d i f f e r e n t i a t i o n and buffer amplifier for
bipolar
out put ; and
b)
O u t p u t buffer stage for unipolar output; it is reached by
e i t h e r direct feed or via delay line; the o u t p u t driver
stage is controlled by the baseline-restorer-circuitry.
The
a m p l i f y i n g stages
1, 2 and 3 are
similar
in their
c o n f i g u r a t i o n and must be made out of d i s c r e t e components, due to
noise rise
time and overload
recovery r e q u i r e m e n t s .
In
the
integrators monolithic operational amplifier circuits can be used
because the pulses are already slowed down.
In the baseline
r e s t o r e r , comparators are used.
The
pile-up
rejector
receives
a
signal
from
the
third
amplifying stage and provides a TTL compatible signal.
For troubleshooting it is necessary to follow the rules listed
be low :
1.
Switch off baseline restorer.
2.
Switch off pile-up rejector.
(137)
Chapter 6
Fig.
(138)
-34-
6.23a:
Spectroscopy a m p l i f i e r circuit diagram
C h a p t er 6
-35-
'9
lïSrV.î'^l'lrijy u ni. !?ïïï*!a
S? «m
Wl.i
* MM
Mîw
5MU?
£
L
? î«1 < *
Fig.
6.23b:
i J
ji
_.
Spectroscopy amplifier circuit diagram
(139)
Chapter 6
-36-
'.-ay^rgfl
3
hrr^ ^^^
Fig. 6.23c:
(140)
Spectroscopy a m p l i f i e r circuit d i a g r a m
-37-
Chapter 6
3.
üo not connect
a BNC-cable to the front
panel connector,
because
sometimes the o u t p u t s are not p r o p e r l y t e r m i n a t e d and
t h e r e f o r e o s c i l l a t i o n m i g h t occur.
4.
Place the input selector switch in the p o s i t i v e position.
Feed a p o s i t i v e signal
from a pulse generator through the
input-plug
to the a m p l i f i e r (pulse r e p e t i t i o n IkHz, pulse w i d t h
a b o u t 100 m s e c . ; to achieve a stable p i c t u r e on the scope, trigger
your
scope
by
the e x t e r n a l t r i g g e r from
the pulse generator).
Follow this signal w i t h an oscilloscope-probe
10:1 stage by stage
and compare the p i c t u r e s between circuit diagram and
oscilloscope.
If at the o u t p u t
of
a stage
there
is a d i s c r e p a n c y , there is
usually
a f a u l t in this stage. S t a r t measuring with the d i g i t a l
v o l t m e t e r the DC levels of the base, e m i t t e r and c o l l e c t o r of the
various
transistors.
A d e f e c t i v e component can be
detected
by
these m e a s u r e m e n t s .
A f t e r the a b o v e - m e n t i o n e d
p r o c e d u r e , an o u t p u t signal of the
r i j; h t shape
should be v i s i b l e at the unipolar as well as
at
the
bipolar o u t p u t .
Now
switch
on the baseline-restorer and compare the signal
with
the previous one. If
its p o s i t i v e going
lobe
is left
unchanged
and only the recovery towards the baseline is a f f e c t e d ,
you can assume that the BLR is working.
Connect the oscilloscope to the o u t p u t , feed a pulse to the
amplifier
and change the gain reducing the a m p l i t u d e of
the test
pulse. In this way the various feedback loops are checked.
The next step is to check, by setting the several
constants, that
the o u t p u t
a m p l i t u d e varies only
range. If t h e r e is a big change in a m p l i t u d e , you can
either the sv;itch gives a bad c o n t a c t or one of the RC
defect ive .
The pile-up rejector
cannot
be
checked
shaping time
over a small
assume
that
networks are
with a normal pulse
generator.
There are two types of pile-up: trailing edge pile-up
and leading edge pile-up. A
test
is possible w i t h a short double
pulse
fed into a RC-network (time constant about 50
usée.).
To
compare the signals w i t h the m a n u a l , the scope m u s t be triggered by
the first pulse; Fig. 6.24 i n d i c a t e s the pulse forming network.
The s e t t i n g s of the double pulse generator w i t h o u t load should
be the following:
1.
2.
3.
ampli tude 5V
pulse w i d t h lusec.
r e p e t i t i o n frequency IkHz
4.
5.
d o u b l e pulse selection
pulse delay variable from 3usec. up to lOOusec.
Check the several signals at the
with the manual.
test points and compare them
(141)
-38-
Chapter 6
from
double pulse
generator
HI
Dl
M
"^
V
5 OR
ouble P ulse jenerator
etting s wit h 3ut load
ci
lOOnF
:B
1O S
s"
to amplifier input
t
5V
lus
Ins
var. 3 - lOOps
Fig.
(142)
6 . 2 4 : Check c i r c u i t , which enables you to check the
pile-up rejector
Chapter 7
DISCRIMINATORS,
SINGLE CHANNEL ANALYZERS,
TIMING CIRCUITS
-1-
Chapter 7
7
DISCRIMINATORS, SINGLE CHANNEL ANALYZERS, TIMING CIRCUITS
7.1
INTRODUCTION
The fundamentals of a m p l i t u d e analysis and time measurement in
nuclear e l e c t r o n i c s have been d e a l t w i t h
in IAEA TECDOC-363 to
which
the
reader
is
referred.
Here we
limit
ourselves
to
a
discussion of commercially available instruments from the v i e w p o i n t
of maintenance and troubleshooting.
Apart
from a few examples, d i s c r i m i n a t o r s are available
just
as part of
single channel analyzers.
The
e x c e p t i o n s generally
apply to i n s t r u m e n t s intended to accept pulses coming d i r e c t l y from
photomultipliers in fast a m p l i t u d e and time m e a s u r e m e n t s . A l t h o u g h
they are not referred to e x p l i c i t l y in what follows, the discussion
is, in broad t e r m s , also r e l e v a n t to them.
To be useful in d i f f e r e n t a p p l i c a t i o n s , the o u t p u t signal from
a single channel analyzer
f r e q u e n t l y contains b o t h a m p l i t u d e and
timing
information.
Circuits r e l a t e d
to each
one
of
these
parameters are discussed below.
7.2
AN EXAMPLE:
EDGE/CROSSOVER TIMING SCA, C A N B E R R A MODEL 2037A
A block diagram of the instrument is shown in Fig. 7.1.
CHANNEL
—«1 CHANNEL
LEVEL
AB
(DISC.
p1A6a
\
SCA
r*
BASELINE
-JjASELINE
LEVEL
-JDISC
A,
Fig.
GATE
A2
t1
\
H
1
OUTPUT
I—« DRIVERS
07-011
|
1
r*|
A6b |
1 TIMING
I INPUT CKT
—«]OHAIO
SYNCHR.
LDGIC
»Ml«***3
TIMN G
DELAY
«1
RESET
»7 ja
7.1: Block diagram of t i m i n g SCA
The
baseline and channel
discriminators compare
the input
analog
signal a m p l i t u d e
to the preset voltage
levels V(E) and
V(E •»• AE). The
o u t p u t of the discriminators is
combined
in
the
block labeled SCA logic to p r o d u c e an o u t p u t if the SCA conditions
are
satisfied (that is, if the baseline d i s c r i m i n a t o r is t r i g g e r e d
and the channel discriminator is not triggered). The output signal
is d e l i v e r e d to the o u t p u t d r i v e r s at an instant d e t e r m i n e d by
the
timing d i s c r i m i n a t o r .
A more
detailed
analysis
of the various
circuit blocks follows, together w i t h troubleshooting guides.
7.2.1
the
Level Setting Circuitry
The comparison level for the baseline
d i s c r i m i n a t o r is set by
circuitry
in a
around
A9, which is wired
voltage
follower
(145)
Chapter 7
-2-
configuration.
From
Fig. 7.2 you
v o l t a g e to A9 (pin 3) comes
from
voltage
c h a n g e s , for example due to
(see C h a p t e r
5, Power
Supplies),
change .
can
the
load
the
see then
that
the input
NIM +24V s u p p l y ; if this
changes in the NIM
crate
output
of A9 is bound to
To
check w h e t h e r the circuit is working p r o p e r l y , verify
if
the o u t p u t v o l t a g e (pin 6) is equal to the input voltage (pin 3) at
the
e x t r e m e positions
of
the RV8
helipot
range;
the
trim
p o t e n t i o m e t e r RV5 should be adjusted
so t h a t at the lowest v o l t a g e
in
the range, the o u t p u t v o l t a g e equals the v o l t a g e at pin
3; at
the upper end of the range, RV6
should be adjusted so that the
output
v o l t a g e of A9
is + 5V.
If the circuit
is not working
properly,
follow
the
procedures
described
in
C h a p t e r 6,
(Preamplifiers, Amplifiers) to identify the fault.
The c h a n n e l
(window) c o m p a r i s o n
level is established by A8,
which is w i r e d as an a m p l i f i e r of gain
2; the gain may be changed
t h r o u g h the a d j u s t m e n t of
RV2
by jh2%. Trim pot RVl adjusts the
offset voltage
as in the c i r c u i t around A9.
The v o l t a g e at the
n o n - i n v e r t ing input
(pin 3)
is very
approximately
given by
V(E) + V(E + A E ) ; thus at
the o u t p u t
of A9
the
voltage
is
2
V(E) + V(E + A E ) . You can
see
that
the c i r c u i t
around AS allows one to move the channel up and down (by adjusting
the baseline h e l i p o t ) w i t h o u t changing its w i d t h . To
check
the
c i r c u i t f i r s t set the baseline and
channel h e l i p o t s to zero; check
if
the
i n p u t (pin 3) and the o u t p u t are at the same
voltage;
if
n o t , adjust R V l . Then turn the baseline h e l i p o t
full scale (the
o u t p u t of A9 should have +5V); you should measure +5V on the o u t p u t
of A8,
o t h e r w i s e adjust RV2. N e x t , turn the channel
helipot
to
full scale and the E range switch S2
to the 10V p o s i t i o n ; with the
b a s e l i n e h e l i p o t at zero, you should read 5V at the o u t p u t of A8 ,
o t h e r w i s e a d j u s t RV3 . N e x t , w i t h
the same s e t t i n g s but w i t h the
range s w i t c h S2 in the IV p o s i t i o n , you should
observe 0,5V at
the
o u t p u t of A8 , otherwise adjust RV4 .
The
above m e a s u r e m e n t s should be done
carefully
and
the
corresponding
a d j u s t m e n t s , if necessary,
should
be m a d e in the
stated order.
We now refer to the level s e t t i n g for the timing d i s c r i m i n a t o r
(All).
This level is derived from the baseline voltage follower;
R16 connects the o u t p u t of A9 to
the base of e m i t t e r follower Q17
and
to the "ideal diode" made up around A10.
The
non-invert ing
input of AlO is at about 200mV; pin 2 of A10 should have the same
voltage
if the f e e d b a c k loop around the a m p l i f i e r is closed. This
will be so if diode D3 is c o n d u c t i n g , a c o n d i t i o n that is f u l f i l l e d
whenever
the o u t p u t of A9 is above the value imposed at pin 3 of
AlO by the R 1 4 , R15 d i v i d e r
network
(= 200mV). You can check for
the
correct b e h a v i o u r of this ideal diode by looking at the anode
of D3 while the baseline h e l i p o t is
increased from zero; while the
output
of A9 is below 200mV, AlO is s a t u r a t e d
(output
positive);
above this value AlO e n t e r s
the active region and the voltage at
the anode of U3 is c l a m p e d to the 200mV value. Emitter follower
Q17 drives this voltage
to the timing comparator
input
if the
leading
edge
triggering mode
is
selected.
However,
if the
crossover mode is s e l e c t e d , this
level
s e t t i n g c i r c u i t r y is not
used; the comparison
level is then set at a value equal to the
(146)
n
Fig.
7.2: C i r c u i t d i a g r a m of a TSCA.
n
~*i
Chapter 7
-4-
average v o l t a g e of the input
line
(that
is, at the mean value of
the o u t p u t of the a m p l i f i e r connected to the present
instrument),
see Fig. 7.3.
7.2.2
Analog Input and Discriminator C i r c u i t s
The i n p u t analog signal is divided
by 2 before it is applied
to
the c o m p a r a t o r s A6a (channel) and A6b (baseline). The input is
dc c o n n e c t e d , and a first
check
can be made
by applying a dc
v o l t a g e to the i n p u t . For e x a m p l e , if the baseline h e l i p o t is set
at half-scale p o s i t i o n , the c o m p a r a t o r
should t r i p when the input
v o l t a g e is a p p r o x i m a t e l y 5V.
The
hysteresis of the c o m p a r a t o r is fixed at roughly 5mV by
the r e s i s t o r network R26-R9. This may be checked by v e r i f y i n g that
the v o l t a g e at pin 9 of A6b takes values d i f f e r i n g 5mV from each
o t h e r a c c o r d i n g to w h e t h e r the o u t p u t of the c o m p a r a t o r is at logic
0 (^ UV) or at logic 1 (- 5V).
The
comparator inputs are p r o t e c t e d by diodes Dl and D2,
and
also by the c i r c u i t
around Q 1 5 , which does not
allow the input
d i f f e r e n t i a l voltage to exceed the l i m i t s specified for the LM319
(jf 5V). The
protection circuit
should
be checked if the input
analog signal fails to appear at the comparator.
The
analog signal is a p p l i e d at the inverting terminal of the
t i m i n g c o m p a r a t o r All t h r o u g h e m i t t e r
follower Q13; Q16 works as a
10mA
c u r r e n t source. The c i r c u i t up to the All i n p u t s , when
the
crossover m o d e is s e l e c t e d , is shown
in Fig. 7.3.
N o t e that this
mode of
o p e r a t i o n may be s e l e c t e d only if the
input
signal
is
bipolar .
6MB
Fig. 7.3: Timing d i s c r i m i n a t o r input c i r c u i t in crossover
mode
In this m o d e of
o p e r a t i o n , the input
analog signal and the
comparison v o l t a g e level are b o t h derived from the e m i t t e r of Q 1 3 ;
the comparison level is o b t a i n e d
by
f i l t e r i n g the signal out with
capacitor
C18.
Thus the c o m p a r i s o n v o l t a g e follows eventual slow
f l u c t u a t i o n s of
the analog
input
baseline v o l t a g e , as may be
v e r i f i e d by a p p l y i n g a dc v o l t a g e at the input. N o t e t h a t the
1mA
c u r r e n t sink Q20 allows the d i f f e r e n t i a l dc level at the All inputs
to be v a r i e d by l O O m V t h r o u g h a d j u s t m e n t of RV10; this is easily
checked w i t h a m u l t i m e t e r if no
input
signals are p r e s e n t . This
a d j u s t m e n t is i n t e n d e d to m i n i m i z e the time walk; this p a r a m e t e r is
d i s c u s s e d in d e t a i l in IAEA TECDOC-363.
(148)
-5-
Chapter 7
The working c o n d i t i o n of the comparators is easily checked. In
the
absence
of
input
pulses, the output of
the
baseline
d i s c r i m i n a t o r (pin 7 of A6) should be at logic 1, and the o u t p u t of
the
channel discriminator should be at logic 0.
With a scope
it
should be observed that the o u t p u t s of the comparators go to the
complementary
logic
state while the analog
signal exceeds
the
respective comparison voltages.
7.2.3
The SCA Logic and Timing Circuits
The SCA logic is mainly based
on RS
latches made with gates
i n c l u d e d in Al, A3, A5, and on gate A2b. You may quickly perform a
first check on these and the other
latches^ present in the circuit
by
just
observing if their o u t p u t s Q and Q are c o m p l e m e n t a r y , as
they should be. The
logic behaves
in a somewhat d i f f e r e n t way
according
to whether the leading edge or the crossover mode
is
selected. We first refer to the leading edge mode operation, which
may be checked as follows.
A p p l y pulses of about 5V a m p l i t u d e and with a shape similar to
lus amplifier pulses. Set the baseline helipot to a level of about
4V, and the channel helipot
to a channel w i d t h of 2V; only the
baseline discriminator should be t r i g g e r e d by the input pulse. Look
at the timing d i s c r i m i n a t o r All o u t p u t
(pin 5); observe that it
changes before the baseline discriminator output, because it is set
to a trigger level of around 0,2V while
the baseline d i s c r i m i n a t o r
level is set at about 4V.
Note that for All to be t r i g g e r e d , it is
necessary that
pin 4 be at logic
1; the state at pin 4 is
controlled by l a t c h A4 pin 6.
You may now observe the actions initiated by the output pulse
of A l l . Through
inverter Q19
it triggers monostable A 5 b ; the
c o m p l e m e n t e d , roughly O.lus wide o u t p u t pulse of this monostable
(pin
12) acts as
a
reset
pulse;
in
particular,
it
resets the
baseline latch (A3a,b). This may be observed at pin 6 of A3 which
changes from logic 1 to 0; soon a f t e r , it is again set to 1 by the
baseline d i s c r i m i n a t o r
signal.
Note that the discriminator
is
reset at the b e g i n n i n g of the analysis cycle, not at the end. The
monostable
also
resets
latch A4a,b disabling
the
timing
d i s c r i m i n a t o r o u t p u t ; pin 6 of A4
then goes from logic 1 to 0,
which causes the delay monostable A7 to be triggered. At the end
of the delay p e r i o d , 0.5us monostable A5a
is
triggered and its
o u t p u t pulse does two jobs: it t r i g g e r s the SCA o u t p u t c i r c u i t s if
pins 4 and 5 of A2b are
at
logic
1 (as
they should be if the
baseline
latch was
set
by the baseline d i s c r i m i n a t o r and
the
channel discriminator has not been triggered); and it sets latch
A4a,b, enabling All for the next
analysis cycle. You should also
test
the c i r c u i t
response when the channel discriminator
is
t r i g g e r e d ; in this case pin 4 of gate A2b should be at logic 0 and
no SCA o u t p u t pulse will be generated by the A5a monostable pulse.
The checking procedures just described are similar to the ones
that may be a p p l i e d to check the logic when the crossover mode
is
selected. In t h i s m o d e , the o u t p u t of All (pin 5) is at logic 1
when no input pulse is present. N o t e , in p a r t i c u l a r , that the SCA
o u t p u t signal is always synchronized with the timing d i s c r i m i n a t o r
o u t p u t , and
t h a t it is delayed by A7 for a preset
time interval
r e l a t i v e to the All triggering time.
(149)
Chapter 7
7.2.4
-6The O u t p u t Circuitry
The output pulses from this unit are intended to actuate the
inputs of coincidence plug-in units or of time to a m p l i t u d e
converters.
The
output
circuitry
should
be able
to drive
terminated 50fl cable; discrete circuits are used here for this
purpose. You should check if the positive-going pulse at the
output
connector J2 complies with TTL standards.
A c t u a l l y , from
the circuit you
should e x p e c t
logic 0 to be less than 0,2V for
currents as large as 50mA (Q7 and Q9 are s a t u r a t e d , Q8 is cut off),
and logic 1 to be higher
than 2.5V for c u r r e n t s larger than 50mA
(Q7 and Q9 are cut o f f , Q8 acts as an emitter-follower).
The circuit for
the negative-going NIM-pulse should sink a
current of roughly 17mA; if this current flows through a 50-Ohm
resistor, a signal of -0,8V develops; this signal is rather narrow
due
to the d i f f e r e n t i a t i n g time constant of a b o u t
20ns at
the
circuit's input. If the discrete o u t p u t circuits are suspected of
malfunctioning,
the
t e c h n i q u e s given in Chapter 6 for
the
t r o u b l e s h o o t i n g of transistor c i r c u i t s may be followed.
7.2.5
Constant Fraction Timing Discriminator
Another common m e t h o d of obtaining accurate time marks, with a
small a m p l i t u d e d e p e n d e n t
time walk, is
the constant
fraction
method. A c i r c u i t example may be taken from Canberra's Model 2035A
constant
fraction
timing SCA, see
Fig. 7.4. The
plug-in unit
differs from the one
previously
discussed e s s e n t i a l l y in the All
input c i r c u i t s .
To check the circuit observe the input pulse at the e m i t t e r of
Q13;
at test point TP6, which is the negative input of A l l ,
the
pulse occurs at the same time than at the emitter of Q13 and with
approximately
half
amplitude.
At test point T P 7 , which
is
the
positive input of A l l ,
the pulse
is delayed
by 0.5, 2 or lOus
according
to the delay line s e l e c t e d , and its a m p l i t u d e is reduced
to about 1/3. The All timing comparator responds
to the voltage
difference between these
two pulses, changing s t a t e when the
difference crosses zero. As b e f o r e , it may be seen that RV10 is a
walk adjustment p o t e n t i o m e t e r .
Note that when no input pulse is
p r e s e n t , the o u t p u t of All (pin 5) is at logic 1.
(150)
o
S"
rt
(B
Fig.
7.4: Constant fraction timing c i r c u i t
Chapter 8
SCALERS, TIMERS, RATEMETERS
-1-
Chapter 8
SCALERS. TIMERS. RATEMETERS
8. 1
TYPICAL STRUCTURES OF SCALERS. TIMERS AND RATEMETERS
To be able to service and repair sealers, timers and digital
ratemeters, it is essential to correctly identify the d i f f e r e n t
blocks in which the instrument may be logically d i v i d e d , and to
u n d e r s t a n d their interactions. To help in this procedure, a short
d e s c r i p t i o n of typical s t r u c t u r e s is given below.
A detailed
d e s c r i p t i o n of
examples of
commercially
available
instruments
completes this c h a p t e r .
8.1.1
Introduct ion
The general block diagram of a t y p i c a l counter
count c a p a b i l i t i e s is shown in Fig. 8.1.
INPUT
GATE
COUNTING
STAGES
with
preset
DISPLAY
PRESET
CIRCUITRY
Fig.
8.1: Block diagram of a typical counter
The
signal to be counted can, in general, be easily
followed
through its path in the counter, and
d e f e c t i v e ICs can be located
without
too much d i f f i c u l t y . C i r c u i t r y associated with preset
count and gating is generally more d i f f i c u l t to troubleshoot
because of interactions among several c o n t r o l signals.
As an example of faults which are hard to f i n d , we may quote
the spurious counts due to bad filtering by a defective capacitor.
8.1.2
Input circuits
Most input circuits accept NIM logic signals, positive and/or
negative. Some counters have discriminat or-type inputs, allowing
analog signals of small amplitude to be counted.
One must check whether the input circuit passes the signals
for counting in a proper way. In p a r t i c u l a r , this involves
checking for t r i g g e r i n g at the a p p r o p r i a t e levels, and for m u l t i p l e
triggering due to cable reflections or other causes. NIM positive
signals are frequently received at a Ik-Ohm impedance; significant
reflections are absent provided
the cables are not too long (and
the signal is not too fast). NIM n e g a t i v e signals must be received
(155)
Chapter 8
-2-
in an i m p e d a n c e t h a t closely m a t c h e s
the cable. F r e q u e n t l y , this
impedance
is m a d e
up of a resistor in series with
the input
impedance of a common base
transistor; a significant impedance
m i s m a t c h may i n d i c a t e a d e f e c t i v e transistor.
D i s c r i m i n a t o r - t y p e input circuits most f r e q u e n t l y use the 710
or o t h e r fast d i s c r i m i n a t o r . Good power s u p p l y d e c o u p l i n g near the
d i s c r i m i n a t o r , and a not too small hysteresis, are needed to avoid
multiple triggering.
8.1.3
O u t p u t Circuits
Output
circuits
are
prone
to be damaged
by
improper
connections. Most
f r e q u e n t l y , however, they are just unable to
cope w i t h the demands put on t h e m . For e x a m p l e , a TTL g a t e , or a
CMOS d r i v e r , have a driving c a p a b i l i t y that can easily be e x c e e d e d .
To
d r i v e a t e r m i n a t e d 50-Ohm cable, one
needs a transistorized
o u t p u t stage or a special
integrated circuit.
A check should be
made to verify whether legal logic levels are b e i n g delivered.
O u t p u t c o n n e c t i o n to p r i n t e r s or e q u i p m e n t buses is
eIsewhere.
8.1.4
addressed
C o u n t i n g Gates
Counting
gates are open/closed manually or by signal-levels
and/or pulses. Debouncing of manual switches is m a n d a t o r y ; the
corresponding
c i r c u i t r y is easily checked and the same is true for
the latches a s s o c i a t e d w i t h pulse control. These b i s t a b l e elements
are
easily t r i g g e r e d by very short pulses; this may be a cause of
t r o u b l e if d e c o u p l i n g is i n e f f e c t i v e , or if g l i t c h e s are allowed to
occur and are not f i l t e r e d out.
8.1.5
The Counting Stages
The
counter generally provides several counting decades whose
o u t p u t is a v a i l a b l e in BCD
format.
N o r m a l l y , only the first
(faster) d e c a d e , or decades d i r e c t l y associated with some types of
preset c o n t r o l , are of the synchronous type. F r e q u e n t l y , ICs w i t h
two d e c a d e s are u s e d , but LSI c i r c u i t s w i t h four or six d e c a d e s are
already p r e s e n t in various i n s t r u m e n t s .
Preset count
is f r e q u e n t l y limited to the c o m b i n a t i o n p q x l O r
where p.q and r represent decimal numbers. Generally it involves
e i t h e r a comparison of the BCD-coded numbers p and q with
the
contents of the s e a l e r , or
the zero d e t e c t i o n in a count-down
process of
two decades p r e v i o u s l y loaded with contents p and
q.
Troubles may easily arise
from
g l i t c h e s or spikes
in the power
lines.
(156)
Chapter 8
-38.1.6
Display Circuits
Seven-segment
digits,
implemented in LED or LCD arrays,
generally used in displays.
In LED
are typical, one from the decades to the BCD-to-7 segment
another from the
decoder
to
the
are
displays two multiplexed buses
digits.
In
decoder,
LCD displays, all
digits should be permanently a c t i v a t e d ; circuits including a
latch and a converter for each digit are generally used.
4-bit
Scanning
c i r c u i t s are typically made out of a binary
counter
and a decoder (for example, an LS93
and an LS138) whose o u t p u t
lines
successively
activate
the open collector
or
tri-state
circuits that connect
the
various
decades to the b u s , and select
the
appropriate
digit.
Some LSI c i r c u i t s , like the
7301, already include the scanning circuits.
8.1.7
six
decade
Time Generators
In
timers or counter/timers, time marks are derived from
the
mains
(typically
at
1/100
or
1/120 second
period) or, most
frequently,
from a crystal oscillator. These generally
oscillate
at a f r e q u e n c y of several MHz, which
is suitably d i v i d e d to give
time marks either in a seconds or in a minutes scale. In preset
type count er/1imers, the preset count can be associated with either
the counting events or the time marks.
Rat erne t er s
8.1.8
A typical d i g i t a l ratemeter has a structure similar to that of
a preset count er/1imer.
preset
The
integration
period
corresponds to a
t i m e ; the displayed count, corresponding to the total count
of the previous period, is u p d a t e d once per period.
Analog ratemeters typically shape the input pulses in a
we 11-de f ined
form,
and
integrate them to
have a
voltage
to
the
incoming
rate
in
either
a
linear
or
in
a log
proport ional
scale.
8.2
TROUBLESHOOTING
The
first step in troubleshooting is to establish
diagram of the
blocks.
This
the
block
instrument
and
to identify
the faulty block or
is generally
easy for an absolute
(no
signal)
failure, but may be rather tricky for the faults related to
d e g r a d e d performance (spurious counts, erroneous preset counting,
etc . ) .
8.2.1
the
Checks at the Component Level
First check whether power arrives at the appropriate pin of
1C.
The
t r u t h table of gates must be seen to be followed,
otherwise the component is
defective.
A check should be made to
verify
if the voltage levels are well w i t h i n the legal l i m i t s
for
(157)
Chapter 8
-4-
the i n p u t s and the o u t p u t s ; t h i s a p p l i e s to a^l 1 d i g i t a l c i r c u i t s .
In latches and f l i p - f l o p s , check if the Q and Q o u t p u t s are in fact
c o m p l e m e n t a r y ; in general, check
the t r u t h table of the component.
If the c o m p o n e n t is supposed to b e h a v e in a dynamic w a y , like a LSI
counter with m u l t i p l e x e d o u t p u t , verify whether the scanning and
r e l a t e d signals are being sent o u t .
Analog
c o m p a r a t o r s and
t r a n s i s t o r s are
sometimes
found,
e s p e c i a l l y in input
and
output
circuitry.
Measurement
of
the
v o l t a g e levels at the input and o u t p u t will generally show whether
a c o m p a r a t o r is working p r o p e r l y .
Transistors are easily checked
as
discussed elsewhere; f r e q u e n t l y they will be either on or o f f ,
and the v o l t a g e levels they give
to the following c i r c u i t s should
be seen to comply with the appropriate legal levels under the load
c o n d i t i o n s in w h i c h they are e x p e c t e d to o p e r a t e .
D i s p l a y s may easily give
short
trouble.
F r e q u e n t l y this is due to
c i r c u i t s or m i s e o n n e c t ions in LED d i s p l a y s , and to
improper
contact
in
the c o n d u c t i n g p a t h
(usually
at
the
rubber/glass
junction) of LCD d i s p l a y s . These are also prone to m a l f u n c t i o n
if
excess h u m i d i t y p r o v i d e s leaky p a t h s for the segment v o l t a g e s .
Faulty c a p a c i t o r s may
be
responsible for a t o t a l failure if
they are
s h o r t - c i r c u i t e d , or just to d e g r a d e p e r f o r m a n c e of
the
i n s t r u m e n t if they do not
a d e q u a t e l y by-pass spikes or g l i t c h e s to
ground (a much more d i f f i c u l t s i t u a t i o n to t roubleshoot ) .
8.2.2
Checks at the Block Level
The
input block must be seen to give a single pulse for each
pulse r e c e i v e d , even if the pulse has a reasonable amount of noise;
a pulse generator will h e l p to d e t e r m i n e the t r i g g e r i n g
threshold.
If the input
is supposed
to work
on analog signals, m u l t i p l e
signals; a sine
triggering
is more
likely to occur w i t h slower
to ground by a d i o d e , is
wave, with the n e g a t i v e half wave c l i p p e d
a suitable test signal. If m u l t i p l e t r i g g e r i n g occurs (with
the
t h r e s h o l d well above noise level), the h y s t e r e s i s circuit of the
c o m p a r a t o r must be checked.
Pulse p a i r r e s o l u t i o n is d e t e r m i n e d e i t h e r by the input
block
or by the first
counting circuit.
If otherwise unavailable, a
simple
test instrument can be b u i l t around two 74LS221 i n t e g r a t e d
monostables and an OR g a t e , as shown in Fig. 8.2.
100nS
500 nS
*••»
— »»
Fig. 8.2:
(158)
1
llOOnS
——
Hr
Double pulse generator with adjustable separation
between pulses
Chapter 8
-5-
The f u n c t i o n i n g of the counting circuits can be easily checked
if they
consist
of
individually
accessible
decades or
binary
counters. When one is dealing with LSI c o u n t e r s , where the output
lines are t i m e - s h a r e d by several d e c a d e s , the functioning of each
d e c a d e may be observed by externally t r i g g e r i n g a scope with
the
scanning clock signal.
Preset count
circuitry may be
checked starting from the
storage of
preset
i n f o r m a t i o n , followed by observation of
the
c o u n t - d o w n , or by examination of the comparator c i r c u i t s that test
when preset count is r e a c h e d , according to the approach used in the
i n s t r u m e n t to implement the preset function.
8.3
TROUBLESHOOTING EXAMPLE:
TIMER/ SCALER, CANBERRA. MODEL 2070
A block diagram of this t i m e r / c o u n t e r is shown
in Fig. 8.3;
some s e m i c o n d u c t o r components
(not all)
are i n d i c a t e d to help in
identifying the blocks.
PRESET COUNTER
GATE CONTROL
INPUT
EVENT GATE
Q6,Q9,Q10,A56C
EVENT/TIMEMUX
A45-46.A52
«—
—> D20, D21
A42-43,A51
A31.A41, A6C
à i.
TIME BASE
TIME GATE
A33-36, A15C
A46b
Fig.
8.3.1
8.3:
START/ STOP
4 ——
SINGLE/RECYCLE * ———————————.
4 ————————
A15a,A14,A3
A24.A41
Block diagram of Canberra 2070 timer/counter
Input and Event Gate
The dc voltage at
the input is approximately 0V for it is
by Q9 whose base is at about 0,7V, as set by
the diode
connected transistor Q6.
NIM positive logic pulses (actually, any
p o s i t i v e pulse larger than about 0,7V) saturate Q10 and cut off Q9;
NIM negat ive pulses do not
a f f e c t Q10, but draw enough current
t h r o u g h Q9 for a logic 0 voltage to be d e v e l o p e d at the
input
of
NAND g a t e A56c.
Input pulses should appear at pin 8 of A56 if pin
9 is at logical 1; this level should appear if PRESET COUNTS mode
is chosen (if n o t , check A 1 5 c for logic 0 at pin 10 and A52 for
logic 1 a t pin 2).
imposed
8.3.2
Time Generator and Time Mark Gate
Check the output of the 12 MHz generator and the divide by 12
1C (A33); the o u t p u t of A33 is t r a n s m i t t e d to the ck input
(pin 5)
of A35 if gate A 1 3 d is
opened;
this
should occur if the external
GATE
input
is left u n c o n n e c t e d , and should stop if this input
is
g r o u n d e d . The 1 MHz o u t p u t of A33 is d i v i d e d by 10 by A35, giving
(159)
Chapter 8
-6-
time marks spaced by 0,01s; A34
provides a division by 60,
producing
time marks separated by 0,01 min. A36 a and b, together
w i t h A46c, form a 2-to-l m u l t i p l e x e r
allowing the selection of the
time scale by a switch. In TP8, a time mark wave should be seen of
either 0.01 min or 0.01s (in the PRESET 0.01s or COUNTS position).
A46b is the time base g a t e ; the control input of this NOR gate (pin
6) is c o n n e c t e d to the same line
as
the control input of the NAND
event
gate (pin 10 of A56), thus assuring that one, but only one,
of these gates is always open.
8.3.3
Gate C o n t r o l and Event/Time Multiplexer
The e x t e r n a l GATE signal actuates
through NOR gate A45c. If
the gate
is closed (logic 1 at pin 8), no time pulse will pass;
again, the o u t p u t (pin 10) will
force m u l t i p l e x i n g d i o d e D20 into
c o n d u c t i o n , therefore
fixing
the o u t p u t of the MUX
(the common
p o i n t of D20 and D21) to logic 0,
and
thus
also disabling the
counting
of event pulses. If jumper A is moved to the B position,
the e x t e r n a l gate signal cannot disable counting.
The external gate also actuates on the time generator by
closing A 1 3 d (provided jumper 5 is in the appropriate position);
t h u s , time c o u n t i n g can be controlled in Ips intervals.
Note t h a t A45c is a c t u a t e d
through
A45b, being open when the
output
of A45b is at logic 1 ; this requires pin 5 of A45b to be at
logic 0 - a c o n d i t i o n that is satisfied as discussed below.
8.3.4
S t a r t / S t o p and Enable C i r c u i t r y
The s t a r t signal, from
the START input or
from the manual
s w i t c h , sets f l i p - f l o p A3.
While the flip-flop is s e t , counting is
e n a b l e d , e x c e p t for an initial 10;is dead time period set by the A24
monostable
to reset the A51 six decade counter. To allow a reset
to follow q u i c k l y , the set
signal should
be narrow; this is
accomplished
in
the manual starting mode by keeping pin 2 of A13
only m o m e n t a r i l y at logic 0- only while R15 charges up C5.
Through
the
output
of
A 1 3 a , the start
signal
triggers
the
lOus
A24
monostable to clear the A51 c o u n t e r and to enable the parallel load
of
the A42 and A43 down counters. N o t e that a lOps dead time
is
thus i n t r o d u c e d in the c o u n t i n g time.
The o u t p u t of the A3 f l i p - f l o p is ANDed with the preset
count
signal (output of A6c) in A 1 4 d , and wired-OR through this gate with
the e x t e r n a l
ENABLE signal to control counting gate A45c
through
A45b. Note that the ENABLE port may be used as an o u t p u t driven by
open collector gate A l 4 d .
The
s t a r t signal line at the o u t p u t of A14d is also used
to
keep the A35 and A34 time generator counters reset while the line
is at logic 1 (counting not started); simultaneously, it also keeps
the parallel load of the down counters enabled.
(160)
-7-
Chapter 8
A2e and d form a falling edge triggered monostable that starts
a counting
cycle t h r o u g h A13a if D2 is moved to the Z position.
This is necessary if one wants
to allow counting to s t a r t at the
falling edge of the external ENABLE signal.
The
stop signal, either from the manual switch or the preset
count o u t p u t , resets the A3
flip-flop
t h r o u g h the A15a g a t e ; t h u s
counting
is disabled. When pin 11 of A14 goes to logic 1, the 10s
monostable
implemented
in one-half
of A24
is a c t i v a t e d if the
RECYCLE mode
is s e l e c t e d . A f t e r the 10s d e l a y , the 10 us A24
monostable is t r i g g e r e d
to clear
the A51 sealer and to parallel
load
the A42
and A43 c o u n t e r s , while the A3
flip-flop
is set
through A46d and A 1 3 a . This keeps the counter recycling w i t h a 10s
interval from the end of one counting period to the beginning of
another.
8.3.5
The C o u n t i n g and P r e s e t C i r c u i t
The
p r e s e t count value takes the form p q x l O r where r is
set
to a value in the range 0 to 6, and pq is a t w o - d i g i t number taking
values
from 0 to 99.
This information is loaded
in the down
c o u n t e r s A42 (units) and A43
(tens).
The
power
r is used to
control
the o u t p u t of the six d e c a d e c o u n t e r A 5 1 ; if r = 0,
the
o u t p u t has the same number of pulses as the i n p u t ; in general, the
o u t p u t c o r r e s p o n d s to a division by 10 r.
Pulses
for
the down counters come from the o u t p u t
of
the
d e c a d e d i v i d e r A51 t h r o u g h g a t e A 4 1 d . This g a t e is enabled by the
output
of A 4 1 b while the p r e s e t count is not r e a c h e d .
When
the
preset value is reached, the
output
of
A41 becomes 1 and t h r o u g h
A28d
and A6 stops the counting process. C a p a c i t o r C3 f i l t e r s out
e v e n t u a l n e g a t i v e spikes that may
t r i g g e r the following c i r c u i t s .
Gate A28d may
be closed by A 3 1 , allowing the counter to be
open
i n d e f i n i t e l y ; A 3 1 , an 8-input NOR
g a t e , closes A28d if the preset
value to the down c o u n t e r s is zero.
8.4.
TROUBLSHOOTING EXAMPLE;
C A N B E R R A MODEL 2071
LCD DISPLAY OF DUAL COUNTER/TIMER,
Here we just refer to the d i s p l a y section of this i n s t r u m e n t ,
the block diagram (channel A) of which is presented in Fig. 8.4.
The 6-digit display can e i t h e r show the c o n t e n t s of decades D5
to DO, or of d e c a d e s D7 to D2, a c c o r d i n g to w h e t h e r the c o n t e n t s of
the most s i g n i f i c a n t decades (D7, D6) is zero or non-zero; this is
i m p l e m e n t e d by
the scaling block.
The
c o n t e n t s of the various
decades
is
transferred
t h r o u g h a bus to the l a t c h - d e c o d e r
ICs
associated w i t h each d i g i t ; the latches are successively enabled by
the
strobe
signals
from A8, a counter that moves w i t h
the same
clock (A32) t h a t a c t u a t e s the scanning c i r c u i t s included in the LSI
counter A54. The scan reset input of the LSI (pin 1) is used
to
assure the synchronism necessary for each digit to display the
corresponding
decade.
The
scan reset input signal can be made
synchronous w i t h the signal
strobe
1, in which case the display
(161)
Chapter 8
-8-
shows decades 05 Co DO; or with a signal that appears two scanning
clock pulses before strobe 1, in which case the display shows
decades D7 to D2. This scale selection is made through multiplexer
A38, the address inputs of which are driven by the a p p r o p r i a t e
signal
from
the LSI
sealers of channels A and B (through a
flip-flop and a gate as shown in the block diagram).
Fig. 8.4: Block diagram of the display section of dual
counter (Canberra Model 2071)
A short guide to troubleshooting the main
display section of the instrument is given below.
8.4.1
points
of
the
Scanning Oscillator and Counter, Scaling Circuit
Check for the oscillator signal at the clock input of counter
A8 (pin 14); if it is not t h e r e , check at the output of A32 (pin 3)
where a r e c t a n g u l a r wave w i t h the high state twice the w i d t h of the
low
s t a t e should be seen. If the oscillator is working and no
s ignal is present at the clock input of A8, go back to the
oscillator through gates A l 5 a and A12 (disconnect the daisy chain
option if necessary).
W i t h the clock signal on, observe the
outputs of A8; the o u t p u t s of A6 should be in the same state as the
0 and 2 o u t p u t lines of A8.
Observe the scanning reset of channel
A (SCNRA signal at pin 13 of A38;check that it is synchronous with
the strobe 1 signal (pin 3 of A8) if at least one of the decades D7
and D6 has non-zero c o n t e n t s , and that it is synchronous with pin 2
of A8 otherwise. If this is not the case, m u l t i p l e x e r A38 must be
from
checked
a f t e r v e r i f y i n g that it receives the correct signal
flip-flop A29 (through gate A36a)
8.4.2
Channel Data M u l t i p l e x e r , Latches/Decades
The BCD-coded data from
the decades of channels A and B is
multiplexed by A37 to a bus connected to all the latch/decoder
circuits. These are to be checked, including the operation with
the phase signal (pin 6 of the latch/decoders) generated
through
(162)
Chapter 8
-9-
A23. The phase signal should be out of phase with the back-plane
signal (pin 3 of A18) whenever asegment is to be ON, and in phase
if it is to be off. If a segment is not ON while it is seen out of
phase with the back-plane signal it is possible that there is a
misconnect ion, or a ground
p a t h , to inhibit the display of the
segment; most
often this occurs at the conducting rubber
contact
with the crystal. Otherwise, the display is defective.
8.5
TROUBLESHOOTING EXAMPLE;
CANBERRA MODEL 2081
DIGITAL LINEAR RATEMETER,
In this instrument the input signals are initially d i v i d e d
by
2 in a f l i p - f l o p , and further d i v i d e d by a series of d e c a d e
counters according to the range selected. It is the rate of
the
o u t p u t signal from this block t h a t is determined by the instrument.
We
just
refer to the p a r t of the instrument responsible for
this
d e t e r m i n a t i o n , for
the other
one
is
just
another example of a
counter.
The block diagram of this part is shown in F i g . 8.5; a
short d e s c r i p t i o n of its operation follows.
EVENT
B-BIT
LATCH
6-BIT
r-»
COUNTEF
RESET __ t A8-9
f LIP -FLO»
TIME MARK
A19a
A6
ADDER
AVERAGING
D AC
CIRCUIT
r
A1-2
L*
A3
AS
8-BIT
R 20
LATCH
CI ,C2
A7
A18 Cyd
Fig. 8.5: Block diagram of part of d i g i t a l linear r a t e m e t e r
Event s are
counted in the 6-bit counter during the interval
between time marks (0.5s, or 5s in the lowest rate scale). At each
t ime
mark , the contents of the counter is t r a n s f e r r e d to e i t h e r
latch A or la tch B, according to the state of the flip-flop; after
transfer the counter is reset and a new cycle starts. The contents
of the two la tches, corresponding to successive time i n t e r v a l s , are
added and the result is applied to the digital-to-analog converter.
The o u t p u t of the converter is read
by a meter either directly or,
in the lower ranges, after averaging.
8.5.1
C o u n t e r , Latches and A d d e r
Start
by checking if the event signal is present at pin 5
of
4-bit counter A8.
Reset pulses must be seen at pin 14 of A8, and
pins 1 and 13 of 2-bit counter A9 (A8 and A9 form a 6-bit counter);
these reset pulses are separated
by 0.5s (5s in the 10 counts/sec
range), and care must be taken if they are to be observed in a
scope. Observe also the clock inputs of the A6 and A7 latches (pin
1):
they must
be complementary and change state at every
time
mark .
(163)
Chapter 8
-10-
If the above signals are
c o r r e c t , check the behaviour of the
A8 binary
counter and of the A9 dual D flip-flop that works as a
2-bit
binary
counter.
Observe
if
the latches A6
and A7 are
c o r r e c t l y l o a d e d ; if t h i s is felt to be d i f f i c u l t , observe a bit at
a t i m e or keep the rate constant and
verify
that the two latches
have
the same c o n t e n t s . The working c o n d i t i o n of
the adder
is
easily v e r i f i e d .
[NOTE t h a t
the
connections shown in the
s c h e m a t i c s from l a t c h A6 to the adders Al, A2 are wrong. ]
8.5.2
PAC and Averaging C i r c u i t
Start
by keeping a constant input to the DAC and
check
the
o p e r a t i o n a l a m p l i f i e r s as discussed elsewhere in this manual. Then
adjust
the
input
pulse generator in such a way that
the lowest
seven i n p u t lines of the DAC
are at
logical 1. The panel meter
should
read
f u l l scale; adjust
the o f f s e t
potentiometer
to
c o m p e n s a t e for small d e v i a t i o n s . I f adjustment cannot be o b t a i n e d ,
measure
the
v o l t a g e at the o u t p u t of A4, then reduce
the pulse
g e n e r a t o r r a t e in a way that only the lowest six input lines of the
DAC
are at logic 1; the o u t p u t vol tage of A4 is now
approximately
one-half of the previous v a l u e , o therwise
the DAC is not working
p r o p e r l y . If it does, check the ci rcuit around A5 and the meter.
To examine the b e h a v i o u r of the averaging c i r c u i t (R20, Cl and
C2),
i n t r o d u c e a large step at the DAC input
(for example, by
s t o p p i n g or c h a n g i n g the scale of the pulse generator), and observe
the
output
of A5.
It should move with a time constant of either
1.5s or 3s, a c c o r d i n g to the s e l e c t e d range.
T r o u b l e can also come from the c i r c u i t r y
associated with the
d e t e c t i o n of overrange. W h e n e v e r the c a p a c i t y of the 6-bit counter
is e x c e e d e d , f l i p - f l o p All is
set
by a signal
coming
from
m o n o s t a b l e A 1 2 b ; the o u t p u t of the f l i p - f l o p sets the 2-bit counter
Ay and p a r a l l e l loads A8 to 0101; the content of the 6-bit counter
is
110101. This is s i m u l t a n e o u s l y transferred to latches A6
and
A7, and a d d e d to apply
1101010 to the DAC (the LSB of tue DAC is
g r o u n d e d ) , and the m e t e r will be d e f l e c t e d almost full scale, until
reset
occurs
(5
seconds later).
A failure of
the overflow
f l i p - f l o p All may have various consequences, because of
its many
c o n n e c t i o n s ; for e x a m p l e , if pin
i! of All stays at logic 0 (which
is the s t a t e a f t e r overflow), counters A8 and A9 would be unable to
count. All is reset by a signal from m o n o s t a b l e A 1 7 b .
(164)
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Chapter 9
MULTICHANNEL ANALYZERS
-1-
Chapter 9
MULTICHANNEL ANALYZERS
INTRODUCTION
9. l
The most frequent task of a m u l t i c h a n n e l analyzer is to search
for
the pulse h e i g h t d i s t r i b u t i o n of the incoming pulses
(pulse
height
analysis, PHA). In
order
to get
the pulse height
d i s t r i b u t i o n or
s p e c t r u m , the pulse height
of each pulse
is
m e a s u r e d , and the result is expressed as a binary number. This is
the
task of the ADC u n i t . The measured binary number serves as an
address to the random access memory
(RAM) location. The contents
of
the c a l l e d memory location is pushed into the ADDER where it is
increased by one. The new n u m b e r is r e t u r n e d to the same location.
As a r e s u l t of this p r o c e d u r e , the RAM can tell how many
times a
pulse of a given h e i g h t has
appeared
during the measurement. The
whole o p e r a t i o n is controlled by the steering logic (Fig. 9.1).
A
^DATA
ADDR^
ADC
RAM
————S
<\
t
f
N————^
r
ADD.
*
LGGIC
Fig. 9.1: S i m p l i f i e d MCA
During
the m e a s u r e m e n t , or after
it, results are
either
d i s p l a y e d by the CRT, and/or
transmitted
to a peripheral d e v i c e ,
like
printer
or
plotter. The display is always a part
of
the
system while external u n i t s are
connected
according
to our
requirements.
Communications with an external p r i n t e r or
plotter
are realized either by
the serial RS-232 i n t e r f a c e or
by the
parallel Centronics. In order to p e r f o r m the d e s c r i b e d
tasks, a
r a t h e r c o m p l i c a t e d system, composed of the analog p a r t followed by
the d i g i t a l one, is used. In c o n t e m p o r a r y systems the d i g i t a l p a r t
o p e r a t i o n is steered
by a microprocessor reading
the o p e r a t i n g
instruction from a large ROM (Read Only Memory). The measured data
are stored in the RAM. For the CRT d i s p l a y , a system similar to a
t e l e v i s i o n screen is used.
For
effective
repair you should become
f a m i l i a r with the
m u l t i c h a n n e l analyzer o p e r a t i o n .
Troubleshooting
instructions
cannot d e s c r i b e every d e t a i l ; some imagination is r e q u i r e d .
Useful
information on
the
o p e r a t i o n of
the m u l t i c h a n n e l
analyzer can be o b t a i n e d
from
the p u b l i c a t i o n
IAEA TECDOC-363,
"Selected
Topics
in Nuclear E l e c t r o n i c s " , pages
117-168.
The
general i n s t r u c t i o n would be out of place here. T h e r e f o r e , we will
concentrate
our
a t t e n t i o n on a t y p i c a l M C A , Canberra
35 Plus
(Fig. 9.2).
(173)
Chapter 9
-2-
HICROPROCESSDR
BUS
COLLECT
WTDVME
Memory
Display
i
Fig. 9.2:
Signal
Processing
i
t
Operator/
Analyzer
InterfaceI
Data I/O I
I
I
I
Block Diagram of Series 35 Plus MCA
9.2
MCA TROUBLESHOOTING AT THE UNIT LEVEL
9.2.1
Low Voltage Power Supply
We have removed the top cover of a C A N B E R R A 35 Plus MCA. The
location of i n d i v i d u a l printed boards is indicated in Fig. 9.3
If
after 20 seconds the screen is still black the power supply should
be checked. On the power supply board are 5 LEDs in line, all
lighting
if supply voltages are present
This v o l t a g e can be
measured between the ground at TP7 and TP1 to TP6.
L.V.SUPPLY
MONITOR
GND = TP7
-12 = TP2
+ 5 = TP6
+15 = TP1
+12 = TP3
+ 24
TP4
-12
-24
TP5
TP2
Fig.
9.3:
Board locations
In the case of failure, follow the instructions given in
Chapter 6, Section 6.14. A d d i t i o n a l useful r e a d i n g about this type
of
the power supply is given in the previously-mentioned
IAEA
TECDOC-363, Page 199.
If the screen is dark while voltages are a d e q u a t e , the reason
might be a bad monitor. Continue troubleshooting at MONITOR.
(174)
Chapter 9
- 3—
Central Processor Board
9.2.2
If a p i c t u r e appears on
the screen, we can check the CPU
(Central Processor Unit) board, The analyzer has three b u i l t - i n
self-testing functions:
1.
2.
3.
ROM test
RAM test
A l p h a n u m e r i c generation test
the
to
press YES to prepare
To run
test you have
to
accept
your
next
command.
Press
the
m u l t i c h a n n e l analyzer
shown
in
Fig.
9.4.
You
will
hear
a
click,
hidden key s i t u a t e d as
and the display shows:
DIAGNOSTICS:
* CHECKSUM RAM CHAR
The CHECKSUM test
can be run by pressing YES
seconds, the first result will appear on the screen as
After 12
A28 55E3
or similar.
The
full
test takes about one m i n u t e .
The
q u e s t i o n marks
following the chip label are
displayed if either the corresponding
1C is bad or not inserted:
A30 ????
The results of the test can be compared
values given in Tables 9.1 and 9.2.
with
the
expected
KEY
Fig. 9.4: The hidden key position
Select ing the RAM test programme causes the CPU to clear RAM
and write known data into it; then the content of the memory is
verified to see if the result is correct
The part of the RAM
to
be tested
is selected by
sett ing
the MEMORY switch.
If the
VERTICAL RANGE switch is set to 1048K, the data p a t t e r n will appear
as a ramp with a positive slope and will be changed to a ramp with
a negative slope.
(175)
Chapter
9
TABLE 9 . 1 :
Series 35+ checksums for
A27
A3!
A50
Option
Number
BASIC
BASIC
BASIC
BASIC
A77
A80
A64
Chip
Number
A28
(176)
-4-
V-2.00
10
Number
900069B
900069B
900070A
900070A
CHECKSUM
55E3
8500
F89C
OD07
PROM TYPE
27128
27128
2716
2716
3541
3541
3541
900071A
900071 A
900071A
D994
03EO
6200
2716
2716
2716
A77
A80
A64
3541N
3541N
3541N
900072A
900072A
900072A
0994
03ED
5616
2716
2716
2716
A77
ABO
A64
3541C
3541C
3541C
900073A
900073A
900073A
0994
03ED
537D
2716
2716
2716
A30
A49
3543
3543
900074A
900074A
6552
9250
2732A
2732A
A53
3551
900076A
906F
2716
A53
3552A
900077A
3C86
2716
A53
3553
900078A
26E2
2716
A64
3554
900079A
EFEC
2716
A67
A74
3571
3571
900080A
900080A
1C50
4170
2716
2716
A67
3572
900081 A
6E3C
2716
A67
A74
3573
3573
900082A
900082A
8D9F
FE2C
2716
2716
A67
A74
3573B
3573B
900083A
900083A
9548
9063
2716
2716
A67
3574
900084A
5CB8
2716
A52
A63
3575
3575
900097
900097
D8E1
4443
2732A
2732A
A64
3576
900098
9F32
2716
Chapter 9
-5-
TABLE 9. 2:
Series 35 -i- checksums for
V-l .00
Chip
Number
A27
A28
A31
A50
Option
Number
BASIC
BASIC
BASIC
BASIC
Number
900069
900069
900070
900070
CHECKSUM
9306
EDAD
FCC2
230D
PROM TYPE
27128
27128
2716
2716
A77
A80
A64
3541
3541
3541
900071
900071
900071
E700
OA44
6200
2716
2716
2716
A77
A80
A64
3541N
3541N
3541N
900072
900072
900072
E700
OA44
5616
2716
2716
2716
A77
A80
A64
3541C
3541C
3541C
900073
900073
900073
E700
OA44
5370
2716
2716
2716
A30
A63
3543
3543
900074
900074
146D
AC02
2732
2732
A30
A63
3544
3544
900075
900075
1649
E64E
2732
2732
A53
3551
900076
913C
2716
A53
3552A
900077
3FE8
2716
A53
3553
900078
2924
2716
A64
3554
900079
EF6C
2716
A67
A74
3571
3571
900080
900080
2AOF
45BD
2716
2716
ID
A67
3572
900081
7709
2716
A67
A74
3573
3573
900082
900082
B012
E3CF
2716
2716
A67
A74
3573B
3573B
900083
900083
B795
7B70
2716
2716
A67
3574
900084
5C95
2716
(177)
Chapter 9
-6-
The following messages are e x p e c t e d as a result of testing:
RAM TESTED O.K., if good or
RAM FAILURE N, if bad
N i n d i c a t e s the f a i l u r e t y p e , and has the following
meanings :
iMemory won't clear
ZMemory integral incorrect positive slope
3Memory i n t e g r a l incorrect negative slope
4Flag bit incorrect
S e l e c t i n g the CHAR test
programme will cause the display to
show two lines of the a l p h a n u m e r i c character across the b o t t o m of
the display.
By pressing the h i d d e n key again, the diagnostic is finished.
If all t h r e e tests have been completed
s u c c e s s f u l l y , then we
can assume that the CPU board is working p r o p e r l y .
9.2.3
Monitor
The MONITOR is a self-contained a s s e m b l y , which includes the
CRT and the necessary electronics
to present a TV raster display.
It
should
be
considered as a replaceable component
and
is not
i n t e n d e d to be r e p a i r e d in the field. It a c c e p t s three signals:
v i d e o , VID
line synchronization, L S Y N C , and
field s y n c h r o n i z a t i o n , FSYNC
Signals can be o b s e r v e d
foilows:
(Fig.
GND
LSYNC
PIN 1
PIN 6
PIN 7
PIN 8
+ 15 V
PIN 9
PIN 10
VIDEO
FSYNC
VRTN (Video ground) PIN 10 is the lower one
Even Field
LSYNC
Line Sync (Vertical)
FSYNC
Field Sync (Horizontal)
Odd Field
LSYNC
FSYNC
Fig. 9.5: Synchronizing signals
(178)
9.5) at the 10-pin connector as
Chapter 9
-7-
In the absence of any of these signals, the MONITOR screen
will be black.
Troubleshooting should be concentrated on the
d isplay board.
It is useful to know that the MCA is able to show the display
field
on the MONITOR with the LOW VOLTAGE POWER SUPPLY and DISPLAY
BOARDs inserted, even if all the other boards removed.
9.2.4
Display Board
On the DISPLAY BOARD three signals for the MONITOR control are
produced: LSYNC, FSYNC and VID as already mentioned.
The first two signals are used to control the MONITOR p i c t u r e .
Analogous to a TV monitor, a MCA
display
is an interlaced raster
scan format. The major difference is that the CRT yoke is turned
90 degrees, causing
the v e r t i c a l field
to force the beam to run
fast from the b o t t o m to the top of the screen, and slowly advancing
from left to right making the
first
256 runs before it returns to
the left where it starts to make the second set of runs between the
lines written d u r i n g the first set (see Fig. 9.6).
49«
.02'
6 8 10.
EVEN FIELD
M? 506 5'0
4JS08J
•INTERLACING'
MASTER
TYPE DISPLAYS
BEAM
TRAVEL
13
5 7 9 1 1
LINE
f
U.FIELD
U.F
FIELD———-497)60 '^605^689)
«99 503 507 511
Fig.
9.6: Resultant "interlaced" frame o b t a i n e d from odd and
even field scan
During scanning, the electron beam striking
the CRT screen
should be properly intensified to draw the wanted p a t t e r n . This is
done by applying
the m o d u l a t e d VID
signal
to
the CRT control
electrode.
The modulation information is g a t h e r e d from d i f f e r e n t
sources. Text is provided from ROM. Spectra representing p o i n t s
are contained in the RAM. Both information sources are on the CPU
board. Fast operation is obtained through direct memory access
(DMA) while the 8085 processor is in the HOLD state. See Fig. 9.7
for the connections between the DISPLAY UNIT and the remaining
system.
The
dead time counter signal is produced on the DISPLAY
board, using the LT line from the ADC board.
(179)
Chapter 9
-8-
BAM
BAD7
A19
n i ii nun
I—— DATA ——J
!
I-— ADDRESSES —J
r
I want DMA
You hove It
DMA
L
TP15
TP26
REQEN ——9*
TP27
Data valid On DMA) DATA VI ——+- TP20
I finished
sending data
ADC dead tine data
Fig.
9.2.5
DCARl •* ——
DCHA1 —— t»
RST 75 ^— -
TP10
PRS
——»•
GUT
LT
———— TP23
— — A27 pm 11
TP88 reset when power on
TP2
TP1
——^
VID
——^
LSYNS
——»*
FSYNS
9 . 7 : Display board connections
Principles of
the
ADCOperation
ADC t e l l s t h e h e i g h t o f t h e
input
simple
example
of
the
operation
of
c o n v e r t e r i s g i v e n i n F i g .9 . 8 .
pulse
in a b i n a r y way.
A
a
t w o -a n d
four-channel
U
00 01 10
Fig.
9.8: Two-
11
and four-channel pulse d e t e r m i n a t i o n
In the first case the h e i g h t of the input pulse U was
grouped
into channel 0 while in the second
case the v o l t a g e was found in
channel
lu (10 = 3, e x p r e s s e d decimally). However, to d e t e r m i n e
the pulse
h e i g h t more p r e c i s e l y , more channels are
required.
C o n t e m p o r a r y m u l t i c h a n n e l analyzers have up to 16384 channels. To
express this number in a binary way, 15 binary d i g i t s are r e q u i r e d .
Conversion from the analog to the d i g i t a l form takes some tens
of m i c r o s e c o n d s .
The
conversion time of course d e p e n d s on the
conversion m e t h o d .
The widely
adopted
converters
in nuclear
i n s t r u m e n t a t i o n , for w h i c h the o p e r a t i n g principles w i l l be b r i e f l y
d e s c r i b e d , are of the Wilkinson type.
(180)
Chapter 9
-9-
OUTPUT
U
PULSE IS
OVER
INPUT
Fig. y . y
What STRETCHER does
Fig. 9.10: Simple STRETCHER
followed by 90% discr
Gaussian pulses
(Fig. 9.9) from amplifiers keep their maximum
value
only
for a short
moment.
T h e r e f o r e , for any
precise
d e t e r m i n a t i o n , which is time consuming, t h e i r a m p l i t u d e should be
stored.
This
is done by using the special unit
STRETCHER.
The
form of the obtained pulse is given
in Fig. 9.9.
Such o p e r a t i o n
can be realized by using the c i r c u i t given in Fig. 9.10.
However,
its p r o p e r t i e s are
not
adequate.
The
real s t r e t c h e r has
incorporated
more than
20 20 t r a n s i s t o r s and a few
integrated
circuits .
Now
the pulse is properly
formed.
Shall we
start
the
measurement? Let
us be sure t h a t the peak is over. For
this we
wait t h a t the instantaneous pulse value drops below 90% of its peak
value w h i c h is s t o r e d in the c a p a c i t o r . A d i s c r i m i n a t o r c o m p a r i n g
90% of the s t o r e d voltage w i t h the instantaneous signal value will
tell us the right moment (Fig. 9.10) to s t a r t .
The
height
d e t e r m i n a t i o n s t a r t s using the c i r c u i t
shown
in
Fig.
9.13. When the c o n s t a n t current is applied to the c a p a c i t o r ,
the voltage on it s t a r t s to go linearly to the zero
(Fig. 9.11).
During d i s c h a r g i n g , we
count: one, two, three ....
C o u n t i n g is
s t o p p e d when zero v o l t a g e is r e a c h e d . The
c o u n t i n g result
is
p r o p o r t i o n a l to the pulse h e i g h t . The analog to d i g i t a l conversion
has been p e r f o r m e d . Really? No, the conversion p r o c e d u r e was
o v e r - s i m p l i f i e d . There are
a lot of
tricks to squeeze out the
1/8000 a c c u r a c y using the analog c o m p u t i n g t e c h n i q u e . The signals
in the t y p i c a l ADC
unit
have
to pass through more
than 80
i n t e g r a t e d c i r c u i t s and 50 transistors.
r
CONSTANT
CURRENT
APPLIED
IN
1————I
U
UNIDIRECTIONAL
AMPLIFIER
Fig. 9.11:
Conversion
Fig. 9.12:
STRETCHER
(181)
-IQ-
Chapter 9
90 X
UNIDIR.
AMPL.
ZERO
_d
K3DFF
CLOCK
O-
START
STOP
E BIN
COUNTER — DUT
Fig. 9.13: S i m p l i f i e d ADC
9.3
CHECK AT THE UNITS LEVEL
9.3.1
ADC B o a r d
An ADC board contains an a m p l i f i e r
and a ADC. As amplifier
t r o u b l e s h o o t i n g is not the aim of this c h a p t e r , e x p l a n a t i o n will be
l i m i t e d to ADC only. See Fig. 9.13 for the block diagram.
In p r i n c i p l e , you can check the operation of the unit by using
only the p r e s e n t i n s t r u c t i o n s . However, it is strongly recommended
also to c o n s u l t C A N B E R R A SCHEMATICS SERIES 35 PLUS. The ADC
board
is given in 5 sheets.
Troubleshooting
following
the signal p a t h is more d i f f i c u l t
than in the p r e v i o u s cases. The separate blocks are interconnected
w i t h more
than one line. Thicker lines indicate more
than one
signal going the same way. Arrows i n d i c a t e the d i r e c t i o n of the
d a t a f1ow.
The
s t r e t c h e r , for instance, a c c e p t s input signals, and
its
o u t p u t is i n s p e c t e d by three blocks: s t r et cher/input interrogation
l o g i c , single channel analyzer and zero crossing discrimination.
In
the case of p r o p e r i n p u t , well-defined o u t p u t signals are
also e x p e c t e d , assuming normal block operation.
However, in the
absence
of the a d e q u a t e control signals on lines DUMP, STRET OFF
and RAMP ON, even a good block
cannot work.
All three control
signals are coming from the CONTROL LOGIC block. This block makes
its decisions by e v a l u a t i n g 14 i n p u t signals. A few of them d e p e n d
again on
the stretcher output signal.
In
this way we
find
ourselves inside a c o m p l i c a t e d f e e d b a c k system.
(182)
-11-
Chapter 9
The basic idea for such troubleshooting is to isolate separate
blocks by fixing the auxiliary signals.
E NO AT E
AOC AOC
MCS
AOF
•EI-EASE R£>DY AADV StOTHlC
PHA »ccttjS
n
TRMR
L&
Fig. 9.14: Canberra 35+ ADC
To check the STRETCHER o p e r a t i o n , for example, the STRET OFF
signal must be low when the input pulse appears. If it is not low,
we can simply push it down by g r o u n d i n g
the STRET OFF line for a
short moment. However, such an approach is not possible
in many
cases. The o u t p u t of a TTL c i r c u i t cannot be c o n n e c t e d to the +5 V
w i t h o u t causing permanent damage.
We will s t a r t the ADC test by applying a d e q u a t e pulses to
input. Pulses can come:
the
from a pulser; recommended height 5 V, f r e q u e n c y - if changea b l e - the highest possible;
from a pulse generator; the repetition rate is a few thousand
per second, height
is a p p r o x i m a t e l y 5 V, and the duration is
a few m i c r o s e c o n d s ;
good pulses can be produced by applying rectangular signals to
Even the
tlie input of any
nuclear spectroscopy amplifier.
front
plate
probe
adjusting signal 0.3 V, 1 kHz,from the
c o n n e c t i o n of the TRIO oscilloscope, is s u i t a b l e .
and
if
the
If
such
pulses are
applied
to the i n p u t
c o r r e s p o n d i n g binary number appears at d i g i t a l adders 74LS283 (A63,
pins 4 , 1 , 1 3 , 1 0 ;
pins 4 - Least significant bit 1, 13, 10; A65
(183)
Chapter 9
-12-
A64, pins 4, 1, 13, 10; and A63, pins
1 and 4 - most significant
bit)
we can assume that the ADC is working properly. In order
to
observe the binary o u t p u t , the ADC
board should be e x t r a c t e d and
e x t e n d e d by using the extension c a r d .
In the case of
failure,
followed step by step.
1.
the
signal
processing
should
be
Connect
the
oscilloscope probe to TP5
and
clip
the
oscilloscope ground to TP9 to observe the input signal. Then
repeat the observation moving the probe to the TP5. Note that
the signal at TP5
follows
the ascending
side of the input
pulse and
remains c o n s t a n t after the peak has been reached.
This is the i n d i c a t i o n t h a t the s t r e t c h e r works properly.
If
not, skip the next steps and go to 2.
One or two microseconds l a t e r
the discharge process starts.
The
slope
of the linear a p p r o a c h to zero d e p e n d s on
the
conversion gain.
A few
hundred
m i l i v o l t s below zero,
conversion is s t o p p e d and the ramp r e t u r n s to zero. The analog
part of the conversion was successful.
Note
the case when the input signal exceeds the u p p e r
level
d i s c r i m i n a t o r (A28) s e t t i n g . The c o n t r o l logic then sets DUMP
to logic 1; Q24 and Q25 are b o t h c o n d u c t i n g and fast d i s c h a r g e
follows. During the i n t e r v a l
of the linear d e s c e n t , the zero
pulses
from
the 100 MHz clock are c o u n t e d .
The
clock
is
running
permanently.
Its o p e r a t i o n
can be
observed
by
c o n n e c t i n g the oscilloscope to TP6 and using the c o r r e s p o n d i n g
ground
TP7
to
get
a
representative
picture
without
superimposed
oscillations. In the pulse analyze m o d e ,
clock
pulses are passing NOR gates
in A89. They can be o b s e r v e d at
pin 6 and pin 11 of the b e f o r e - m e n t i o n e d c i r c u i t .
When
all
the c o u n t i n g
c o n d i t i o n s have been
m e t , the
synchronizing unit is a c t i v a t e d . The SYNC/ signal is delivered
at pin 7 , A83.
Counting s t a r t s . The address counter
consists of A79 t h r o u g h
A82
and is c o n f i g u r e d as a 14-bit r i p p l e counter.
The
14th
bit is used for under and
overflow d e t e c t i o n . During rampdown
sequence, the ramp counter and ramp current are
enabled
and synchronized to the 100
MHz clock. The r e s u l t a n t d i g i t a l
a d d r e s s in the address c o u n t e r r e p r e s e n t s the m a g n i t u d e of the
ADC analog i n p u t . The
c o u n t i n g can be followed from stage to
stage if the i n p u t
signal
is
high
enough
to a c t i v a t e the
higher
bits
too and if the s e l e c t e d number of
channels
is
s u f f i c i e n t . For i n s t a n c e , if
the channel number was set to
512,
then only
the lower 9 b i t s will be active.
Set
the
oscilloscope time base
to smaller
t i m e / d i v i s i o n values
to
display only a few periods of the observed signal.
Then move the probe to pin 5, A82
and set t i m e / d i v i s i o n value
to 1 or 2 microseconds per division.
full length of the pulse train
of the input pulse.
(184)
Now you can observe
the
w h i c h should follow the height
-13-
Chapter 9
When counting is f i n i s h e d , the resulting number is transferred
into s h i f t r e g i s t e r s A85, A86, A87 and A88.
F i n a l l y , the d a t a
are m o d i f i e d in four A - b i t adders A63, A 6 4 , A65 and A66.
The
binary n u m b e r w h i c h was loaded into shift registers A85,
A86,
A87 and A88 from the sliding scale counter is now s u b t r a c t e d .
2.
There is no signal at TP5.
STRET OFF m i g h t be high. In this
case the u n i d i r e c t i o n a l a m p l i f i e r c h a r g i n g Cll is d i s a b l e d . To
v e r i f y this p o s s i b i l i t y , join pin 9, A86 for a moment to the
ground.
If an a d e q u a t e signal at TP5 appears, the fault
is
w i t h i n the ADC b o a r d , c o n t r o l
logic
part.
If the g r o u n d i n g
does
not
restore
normal
operation,
the
unidirectional
a m p l i f i e r is b a d . Check all t r a n s i s t o r s .
The next p o s s i b i l i t y is the v o l t a g e following the ascent side
of
the p u l s e , but not dropping back, to zero. In this case, the
discharging
constant
current
source
is
not s w i t c h e d on, or the
source
is bad.
RAMP ON s i g n a l (A29, pin 6) should be high
during
the d i s c h a r g e . If high state is
r e v e a l e d , then the c u r r e n t source
is
bad.
C o n c e n t r a t e your o b s e r v a t i o n s on t r a n s i s t o r s Q20,
Q21,
Q22, Q23, Q26, Q27, Q28 and
c o m p a r a t o r A77 (LF411).
If RAMP ON is
low
and
d i s c h a r g i n g doesn't take p l a c e , the f a u l t is p r o b a b l y
in
the c o n t r o l logic as before.
A good t e s t to check a p a r t of the d i s c h a r g i n g system
is
to
use the f a s t
discharger
by
injecting
a bigger
current through
t r a n s i s t o r Q29 c o n t r o l l e d by t r a n s i s t o r Q24.
If the DUMP signal,
which
is nornally
low,
can
be
m a d e high
by joining
the
c o r r e s p o n d i n g g a t e o u t p u t to 5V.
However, by s h o r t - c i r c u i t i n g
the
emitter
and
collector
of
transistor
Q29,
the
same
e f f e c t is
achieved.
The
command
to s t a r t
c o n v e r s i o n , i.e. to
inject
the
d i s c h a r g i n g c u r r e n t i n t o C l l , is given by the d i s c r i m i n a t o r A16
when the input signal has d e c r e a s e d to 90% of its a m p l i t u d e .
Two a d d i t i o n a l tricks used
a c c u r a c y w i l l be d e s c r i b e d .
during
the c o n v e r s i o n to i m p r o v e
When c o n s t a n t c u r r e n t
g e n e r a t o r is c o n n e c t e d to discharge the
storing
c a p a c i t o r , t h e r e s u l t i n g ramp voltage m i g h t d e v i a t e
from
linearity b e c a u s e of the t r a n s i e n t
phenomena. T h e r e f o r e , counting
s t a r t s later. The lost time is c o m p e n s a t e d for by c o u n t i n g longer.
C o u n t i n g is s t o p p e d by the zero crossing d i s c r i m i n a t o r w h i c h is
actually
set about
80
mV
below the zero
level.
Instead
of
i n t r o d u c i n g the time delay T b e f o r e c o u n t i n g , we can count from the
very
b e g i n n i n g , but s e t t i n g the c o u n t e r to the a d e q u a t e n e g a t i v e
v a l u e - N . Then a f t e r NX clock
period
= T, the counter will be at
zero.
The
l o a d i n g , w h i c h is d i f f e r e n t for d i f f e r e n t
conversion
rates, is realized t h r o u g h the p a r a l l e l input of c o u n t e r s .
The
second trick is the use of the so-called
sliding
scale.
Because of the i n t e r f e r e n c e of
the s u r r o u n d i n g d i g i t a l c i r c u i t s ,
the produced ramp v o l t a g e m i g h t be m o d u l a t e d . This would result in
spectra d e f o r m a t i o n . The idea is to use in successive m e a s u r e m e n t s
different
p a r t s of
the ramp
voltage.
T h e r e f o r e , the binary
(185)
-14-
Chapter 9
c o u n t i n g system is additionally
preloaded with 1, 2, 3 ..., up to
18.
The r e s u l t i n g binary number would thus be too big by 1, 2,
3 .... To get the correct v a l u e , this number is s u b t r a c t e d in the
adders A 6 3 , A64, A65 and A67. S u b t r a c t i o n is implemented
by the
a d d i t i o n of the two's c o m p l e m e n t .
Lines QLO to Q L 1 2 , with the signals representing the result of
the c o n v e r s i o n , have to be connected to the a d d r e s s - d a t a bus (BADO,
BAU1,
BAD2,
BAU4,
BAD6, BAD?, A8,
A9,
A10, A l l ,
A12). The
c o n n e c t i o n is done t h r o u g h three-state b u f f e r s A33 and A44 are used
between.
9.4
CPU BOARD
9.4.1
Processor Description
The heart of the CPU board is
the INTEL
w i t h the pin assignment shown in Fig. 9.15.
address and data pins, an a d d i t i o n a l latch must
to create
the normal 16-bit address and 8-bit
m i c r o p r o c e s s o r ADU - AD7 and A8 - A15 lines.
TABLE 9.3: S t a t u s signals
STATUS SIGNALS
SO SI
IO-M/
1
1
0
1
0
0
1
0
0
1
0
1
1
1
0
1
1, 1
1
0
0
Type of Machine Cycle
Op Code Fetch (OF)
Memory Read (MR)
Memory Write (MW)
I/O Read (IOR)
I/O Write (IOW)
Interrupt Acknowledge (INA)
Bus Idle
xi C 1
40
vcc
X2 C 2
39
38
37
HOLD
HLDA
CL K (OUT)
RESET IN
READY
IO/M
RESET OUT C 3
SOD
SID
TRAP
RST75
RST6S
RST55
INTR
INTA
ADo
AC]
A02
AO]
»D«
AD«
AD«
AD7
V»
Fig.
(186)
9.15:
C 4
C 9
C 6
C 7
C 8
C
C
6A
6
C ÎÏC 12
£ 13
C 14
C 15
C 16
C 17
C 11
C 19
C 20
36
35
34
33
32
31
30
29
2l
27
2«
25
Si
RD
WR
ALE
$0
Ais
»14
A13
»12
24 3 An
23 ) Aio
22 1 A9
21 1 *»
8085 microprocessor
8085 microprocessor
Because of the shared
be a d d e d externally
d a t a bus
from
the
-15ALE (Address
Latch Enable)
address to the e x t e r n a l latch.
8085
Chapter 9
is
used
to
load
the low byte
needs a clock, therefore, an external clock signal of
6
MHz is a p p l i e d to XI while X2 is left floating.
The CPU board has a 12 MHz (A43) oscillator d i v i d e d down to 6
MHz in 74LSD292 (A44). The b u f f e r e d
6 MHz is the clock input to
the
8085. The 8085 also divides this clock by two and o u t p u t s
it
as CLK signal .
To execute an i n s t r u c t i o n , the 8085 d i v i d e s the operation into
machine cycles. An instruction might take from one to five machine
cycles.
In each machine cycle there are three to six
time states
T (see Fig. 9.16). Each T-state consists of one clock period.
The a c t i v i t y of 8085 w i t h i n each machine cycle is:
Tl
T2
T3
T4, T5
T1
O u t p u t an address
Switch A/Ü from a d d r e s s to data IN/OUT
Hold data stable
Internal operating time for 8085
T2
T3
T«
T3
T2
T1
T2
T3
CLK
STATUS
AI-AIS
---
AOO-AD7
ALE
y~
AO-7
Oil« I
><*o>
M
OUT)
n.
RD
WR
r
M1
OPCOPE FETCH
Fig.
What
signals.
9.16:
M2
MEMORY READ
M3
MEMORY WRITE
8085 t i m i n g
is going on in the processor can be learned from
RESET IN sets the programme counter
flags; it initializes the microprocessor.
to
status
zero and resets all
RESET OUT i n d i c a t e s that CPU is being r e s e t .
serve as the system reset.
This signal
can
(187)
Chapter 9
-16-
RD, WR.
The b e h a v i o u r is e v i d e n t from
pulses are sent o u t .
Fig.
9.16.
The
shown
HOLD i n p u t when d r i v e n h i g h _ t r i - s t a t e s
the address and
data
lines as well as RD, WR and
I0/M line.
Internal processing can
continue.
HLDA is an o u t p u t i n d i c a t i n g t h a t the HOLD
accepted.
command
has
been
TRAP
i n t e r r u p t is edge and level sensitive and must remain at
a high level until it is acknowledged.
It is non-maskable (there
is no
s o f t w a r e way to avoid it when the signal is a p p l i e d to the
i n p u t ) . When the i n t e r r u p t o c c u r s , the a d d r e s s is branched to 24H.
RST 7.5, RST 6.5, RST 5.5.
These
three i n p u t s have the same
timing
as
INTR e x c e p t
they
cause an
internal
restart
to be
a u t o m a t i c a l l y i n s e r t e d . The
r e s u l t i n g addresses are 3CH, 34H and
23H r e s p e c t i v e l y .
INTR, i n t e r r u p t r e q u e s t , has a similar e f f e c t as TRAP and RST,
with
the r e t u r n a d d r e s s d e p e n d i n g on the i n s t r u c t i o n read
by
the
CPU when i n t e r r u p t is a c k n o w l e d g e d .
NOTE ;
.5
We only discussed the f u n c t i o n of the pins used in
the d e s c r i b e d a p p l i c a t i o n of 8085
G E N E R A L STRUCTURE OF THE CPU BOARD
Microprocessor
8085 has to be s u p p o r t e d w i t h some a d d i t ional
u n i t s to form the c o m p l e t e
c o m p u t e r system. H o w e v e r , b e c a u s e of
the
r e q u e s t for d i r e c t m e m o r y access, more than m i n i m u m a d d i t i o n a l
for the block d i a g r a m of the
c i r c u i t s were
a d d e d . See Fig. 9.17
CPU b o a r d . The c o m p l e t e s c h e m a t i c s of the CPU board are c o v e r e d by
6 sheets:
1.
2.
3.
4.
5.
6.
9.6
RAM o r g a n i z a t i o n
ROM o r g a n i z a t i o n
C u r s o r - m o t i o n c o n t r o l l i n g system
8085 processor w i t h bus lines and s t a t u s decoder
D i r e c t m e m o r y access c o n t r o l l e r ; c e n t r a l clock unit
Remote control interface
8085 C O M M U N I C A T I O N
To
check
the 8085 o p e r a t i o n s t a r t s w i t h 6 MHz
clock
i n s p e c t i o n . The i n d i r e c t test is done by o b s e r v a t i o n of the 1 MHz
clock d e r i v e d from 6 MHz clock at test point TP8.
Another indirect
t e s t is to o b s e r v e CK signal of 3 MHz
leaving the 8085 at TP10.
H o w e v e r , for the d i r e c t c h e c k , your probe should be a t t a c h e d to pin
1 of 8085 (big c h i p , A45 position).
(188)
Chapter 9
-17-
INTEL
The 8085 system bus is created by using A40 and A47 (8282,
p r o d u c t i o n , similar to 74LS374, but d i f f e r e n t
pin config-
ura t i cm to latch the 16-bit addresses).
Latch
is
performed using
the A signal derived from the ALE microprocessor o u t p u t
(A signal
can be observed at TP11). See Fig. 9.18 for the block diagram.
! BADO-BAD7
Il'RECR
MOTION
ZONTRCLLER
C AJ-A15
Fig.
9.17:
CPU board s t r u c t u r e
DI
ABC tOOS
140
LOW A
MM 1 MM
•ADDRESSES
1
MS
Fig.
9.18:
»SPLAY KMID
ADC OMDS
8085 system bus and d a t a flow
(189)
Chapter 9
-18-
B i d i r e c t i o n a l data flow is
provided
using
the 3-state
b i d i r e c t i o n a l drivers 74LS245 (A61 and A62). A61 o u t p u t is enabled
by RDROM signal (observe TP17) and
the d i r e c t i o n is selected by DI
(available at pin l, A61).
The A61 data flow d i r e c t i o n is s e l e c t e d by a signal which can
be observed at TP15. Three-state operation is not used, i.e. pin
19 is grounded.
The
s ignals:
status
decoder
A39 p r o d u c e s
the
input/output
control
WM, at TP9
RM, at TP13
OE, at TP12
and a few others.
state
When working w i t h the slow input or
o u t p u t d e v i c e s , a WAIT
should be inserted into the normal instruction cycle.
The
READY i n p u t is suitable for t h i s , controlled by A25 and A26.
After
the
direct
memory
access
request,
the normal
microprocessor
o p e r a t i o n should be r e s t o r e d . This is done using
RST 7.5 and RST 5.5 input lines of 8085.
The g e n e r a l power failure is i n d i c a t e d by a LOW on PWRF
line.
A f t e r the inversion in A60, the TRAP
i n p u t (highest p r i o r i t y ) is
activated.
9 .7
ROM OPERATION
ROM o r g a n i z a t i o n is given in Fig. 9.19. The
ROM circuit
is
d i v i d e d into four banks of
16k
x 8 b i t s for a total p r o g r a m m e
memory
of 64 k b y t e s . This is also the normal addressing space of
8085.
B e c a u s e we
also
need
RAM
for
data s t o r a g e , the
p r o g r a m m e - c o n t rolled bank switch was i n t r o d u c e d .
The
signals BK1 and BK2 are stored in l a t c h A l l .
Tri-state
driver A48 is enabled if BK1
is
low
or address bit A14 is low.
D r i v e r A29 is enabled if BK1 is high and if the address bit A14
is
high. Signal BK2 is gated with address b i t s A 1 4 , A 1 3 , and A12 in a
74LS10 (location A 1 0 )
to d e t e r m i n e which ROM o u t p u t
is passed
through d r i v e r A48.
The
s e l e c t i o n of the separate ROM chips is
realized t h r o u g h 3 to 8 line d e c o d e r s 74LS138 (A14 and A78).
(190)
Chapter 9
-19-
Fig.
9.8
9.19:
ROM organization
RAM OPERATION
RAM consists of
four
static
RAM
c h i p s , 8k
x
8-bit each.
Because of the low power c o n s u m p t i o n , memory chips can
be powered
from the b u i l t - i n r e c h a r g e a b l e b a t t e r y for up to 30 days.
The
data
exchange goes
through
t h r e e - s t a t e bidirectional
t r a n s c e i v e r 74LS244 (A65, A72). The d a t a from the upper four lines
b e t w e e n transceiver and RAM can be stored in latch 74LS373 (A75) in
order to avoid
false
data
registration
in
some
phases of the
programme.
27.
Read or write access to RAM
is determined by WT signal, pin
The same signal is used to control A65.
Addressing
is achieved t h r o u g h 14 address lines. Lines AO to
A9 are d i r e c t l y connected
to RAM
chips.
They come from latches
8282, sheet
4.
Signals PÜ, PI, P2 and P3 indirectly involved
in
the addressing are generated by the programmable chip A26, sheet 5.
It
a c c e p t s the address lines from AlO to A15 from circuit A47
and
p r o v i d e s o u t p u t s w i t h respect to the memory organization.
Three
of
the generated lines PÜ, P2 and P3 are used
remaining three address lines.
PI line t o g e t h e r with the latched
applied to A62, 3 to 8 lines decoder.
of A62 are used to enable RAM chips.
as
the
A12 and A15 address line is
Output pins 4, 5. 6 and 7
(191)
Chapter 9
' -20-
The b a t t e r y charger (sheet 5) connects the rechargeable NiCd
b a t t e r y to the 5 V source when power
is
applied.
If 5 V is
p r e s e n t , the c u r r e n t to the base of Q4 keeps it
conducting.
The
resulting c u r r e n t sinking from the Q5 emitter into the Q4 collector
through R61 also makes Q5 c o n d u c t i n g .
9 .9
DIRECT MEMORY ACCESS
Direct memory access (DMA) is used to allow high speed
data
exchange b e t w e e n RAM and an external device. The operation of the
system
under
the DMA
conditions
is
shown in Fig. 9.20.
The
c o n t r o l l e r (see sheet 5 for schematics) p r o v i d e s three o p e r a t i n g
modes: R e a d , W r i t e and Read-Mod ify-Write (R-M-W).
<
o
lu
D
E
INI
Encoder and Bufftr
12MH»
• RAM Controli
Timing Generator
• BuNer Control»
Fig. 9.20:
DMA realization
The c o n t r o l l e r can a c c e p t three d i r e c t memory access requests
DCHR2,
coming
either
from
the Display
Board
or
from
Collecting
I n t e r f a c e / M i s c e l l a n e o u s Boards.
The third one, DCH3 ,
m i g h t be used by the boards inserted into the o p t i o n board slot.
DCHR1,
In c o m b i n a t i o n w i t h the clock s i g n a l s , the DMA
request
is
c o n f i r m e d t h r o u g h the DCHA1 line
(TP2)
and t h r o u g h the DCHA2 line
(TP3).
These signals are sent back to the DMA r e q u e s t i n g units.
S i m u l t a n e o u s l y , the microprocessor address and data lines are
t r i - s t a t e d t h r o u g h t h e hold c o m m a n d HDRII.
When 8085 acknowledges the HOLD c o m m a n d , it generates the HDAI
signal. This signal s t a r t s the operation of the timing generator.
In
the t i m i n g g e n e r a t o r (A34 and A3), followed by some logic
(A2,
A7, A18), d i f f e r e n t DMA cycles are composed.
See Fig. 9.21 and
Fig. 9.22 for the generated signals.
(192)
Chapter 9
-21-
"LT
DCHftt
Fig.
9.21:
[
DMA Read T i m i n g
I—— I
OCH« j"~l
M..
-*'
,L—-83nsec
|
i
i
'
'
1
!
Control
SnHI
1
! 1
4
7
6CT«A
3
i
l
!
J
1
i
I
i
_J
i
1
r
i
1
!
1__ }
KÄi
*
|
î
~|
I
•
>
j_____
ROMA M
|
i
i
1 !
CI
TP3
-»,' 1M ni ]«—— Addr«w LllclWd Her*
I
1
ÔÂTÂV
l
I
: i
&
1
l
1———————— 1
—— 'i— 1 i,
•
'
i
n*g
!
i
|
TF8
i
_j
'———————— '
1
TFlU
1
!
1
i
WTOT/ 1
WT
DAT*
• Uf
1
;
y^vV
__ — - -— --—-\>^
i
1
Data Out Valid ——^IMM
1
Fig.
9.22:
\\\x
^___
— I
—^^^^
>.__.—— ^ x
""
J
1—————1
TP25
\\\X >—
-"
IU In VOM
' 1MM
DMA Read-Mod i f y - W r i t e Timing
(193)
Chapter 9
9. 10
-22-
CURSOR CONTROL
The
aim
of
this circuit given in sheet 3, CPU board, is
to
control the cursor position. Its i m p l e m e n t a t i o n into the board can
be seen from Fig. 9.23.
MDTDR
SCR
CDNTRDLLER
DO-D7
Fig.
9.23: Cursor controller connections
The
c o m m a n d , in the form of a binary n u m b e r , to move
the
cursor is given to the CPU
through
the t h r e e - s t a t e
latch A81
(74LS373). The l a t c h receives d a t a from two b i d i r e c t i o n a l four-bit
counters A82 and A83 (74LS191), w h i c h
are p e r i o d i c a l l y scanned for
the data. Counters count pulses from the VFC (Voltage to Frequency
C o n v e r t e r ) A84 (4151).
VFC d r i v i n g v o l t a g e
is provided by a DC
motor
serving
as a g e n e r a t o r when r o t a t e d .
N e g a t i v e or
positive
voltage
applied
to the o p e r a t i o n a l a m p l i f i e r
connected
as
a
rectifier
causes the generation of the clock signal at the o u t p u t
of V F C . The same f r e q u e n c y
r e s u l t s for the left and for the r i g h t
motion.
An a d d i t i o n a l c o m p a r a t o r A86 (LM 3 1 1 ) senses the p o l a r i t y
of the g e n e r a t e d v o l t a g e and selects the c o u n t i n g d i r e c t i o n : up or
down .
FRONT
PANEL
MOTOR
ANALOG
PRDC.
A85
VFC
A84
CLOCK
DIRECTION
8 BIT
COUNTER
A82,A83
CLEAR
LATCH
ZERO
CDMP
A86
STROBE
LATCH
A81
DO-D7
Fig.
9.24: Cursor c o n t r o l l e r block diagram
Some T r o u b l e s h o o t i n g hints follow:
1.
(194)
Check the v o l t a g e , m a g n i t u d e and the p o l a r i t y at pin 3, A86
the ground.
to
-23-
Chapter 9
2.
Is there a change in the p o l a r i t y of the A86
pin 7 when r o t a t i n g left and r i g h t ?
3.
Can you observe the r e c t a n g u l a r signal at pin 3, A84?
4.
o u t p u t signal at
Are t h e r e s t r o b e pulses of n e g a t i v e p o l a r i t y at pin l, A81 and
and pin 11, A83?
pin 11, A82
5.
Are
c o u n t e r s A82 and A83 c o n n e c t e d serially into 8-bit binary
counter a c t i v e ? Can you observe the r e c t a n g u l a r signal at pin
y, and at the higher b i t s , when you r o t a t e the m o t o r r a p i d l y ?
9.11
DISPLAY BOARD - WHAT IT DOES
The
task of the display board is to control the monitor
p r o v i d e t lie i n f o r m a t i o n to be d i s p l a y e d on its screen.
Using the built-in clock, two
digital
FSYNC, are d e r i v e d to synchronize the m o n i t o r .
signals,
and
LSYNC and
The d i s p l a y board also looks into the RAM using the d i r e c t
memory
access DMA
to get the number of
the stored
counts
in
d i f f e r e n t channels. These d a t a are
c o m b i n e d w i t h the i n f o r m a t i o n
about the MCA working conditions w h i c h have to be d i s p l a y e d in text
form in tne lower p a r t of the screen.
For t h i s task, the display
board is i n s t r u c t e d w h i c h l e t t e r should be sent to M O N I T O R
through
the c h a r a c t e r v i d e o line CVID.
Also, the dead
time d e t e r m i n a t i o n
runs
in p a r a l l e l . D a t a
about
the
analog to d i g i t a l c o n v e r t e r a c t i v i t y obtained from
the
ADC unit in analog form are
converted
into
the d i g i t a l one. The
result
is put on the internal d a t a bus. The dead
time
indicator
scale p a t t e r n is also taken from the ROM as a s e p a r a t e c o n t r i b u t i o n
to the screen p i c t u r e .
Data
from
d i f f e r e n t sources are composed t o g e t h e r
video signal, which is used for the CRT beam m o d u l a t i o n .
into
the
The display board containing more than 100 i n t e g r a t e d c i r c u i t s
is too c o m p l e x to be p r e s e n t e d
here, even in block d i a g r a m form.
T h e r e f o r e , the partial block
diagrams will be given
in
the
following c h a p t e r s .
9.11.1
CRT Synchronizer
The
final p r o d u c t of the CRT synchronizer is FSYNC and LSYNC
signals. However, auxiliary
signals d e f i n i n g the w r i t i n g across
selected
parts of the screen, like G T 1 , GT2, CHEN and o t h e r s , are
also g e n e r a t e d . See Fig. 9.25 for the screen s t r u c t u r e .
(195)
Chapter 9
-24LDGD/DT METER
GRAPHIC FIELD
Fig.
CRT DISPLAr REF
9.25: Screen fields
g
All
signals are d e r i v e d from the basic clock
of
14.90 MHz
g e n e r a t e d by the c r y s t a l
oscillator.
A f t e r the division by t w o ,
the DTCK signal (Fig. 9.26), a v a i l a b l e at TP6, is o b t a i n e d
(dead
time clock). Presetable c o u n t e r A12
(74LS161) p r o d u c e s the cell
clock signal (TP7).
134 ns
CELL CK
J
r
Fig.
9 . 2 6 : Clocks
Through
the
further
d i v i s i o n by counters A10 and
All,
and
using the d a t a from ROM (IM 5610), the line s y n c h r o n i z a t i o n trigger
LST
is
formed. Passing t h r o u g h u n i - v i b r a t o r A16
(74LS221), the
line
synchronizing
signal LSYNC can be inspected at TP2. After
inversion in A92 (74LS37), it leaves the b o a r d as LSYNC.
In a n o t h e r
binary counter group A 2 5 ,A26 and A27
(74LS161A
all),
the CELCK s i g n a l is
divided
by 547. Under the name FST, it
t r i g g e r s two serial c o n n e c t e d u n i - v i b r a t o r s in A15 (74LS221 ) .
The
r e s u l t i n g signal F S Y N C can be c o n t r o l l e d at TP1.
The signal leaves
the DISPLAY B O A R D as FSYNC after the inversion in A92.
Inside
the CRT
s y n c h r o n i z e r u n i t , some a d d i t i o n a l
control
signals, like G T 1 , GT2
(dead
t i m e m e t e r gate), CHEN (character
gate), DGT (data gate), and SDMA (start d i r e c t memory access) are
also d e r i v e d . As i n p u t s to A9, ROM
IM 5610, signals from VERTICAL
CELL COUNTER (A10, A l l ) are taken.
A f u r t h e r d i v i s i o n of the field synchronizing signal provides
a 2.1 Hz b l i n k f r e q u e n c y for flashing messages. There is no test
point for it; it a p p e a r s at pin 6, A29.
9.11.2
Dead Time Indicator
The task of this unit schematically presented in Fig. 9.27,
r i g h t p a r t , is
the d e t e r m i n a t i o n of
the ADC dead time and the
g e n e r a t i o n of signals to show the result on the CRT screen.
The
circuit measuring the p e r c e n t a g e of the dead
time
h e l p i n g to present it, is given in sheet 8, DISPLAY BOARD.
(196)
and
Chapter 9
-25-
Ct- C*
-]
____________ » «*••
E!«J
__*]
SkM.
/Alt-<I>1
D IAO T»*C Cawwvm.
____J
(*CmO £7X«0» ID»'/. Bt)
>
*
»rvr r>irf s».B «.
Fig. 9.27: Synchronizers and t i m e r s
Cell clock CELCK (TP7) is counted
in the decade counter A30
during the i n t e r v a l s when ADC is busy. For a d e q u a t e gating, the LT
signal coming
from the ADC
board
is used, A97, pin 13. The
counting proceeds
in dual b i n a r y c o u n t e r s A29 and A 3 1 . Collect
time of s u b s e q u e n t runs are two frame p e r i o d s , which amount to 58.8
ms.
In
this way 4096 counts are c o l l e c t e d each time if the ADC
would be busy all the t i m e , i.e. if
the dead time p e r c e n t a g e is
100.
Only the most s i g n i f i c a n t 6 d i g i t s of the o b t a i n e d result are
used for the i n d i c a t i o n . If the dead
time is s m a l l e r , the number
of the c o l l e c t e d c o u n t s is p r o p o r t i o n a l l y smaller.
The o b t a i n e d r e s u l t i n g number stored in latch A32 is compared
w i t h the six lower bits LO, LI, L2, L3, L4 and L5 of HORIZONTAL
L I N E COUNTER in the 6-bit d i g i t a l c o m p a r a t o r A33, A 3 1 . The
result
of comparison (A34, pin 7) tells which of both numbers is bigger.
If this signal would be used for the CRT beam intensification, then
the p a t t e r n of 8 v e r t i c a l b r i g h t
raws would be seen over the full
screen.
The
w i d t h of the raws would be dead time d e p e n d e n t , but
the full screen is taken for presentation.
A d d i t i o n a l logic is i n t r o d u c e d to limit the p i c t u r e size. The
p a t t e r n is a c t i v a t e d only in the sixth row by applying L6, L7 and
L8
to the N A N D gate A28. The height is defined by the GT1
signal
d e r i v e d in the CRT S Y N C H R O N I Z E R applied to the same gate.
(197)
-26-
Chapter 9
Data Point V i d e o
9.11.3
During the scanning of the display f i e l d , the p r o p e r l y timed
burst
signals are required to draw the pattern. One of these beam
i n t e n s i f y i n g signals makes the p i c t u r e of the s p e c t r u m w i t h i n the
graphic
f i e l d . It is p r o d u c e d inside the d a t a point v i d e o section
(sheet 2 and 3 of the DISPLAY BOARD d o c u m e n t a t i o n ) .
S t a r t at sheet
2 and find the local bus lines ADO to AD7.
From the local d a t a b u s , 20-bit d a t a about the memory content
are
locked in A86, A88 and A89 at 0 to 1 transition of the BYTlCLC
(TP29), BYT2CLC (TP24) and BYT3CLC (TP25). For f u r t h e r processing,
only
8 of
20 b i t s are selected because of
the l i m i t e d
display
p o s s i b i l i t i e s (Fig. 9.28).
1-
LI
• *»•'
t**
"•—
|.
*»r,
0*
uY^I i, VIA
*—"
j....
CUT
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Fig. 9.28: Data p o i n t v i d e o
Selection is done by
multiplexer (A68,
A69),
which
c o n t r o l l e d by another m u l t i p l e x e r A67. M u l t i p l e x e r A67 allows the
selection of linear or
l o g a r i t h m i c d i s p l a y , and the selection of
the v e r t i c a l scale.
The conversion
to l o g a r i t h m i c
format
is
p e r f o r m e d in ROM A 7 1 , and the result is stored in latch A72 (drawn
in sheet 3, top).
Simultaneously, the c o u n t i n g
of DTCK clock signal (TP6) in 8
b i t s line resolution counter A49, A50 starts. The line
resolution
counter is periodically cleared by DGT (TP16). During c o u n t i n g ,
the e l e c t r o n beam
in the CRT climbs up along the line.
At
the
moment w h e n the c o n t e n t of the LINE RESOLUTION COUNTER is equal to
the channel content
number
stored
in A 7 2 ,
the 8-bit binary
(198)
is
-27-
Chapter 9
comparator A53, A54 delivers the pulse at the o u t p u t A=B (A54, pin
6).
This pulse
is used to intensify the e l e c t r o n beam at
the
proper t ime.
In t h i s way a b r i g h t
point
appears on the CRT screen at the
position whose height
is proportional to the number
of counts
stored in the channel.
The signal A > B generated
in
the same d i g i t a l c o m p a r a t o r
tells when
the number of the stored counts is smaller
than
the
c u r r e n t LINE RESOLUTION COUNTER content.
The A > B signal can be
used
to
b r i g h t the bar between zero line and the s p e c t r u m point
when this is r e q u i r e d .
Check the g r a p h i c field
contribution
to the video display at
TP13.
Notice:
to realize the o v e r l a p - f u n c t i o n , the 4-bit adder
A70
(74LS283) and the comparators A52/A53 are used. To achieve t h i s
mode, a constant
offset of 64 is a d d e d to RAM contents.
This
results in a second g r a p h i c d i s p l a y 25% l i f t e d .
9.11.4
S t a t u s Register and Data V i d e o
S t a t u s information is needed to provide the display logic w i t h
system v a r i a b l e s such
as
v e r t i c a l scale, e x p a n d , range, e t c .
Therefore, at the end of
each monitor
f r a m e , an
interrupt
is
introduced
through
the line RST
7.5.
Programme jumps
to subroutine u p d a t i n g the status register.
Related
schematics are at
sheet 5. The block diagram is shown in Fig. 9.29.
Inside the display board its own d a t a - a d d r e s s bus is c r e a t e d .
The
t h r e e - s t a t e b u f f e r A100 takes data from BADO to BAD7 lines and
repeats them as ADO to AD7.
Enable of A100 is c o n t r o l l e d by CHEN
(A10,
pin 1 or pin 19). Lines A8 to A15 are reproduced at o u t p u t s
of A80, also the three-state o u t p u t
b u f f e r . Three lines, BA8 , BA9
and
BA10,
d e m u l t i p l e x e d in 3 to 8 d e m u l t i p l e x e r A78 are
used
to
p r o d u c e the strobe signals RO, Rl, R2 and R4.
M u l t i p l e x e r A78 is
enabled
by OUT (output request generated w i t h i n the CPU
board).
OUT can be checked at TP23. By these signals applied to four 8-bit
latches A 1 0 1 , A103, A84 and A102, four b y t e s of the i n f o r m a t i o n can
be stored. R4
is used
to e n t e r
the d a t a
into A60, the dual
flip-flop.
Through some a d d i t i o n a l processing in two m u l t i p l e x e r s A83 and
A65, the c o m p l e t e d i s p l a y s t a t u s is c r e a t e d . From the l a t c h e d d a t a
you can learn what is the memory range (MÜ, Ml, M2), or the d i s p l a y
mode, or similar.
(199)
Chapter 9
-28-
*mC£S /«»•«
«><»
6»D*. ÛŒ>7
INTERNAL
BUS
tmr
Fig.
y.11.5
case.
tit*
n" C4W4OM«
*c* M ia »on
«cm tu
9.29: S t a t u s r e g i s t e r
C h a r a c t e r Display and V i d e o Mixer
C h a r a c t e r s are d i s p l a y e d in the dot m a t r i x f o r m , 5 x 7 in our
The c h a r a c t e r cell size is b i g g e r ; 8 x 10 d o t s (Fig. 9.30).
Characters
to be d i s p l a y e d are a c c e p t e d from the local d a t a
bus
(Fig. 9.32) during
the d i r e c t
memory access (DMA) cycle and
loaded
into
the 8-bit c h a r a c t e r l a t c h A91 (sheet 4)
followed
by
a n o t h e r 8-bit
latch.
In
this way
a two-byte
fast
m e m o r y is
created.
W h e n the second c h a r a c t e r is a c c e p t e d i m m e d i a t e l y a f t e r
the first one, the first
character
is
t r a n s f e r r e d into l a t c h A90
(Fig.
9.32).
ooooo oo oo oo
QO
O O O
Qo o o o o
000° °o
Qo o o o o
QO O O O o
o
ooooo o
ooooo o
ooooo o
oooooo
ooooo
Fig.
v
oo
oo
oo
oo
oo
oo
oo
oo
oo
9.30: M a t r i x r e p r e s e n t a t i o n
t
Fig.
9.31:V i d e o signal drawing
the first 1ine of E
Having
the c h a r a c t e r code at o u t p u t s (Al to A6) of
the ROM
A108,
the o u t p u t corresponding to the v e r t i c a l line p a t t e r n of the
required
character
appears at Bl to B8.
To
r e p r o d u c e E,
for
e x a m p l e , the thicker points should
be b r i g h t e n e d . When the beam
runs from b o t t o m to top of the screen and arrives at the l e t t e r E,
the p a t t e r n 1 1 1 1 1 1 1 should a p p e a r at the ROM o u t p u t . Then two 4-bit
(200)
Chapter 9
-29-
shift
r e g i s t e r s A106 and A 1 U 7 are loaded w i t h Bl to B8 data.
The
p a t t e r n is then p u s h e d out in the serial
form c o r r e s p o n d i n g to the
first v e r t i c a l line of the loaded c h a r a c t e r (TP30), and the c o m m a n d
shown in Fig. 9.31 is d e l i v e r e d at the C V 1 D line.
LOCAL
ADO-AD7
DATABUS
CHAR. GATE
DTCK
Fig.
SHIRT RCG.
CVID
y.32: C h a r a c t e r d i s p l a y block d i a g r a m
A f t e r t h i s , the second
character
is pushed down from A91 to
A9U
to draw
the p a t t e r n of the u p p e r c h a r a c t e r line.
Only
the
first line of b o t h characters was
used. T h e r e f o r e , we don't need
to change them yet but we have to keep t h e m . The lower
character
stored in A90 is r e t u r n e d
to A91
to be used again in the next
v e r t i c a l line. A f t e r using all e i g h t lines of b o t h c h a r a c t e r s , A91
and A9Ü are r e l o a d e d . However, to w r i t e the second v e r t i c a l line
of the same c h a r a c t e r the LO, LI and L2 a d d r e s s e s are increased for
one .
Signals CCK
(character
clock
enable) ensure proper
timing
in
c h a r a c t e r s blink.
TP17)
and CHEN
(character
data handling.
BOSC makes
Video
m i x e r , sheet 4, DISPLAY B O A R D , a c c e p t s
three
separate
TTL signals (CVID, NVID and 1VID) and mixes t h e m into v i d e o signal
V1D
(pin 8, 10 pins line c o n n e c t o r , MONITOR i n p u t ; use also v i d e o
g r o u n d , pin 10) serving for the CRT beam m o d u l a t i o n in MONITOR. The
c o n t r i b u t i o n of each signal in the composed signal is d e t e r m i n e d by
p o t e n t i o m e t e r s as follows:
C V I D , (TP30) drawing characters is set by RV 5;
N V I D , (TP13) used for the normal s p e c t r u m r e p r e s e n t a t i o n is
set by R V 3 ; and
IVID, (location A20, pin 4) used for the i n t e n s i f i e d lines
like the i n t e g r a t e d areas as set by RV4.
Check also the auxiliary DC voltage at o u t p u t s 1
quad operational amplifier A l l l .
8 and 14 of
Observe signals at collectors of Q6, Q5 and Q4.
They should
follow the c o r r e s p o n d i n g TTL
input
signals but the a m p l i t u d e of
signals depends on the R V 3 , RV4 and RV5 settings.
(201)
Chapter 9
-30-
Borriv.
Fig.
9.11.6
9.33: V I D E O mixer
D i r e c t M e m o r y Access Logic and A d d r e s s Generator
The d i r e c t m e m o r y access (DMA) is used to keep the CRT d i s p l a y
refreshed
and
updated
w i t h real time d a t a c o l l e c t i o n a n d
text.
This is d o n e once per each M o n i t o r v e r t i c a l line, during
the
flyback.
DMA logic g e n e r a t e s w r i t e and read
to r e l i n q u i s h bus c o n t r o l .
commands and asks the 8085
DMA a d d r e s s g e n e r a t o r d e t e r m i n e s the memory a d d r e s s from w h i c h
d a t a should be taken.
The
circuit
block diagram is given
in Fig. 9.34 while
c o r r e s p o n d i n g d e t a i l e d s c h e m a t i c is at sheet 6 and 7.
the
DMA r e q u e s t is sent to the p r i o r i t y encoder (sheet 5) r e s i d i n g
at
the CPU b o a r d . The DMA r e q u e s t signal DCHR1 can be checked at
TP15.
S i m u l t a n e o u s l y t h e m e m o r y a d d r e s s from w h i c h t h e i n f o r m a t i o n
should
be
provided
is set at the MUX (A81, A63, A98
and A99)
o u t p u t . The signal D C H A 1 c o n f i r m i n g
t h a t the DMA r e q u e s t has been
a c c e p t e d and e x e c u t e d is r e t u r n e d a f t e r 1 us.
DCHA1 is at TP26.
The c o n t e n t of the selected memory
l o c a t i o n a p p e a r s on the
bus
and
the local b u s . The c o n t e n t is locked in
BYTE
1
r e g i s t e r (Fig. 9.28, top l e f t ) by the BYTE l CLK (TP29).
system
The
next
memory
a d d r e s s is set and
c o r r e s p o n d i n g memory l o c a t i o n
is
latched
Check TP24 for BYTE 2 CLK.
(202)
the c o n t e n t
from
the
in the BYTE 2 register.
Chapter 9
-31-
The a c t i o n is r e p e a t e d to get the d a t a BYTE 3.
Check TP25 for
BYTE 3 CLK.
To run 3-stage ring
counter A 9 6 , A95
some a c t i v i t y at TP2Ü
(DATA VI) and CNDMA (pin 5, A74) s h o u l d be revealed.
For
the c h a r a c t e r
loading into 2 - b y t e memory
( A91,
sheet 4) signals CCK (TP 17) and CHLU (A26, pin 13) are used.
A90,
D u r i n g d a t a h a n d l i n g the p r o p e r RAM and ROM a d d r e s s e s have to
be s e t . See
Table 9.4
for
the r e q u i r e d
combinations.
The
c o r r e s p o n d i n g s c h e m a t i c s are shown in sheet 7.
CMHt DOMClfc
Fig.
9.34: DMA
TABLE 9.4: DMA a d d r e s s e s
AO
AI
A2
A3
AU
A5
A6
A7
A8
A9
AlO
AI 1
A12
A13
A1U
A15
8K
DO
Dl
0
D2
LO
Ll
L2
L3
Ll
L5
Le
L7
L8
BÖ
Bl
1
UK
DO
Dl
D2
LO
Ll
L2
L3
LU
L5
L6
Ll
L8
0
BÖ
ßl
1
W
2K
DO
Dl
LO
Ll
L2
L3
LU
L5
L6
L7
L&
0
0
BÖ
Bl
1
ft,
1K
DO
LO
Ll
L2
L3
LU
L5
L6
L7
L8
0
0
0
60
Bl
'
CsJ
O
|j
512
LO
Ll
L2
L3
LU
L5
L6
L7
L8
0
0
0
0
BÖ
Bl
1
s
256
Ll
L2
L3
LU
L5
L6
L7
L8
0
0
0
0
0
BÖ
Bl
1
128
L2
L3
LU
L5
L6
L7
L8
0
0
0
0
0
0
BÖ
Bl
1
CHAR
CHO
L3
LU
L5
L6
L7
L8
0
0
0
0
0
0
1
1
1
(203)
Chapter 9
9. 12
-32-
MISCELLANEOUS BOARD
The miscellaneous board is responsible for various a c t i v i t i e s
such as
c o m m u n i c a t i o n with the keyboard and communication w i t h
e x t e r n a l units t h r o u g h the serial interface RS-232 or TTY.
H o w e v e r , the most important task is the m a t h e m a t i c a l operation
during the pulse height analysis (PHA). As it has bean previously
m e n t i o n e d , the ADC board presents the h e i g h t of the input pulse in
the binary code. This binary number is the address of the memory
cell (RAM, residing at the CPU
board) in which the content should
be incremented by 1.
9.12.1
Collect Part
This a d d i t i o n is p e r f o r m e d on the miscellaneous board within
the p a r t schematically given in sheet 4 and 5. See Fig. 9.35 for
the block diagram.
Buffered lines BADO to BAD? (sheet 4) are seen as UDO to DD7
at A-side of the b i d i r e c t i o n a l b u f f e r A81 (74LS245).
Data flow
d i r e c t i o n is d e t e r m i n e d
by signal at pin
l, A81. When data is
p r e s e n t - i n d i c a t e d by DATA-V2 (sheet 5) - it is l a t c h e d into A37 an
8-bit latch 74LS273. O u t p u t s of A37
are connected to A - i n p u t s of
8-bit summer A 2 5 , A38 (two 74LS283 connected serially). In the PHA
(PHA
line, pin 2, A 4 7 , high) m o d e , the a d d i t i o n of "1" is achieved
by applying high s t a t e to the carry
input
(pin 7, A38) from
f l i p - f l o p A7 while B-inputs are g r o u n d e d .
The number of counts for each
bytes. See A P P E N D I X for details.
channel
are contained in three
SYSTEM BUS
JL
X MUX A46,A50
^
A48,A52
A33
A44
DDO-DD7
A81
LATCH
A37
A2S
A38
STATUSREG
1
D Q
A35
A47,A49,A51
CONTROL
FF
LER
DMA REQ.
I/O
Fig.
(204)
DMA ACK.
1 ADDR
DMA
CYCLE
ADC
MISC.
MISC.
sheet 4
sheet 4
sheet 5
9.35: Collect part of the Miscellaneous Board
-33-
Chapter 9
In the first s t e p , "1" is added to BYTE 1. The result is put
in the f l i p - f l o p A7 . The m o d i f i e d
BYTE 1 is r e t u r n e d to the same
memory
cell
through
d e m u l t i p l e x e r s A50, A52 (sheet
4)
and the
bidirectional b u f f e r A81.
N e x t , BYTE 2 is inserted and the procedure is repeated.
Then
BYTE 3 is t r e a t e d .
However, only four LSB are a part of
T h e r e f o r e , by
a p p l y i n g BYTE 3 signal to the gate A12 and A24, the sumation
is
l i m i t e d to four LSB only.
the n u m b e r r e p r e s e n t e d by BYTE 1, BYTE 2 and BYTE 3.
If
during
the processing of BYTE 1 or BYTE 2 carry
p r o d u c e d , f u r t h e r sumation is meaningless and is s t o p p e d .
is
not
When the DMA cycle is o v e r , HOLD command is removed and 8085
is r e t u r n e d to the normal o p e r a t i o n through line RST 5.5.
All
commands controlling
the d a t a flow and
the d e s c r i b e d
a r i t h m e t i c are generated by the circuit containing A l O , A5, A6, A7,
A8, A12, A24 and A65 (situated in sheet 5, left down).
We
have assumed up to now t h a t BYTE 1, BYTE 2 and BYTE 3 are
a v a i l a b l e . However, for
proper
selection the a d e q u a t e address
should be given to RAM.
The
p o s i t i o n of BYTE 1, BYTE 2 and BYTE 3 w i t h i n the RAM
is
given in Fig. 9.36.
For
proper a d d r e s s i n g , the address line A15
should
be
1 all the time when communicating with RAM
(ROM
is
a d d r e s s e d by A15 = 0).
A13 and A14
have
to be set according to
Table 9.5.
TABLE 9.5
A15
A14
A13
BYTE 1
1
0
0
d e t e r m i n e d by base a d d r e s s and range
BYTE 2
1
0
1
d e t e r m i n e d by base address and range
BYTE 3
1
1
0
d e t e r m i n e d by base address and range
A12
.............................AO
The above-mentioned address lines are controlled by flip-flops
A13 through three-state buffer A65 (74LS367, sheet 5).
During the
DMA-cycle
the CPU data bus is t r i - s t a t e d and e x t e r n a l users have
d i r e c t access to the RAM.
Check TP5
for A13 by using DCHA2 as the
trigger.
The
end of the DMA cycle is indicated by DMEND signal
from monostable A5 and triggered by the signal t h a t can be observed
at TP4.
(205)
Chapter 9
-34-
U n t i l now we have talked a b o u t the ADC-gain of 8291 channels.
F r e q u e n t l y we
use
less c h a n n e l s
to have
the possibility for
r e g i s t e r i n g several spectra. In such cases the s t a r t i n g address
also d e p e n d s on the base a d d r e s s .
For a v a i l a b l e base a d d r e s s e s and
their
c o d i n g , see
Table 9.6. The c u r r e n t base a d d r e s s
is kept
w i t h i n the miscellaneous board.
0000
003F
Interrupts 64
0040
Progrim
(Read Only)
16K
3FFF
4000
Program
Bank 0
(Read Only)
16K
7FFF
8000
Data Byte 1 (LSB)
2K. 4K. or BK.
9FFF
AOOO
Data Byte 2
2K. 4K, or BK
BFFF
COOO
Data Byte 3 (MSB)
2K, 4K, or 8K
DFFF
E 000
V a r i a b l e s and
Parameter Stack
3.75K
EF7F
EFBO
Return Stack
126
FOOO
Reserved
FFFF
Fig.
y.36: M e m o r y map
TABLE 9.7
TABLE 9.6
Base
B4
B3
B2
Bl
BÜ
Ü
Ü
Ü
0
Ü
Ü
Ü
Ü
Ü
0
0
Ü
Ü
u
l
l
0
0
0
l
Ü
256
512
768
1024
2048
3072
4096
6144
U
0
Ü
0
0
0
u
l
l
1
1
0
l
l
Ü
0
Ü
Ü
Ü
l
Ü
Ü
Ü
0
0
R2
Rl
RO
0
0
0
0
l
l
0
0
0
l
0
0
0
0
l
l
l
l
Range
256
512
1024
2048
4096
8192
For a p r o p e r d i s p l a y , the m e m o r y a d d r e s s s h o u l d n o t r u n h i g h e r
number
than the s t a r t i n g address plus t h e n u m b e r o f c h a n n e l s . T h e
(206)
-35-
Chapter 9
the
of channels is also kept within the miscellaneous board under
name range. See Table 9.7 for the c o d i n g . The range code is used
by
the
l i m i t logic on s h e e t 4 (A44, A 5 5 , A56 and A57) to p r e v e n t
the AUC from generating an address larger than the memory assigned
to it .
Where
to f i n d , and how to use the base a d d r e s s and the
range
code?
Miscellaneous
board
(sheet 4)
contains its own
status
r e g i s t e r . You can find l a t c h e s A47, A49 and A5l (74LS273, 74LS174,
74LS273) p r o v i d i n g 22-bit
d a t a storage. The last, 8-bit
latch
74LS273 also c o n t r o l l e d by the miscellaneous board
is m o v e d into
the AUC board
(sheet 4, A35). The lowest 5 bits are
the base
a d d r e s s while the t h r e e h i g h e s t b i t s
r e p r e s e n t range code (RO, Rl,
R2).
In A 4 7 , A49 and A 5 1 , i n f o r m a t i o n
is s t o r e d a b o u t the c u r r e n t
o p e r a t i n g mode
(PHA,
MCS)
and o t h e r
system v a r i a b l e s .
i n f o r m a t i o n can be sent to DDO to DD7 lines t h r o u g h the t h r e e - s t a t e
multiplexers
74LS257 (A46, A47, A48, A50). Such c o n f i g u r a t i o n
called I/O p o r t in the c o m p u t e r language.
This
is
D a t a a r e w r i t t e n into o u t p u t l a t c h e s w i t h signals d e l i v e r e d a t
o u t p u t s of A62 (two-to-four line d e c o d e r ) and a c t i v a t e d by the 8085
OUT
c o m m a n d (pin 5, A63). Data from p o r t s are read when the 8085
IN
signal
is given
(pin 9,
A63).
The
port
addresses are
transmitted
through
lines BA8 , BA9 , B A 1 2 , B A 1 3 , B A 1 4 and
BA15.
O u t p u t lines P03 ( o u t p u t ) and PI3 ( i n p u t ) c o n t r o l the p o r t in ADC.
(207)
Chapter 9
-36-
APPENDIX 1
To count up to 2^0
counts each channel has to be three bytes
long.
The
four highest
b i t s of the t h i r d b y t e , not
used
for
channel c o n t e n t , p r o v i d e
the
information
about
the special
t r e a t m e n t for this channel:
bit 21, if s e t , the c o r r e s p o n d i n g point is i n t e n s i f i e d ;
bit 22, if s e t , the bar is d i s p l a y e d instead of the p o i n t ;
bit 23, if s e t , indicates the channel w i t h the biggest
content ;
bit 24, if s e t , channel is w i t h i n the region of interest.
(208)
Chapter 10
INTERFACES
-110
INTERFACES
10.1
GENERAL
C h a p t e r 10
All information delivered in the following chapters assumes
some basic knowledge of interfaces and of Personal Computers
(PC).
It is d e v o t e d
entirely
to problems you will be faced with when
selecting interfaces or working with t h e m .
The three major g r o u p s
of
interfaces
which
w i l l be covered
are :
(i)
Standard Interface Used for Data C o m m u n i c a t i o n
the serial interface RS232 C/V24
the parallel interface Centronics
(ii)
Standard Interface Used for Remote Control of Instruments
(iii)
the IEC 625 (IEEE-488, GPIB) interface
Non-Standard Interface for Remote Control of Peripherals
I/O p o r t s
Motor control
10.2
SELECTION OF INTERFACE
Whenever
questions.
you have to select an interface, ask yourself a
a.
What is available at my laboratory?
b.
Who is going to be the controller (master) of my system?
few
Is it a PC?
Is it a special d e d i c a t e d
device which only p r o v i d e s d a t a
like a multichannel analyzer (MCA), s e a l e r / t i m e r , d i g i t a l
m u l t i m e t e r (DVM)?
c.
What am I going to connect (slave)?
-
Is
it a data
terminal?
communication device like a
Am I going
to control
sealer/timers, MCAs?
different
printer
instruments
or
a
like DVMs ,
Do I have to control peripherals such as sample changers or
motors ?
(211)
Chapter 10
-2-
Are Interfaces already a v a i l a b l e in the master?
If "YES", can I use them for the job I am going to do?
-
If "NO",
market?
what
is
available from the company or
Is an i n t e r f a c e already available
am going to connect?
on
the
in the device (the slave) I
If interfaces are available in both the master and the
slave,
are they c o m p a t i b l e ?
ATTENTION;
10.3
Make your decisions very carefully; you will have
less trouble later on.
SERIAL INTERFACE RS-232C/V24
The a p p l i c a t i o n of the RS-232 interface is widespread in
mainframes, m i n i c o m p u t e r s , microcomputers, p r i n t e r s and all
types
of terminals.
It is used for a
SERIAL
BINARY
DATA
INTERCHANGE
between two
devices. The basic principles of
the standard
are
implemented in such
devices; however, various options may occur
from
device
to device.
These variations become
extremely
significant when interfacing
different
combinations of computers,
printers and
terminals.
A general rule may
be established as
follows.
NOTE ;
When connecting between devices, make sure that an
o u t p u t signal goes to an input signal and vice versa.
First of all, terms which are used for this interface must
explained :
DTE -
DATA TERMINAL EQUIPMENT like c o m p u t e r s ,
terminals or printers
DCE -
DATA COMMUNICATION EQUIPMENT like modems
be
As one can see
from above, two different
types of devices
exist which can be connected together. The first case to connect a
DTE to a DCE should not create any problem; you simply connect them
straight
through. This is because each o u t p u t pin on a DTE has a
corresponding input pin on the DCE.
Signal flow is shown from the
DTE side.
(212)
Chapter 10
-3Lead name (abbreviation)
PC-Pin (DTE)
Modem-Pin (DCE)
P r o t e c t i v e Ground (PG) ---------->
Transmitted Data (TD) ----------->
Received Data (RD) <-------------Request to Send (RTS) ----------->
Clear to Send (CIS) <---_--------Data Set Ready (DSR) <-----------Signal Ground (SG) -------------->
Data Carrier Detect (DCD) <------Data Terminal Ready (DTR) ------->
1
2
3
4
5
6
7
8
20
1
2
3
4
5
6
7
8
20
The second case to connect two devices from the same t y p e ,
namely two DTK's t o g e t h e r , can bring you into trouble.
Therefore,
The
kind
of cable we have to use is
let's do it step by s t e p ,
called a "NULL MODEM CABLE
GROUND:
connect them straight through
Function
PCl-Pin (DTE)
PC2-Pin (DTE)
Funct ion
PG
1 <•
•>
1
PG (Protective Ground)
SG
7
•>
7
SG (Signal Ground)
DATA LEADS
Function
<•
Pin 2 is used for transmitted data and pin 3 for
received data. Data are t r a n s m i t t e d over pin 2 from
one machine and received on pin 3 at the other.
To
allow for proper data transmission and reception at
b o t h machines, cross pin 2 on one end with pin 3 on
the other end.
PCl-Pin (DTE)
PG
1
SG
7 <•___
TD
2
RD
3 *~ "
PC2-Pin (DTE)
<___________>
"~-.--
Func t ion
l
PG (Protective Ground)
7
SG (Signal Ground)
_____ 2
TD (Transmit Data)
^~~^ 3
RD (Receive Data)
______>>
Data
terminal e q u i p m e n t (DTE)-provided signals are all that
are present in a null-modem cable.
This limitation forces us to
provide
DCE signals w i t h available DTE signals. S p e c i f i c a l l y , the
DTE signals (RTS and DTR) must L« used
to p r o v i d e or e m u l a t e the
DCE-provided signals (DSR, r.TS and DCD). Data Terminal Ready (DTR)
pin 20 is ordinarily p r o v i d e d by the DTE to indicate t h a t power is
on
at the terminal.
For
an
indication
that
the line
is
e s t a b l i s h e d , the DCE normally gives a signal on pin 6 Data Set
Ready
(DSR).
As
long as DSR is on, one can assume t h a t DCE
is
a v a i l a b l e for d a t a transmission. If pin 6 is not p r e s e n t , the line
or connection
is not available. To emulate DSR (pin 6) at b o t h
e n d s , we s t r a p the DTR signal (pin 20) at one device across to pin
6
on
the other device. The same strapping is done in the other
(213)
Chapter 10
-4-
direction. By strapping pin 20 across to pin 6, whenever DTR is
high
(the machine power is on), the other end will get indication
t h a t the transmission line
is available.
If power
is o f f , the
o t h e r end will not have ÜSR, indicating that the c o m m u n i c a t i o n p a t h
is not established.
Function
PCl-Pin (DTE)
PG
1 <•
SG
7 <•
TD
2
PC2-Pin ( D T E )
Function
PG (Protective Ground)
•> 7
SG (Signal Ground)
2
TD (Transmit Data)
RD
RD (Receive Data)
DSR
6
DTR
20
DSR (Data Set Ready)
20
DTR (Data Terminal Ready)
The other element of the control function on the interface is
path control. Request To Send (RTS) pin 4 is normally generated by
the DTE. For data transmission to be a l l o w e d , Clear To Send (CTS)
pin 5 must be received by the same DTE. So we loop the RTS signal
back to the o r i g i n a t i n g
DTE
by
wiring
it back
to pin 5 (CTS).
Whenever the DTE - for example PCI - raises RTS, it immediately
receives a CTS
signal i n d i c a t i n g that data transmission
is now
possible. As for the need
of
the receiving device
to have an
i n d i c a t i o n that
data will be arriving, we must provide
for Data
Carrier D e t e c t (DCD) pin 8 to be derived from the same source, RTS.
Thus we also connect RTS (pin 4) at the originating DTE (PCI) to
the Carrier D e t e c t lead (pin 8)
from receiving d e v i c e . By making
these cross connections, not only will a CTS signal be given, but
when RTS is raised, the other end will also receive its DCD signal,
indicating
that
data
transmission
is possible.
Repeat
these
c o n n e c t i o n s at both DTE's to allow two-way transmission.
Func t ion
PCl-Pin
(DTE)
PC2-Pin
PG
! < - - - -- - - - - - > 1
SG
7
TD
2
RD
3 <f~""
DSR
6
DTR
20
RTS
4
CTS
5 «— *-^ <--""
DCD
8 ^"
(214)
<
__
Funct ion
PG (Protective Ground)
.__ —— > 7
SG (Signal Ground)
__-
2
TD (Transmit Data)
"""•^
3
RD (Receive Data)
--7
6
DSR (Data Set Ready)
20
DTR (Data Terminal Ready)
4
RTS (Request To Send)
5
CTS (Clear To Send)
8
DCD (Data Carrier D e t e c t )
^_
"" ~"
(DTE)
^~"^"-
— i
——
^
-5-
Chapter 10
The path control requirements have been met in the null-modem
cable. As we have mentioned above, two different types of devices
(DTE
and DCE) are existing, so we have to ASCERTAIN THE "SEX" OF
THE EQUIPMENT before we can connect them together.
The best way to determine whether an equipment using RS-232
ports is configured to emulate DTE- or DCE-provided signals, is to
review the device documents.
Consult the user's manual for this
information.
If documentation is not available, a break-out box
may be used to determine which leads are provided by a device. For
specifications of a break-out box, see Chapter 3, Section 3.2.17.
Connect the break-out box to the RS-232 port and make sure
that the device is powered and the port in question is active or
enabled. The LED's on the box should display which leads are being
generated from the device. From this display a determination can
generally be made as to whether the device is emulating DCE or DTE;
green LED's indicate negative voltage, red LED's indicate positive
voltage .
REMEMBER;
RS-232 output levels for
LOGIC 0 (SPACE) between +5V to +15V
LOGIC 1 (MARK)
between -5V to -15V
RS-232 input levels for
LOGIC 0 (SPACE) between +3V to +15V
LOGIC 1 (MARK)
between -3V to -15V
INPUTS are ENABLED when positive, DISABLED when negative
OUTPUTS are ASSERTED when positive, FALSE when negative
We will ascertain the "SEX" of the computer first. If the PC
is a DTE device, pin 2 should be the transmitter, and its negative
voltage will illuminate the green LED. The receiver terminal, if
left unconnected, may illuminate an LED
if a pull-up resistor is
present.
If pin 20, Data Terminal Ready, or pin 4, Request To
Send, is on, the port is more likely emulating Data Terminal
Equipment and is expecting to be connected to a modem or a device
emulating DCE signals. On the other hand, if the display shows
that signals such as Clear To Send (pin 5), Data Set Ready (pin 6),
or Data Carrier Detect (pin 8) are present, the port is probably
emulating DCE and will allow a straight-through cable to be used
when connecting a terminal configured as DTE, as if connecting to a
modem.
If you don't have a break-out box, the levels can
determined by an oscilloscope or a DVM.
also
be
If you want you can build up a LED Voltage Detector by
yourself to test your interface pin by pin (Fig. 10.1)
(215)
Chapter 10
-6-
Red LED
FLAT SPOT
Green LED
470 ohm
V
Fig.
BLACK
RED
10.1: Simple LED voltage detector
To avoid confusion, it is best
to use
charts to show the
status of the seven pins. For example, use NEC for negative, POS
for p o s i t i v e . Inactive pins that illuminate neither LED should not
be
left
blank - otherwise you may not know if you have already
tested the p i n . Instead, use an X to represent any undefined logic
state.
Beside each number is the a b b r e v i a t i o n of its name as well
as an 0 for o u t p u t and I for input. When the connector is c h a r t e d ,
you'll have a good idea of what control logic is being used.
Two
examples, one for a DTE and one for a DCE , are given.
DCE:
DTE:
Pin
Pin
I/O
I/O
Volts
3 RD
0
NEC
2 TD
2 TD
3 RD
I
X
4 RTS
0
POS
4 RTS
I
X
5 CTS
I
X
5 CTS
0
POS
6 DSR
I
X
6 DSR
0
X
8 DCD
I
X
8 DCD
0
X
20 DTR
0
POS
20 DTR
I
X
The
important factor is that all requirements of the
with regard to pins' being on or off, must be met.
(216)
ports,
-7-
Chapter 10
You can easily test how the control inputs are working.
If
you apply a negative voltage (< -3V) to DSR (pin 6) of the DTE, you
disable it.
The cable between the d e v i c e s to be connected is
important for
successful
implementation.
However, after a cable
has been b u i l t
or
s u p p l i e d , a n u m b e r of
other
items must
be
c o m p a r e d and set properly before the interfacing will be complete.
Following, you will find a checklist for
found on c o m p u t e r s and peripherals.
options
generally
Item
Opt ions
Speed
Flow control
Parity
character length
# of stop bits
Mode
Echoplex
Line feeds
Transmission mode
75 bps to 19,200 bps
ETX/ACK, XON/XOFF, Hardware
Odd, Even , None
5, 6, 7 , 8 bits
1, 1.5, 2
Simplex, Half-Full duplex
Yes, No
0, 1, 2, CR implies LF, LF implies CR
Asynchronous, Synchronous
Positive, N e g a t i v e
Polarity
In s u m m a r y , all items should be
checked for proper optioning
to p e r m i t successful installation. Both devices have to be
SETUP
to m a t c h each other.
For
example, if one d e v i c e is set to 4800
bps,
the o t h e r one has to be set to the same b a u d r a t e . The
same
has to be done with all other
items
listed above. In most cases
these changes can be done either by DIP-switches l o c a t e d
on
the
i n t e r f a c e board or by software in a PC.
Let us have a closer look at
the basic hardware of such an
interface.
Normally you have data on a parallel data b u s , in most
cases 8-bits wide. This data has now to be converted into a serial
one
which is done in a so-called USART (Universal Synchronous
Asynchronous R e c e i v e r
Transmitter)
This c i r c u i t also takes care
a b o u t the h a r d w a r e handshaking lines as shown in Fig. 10.2.
RS232 29-pin connector
pin W for mod«m/computer
Fig.
10.2: Parallel to serial conversion with a USART
(217)
Chapter 10
-8-
C o m p u t e r RS-232 interface supports via USART (Fig. 10.2), DTR,
DSR, RTS and CIS control
lines.
The
25-pin connector
pins are
labeled
from the terminal's point of view,
computer labels appear on the USART.
Note
works
with
or
without
the control
handshake lines are pulled up.
while the inverted
that this interface
protocol,
since
the
input
Most of the interfaces work in this
way, so you only need to define pins 2 and 3.
connect the ground pins 1 and 7.
Fig. 10.3 shows the widely used
receiver
includes
a
hysteresis
s e n s i t i v i t y . The + /- supply v o l t a g e
be s y m m e t r i c . Positive voltage
can
n e g a t i v e from -3 to -15v.
But don't forget
to
TTL to RS-232 converters. The
input
to reduce
its noise
for the 1488 does not have
to
range from + 7V to + 15V and the
it) «tceivtm
(«I OftlVIN
, 0 IS
MCMM-0.tf.MM in
Fig.
10.3:
Pin-out of RS-232 drivers and receivers
As a t y p i c a l example, we
MCA
35+ from Canberra.
will
take
the interface board of a
Fig. 10.4 shows you the
where A69 is the USART,
receiver.
A57
is
the
circuit
transmitter,
diagram
and A67 is the
One t y p i c a l example is to connect a MCA like Canberra 35+ to a
PC, in our case an I B M - c o m p a t i b l e .
The
MCA is set as DCE, the PC as a DTE.
In this case you can
use a cable w h i c h is connected s t r a i g h t through.
(218)
PC
Pin
Pin
MCA
TD
2
2
RD
RD
3
3
TD
CTS
5
5
DTR
DSR
6
6
DSR
DCD
8
8
DCD
I > pulled to +5V
through 3k
-9-
Chapter 10
aß
Fig. 10.4 :
Interface circuit diagram for the RS-232 port of
a MCA 35+
(219)
-10-
Chapter 10
ATTENTION:
1
|
1
1
Don't connect Pin 20; it can damage your driver
in the PC!
Next you have to provide the options in your c o m p u t e r to the
setup chosen on the serial interface in the MCA. You will find a
detailed description on the setup of the interface board in the
Operators Manual of the MCA. Take care: you cannot exceed a
b a u d r a t e higher than 4800 b p s , because no handshake is s u p p o r t e d .
Some hints if you run
into trouble
interface are presented in Table 10.1.
when
installing
your
TABLE 10.1: Partial list of p r o b l e m s with an RS-232 interface
Symptom
Equipment
Cause/Remedy
No data is being
displayed or
printed
Terminal or
printer
Check to be sure that power is on.
Ensure that device is in on-line
mode. RS-232 requirements may not
be satisfied.
Polarity of signals could be improperly set. Both devices should
have the same setting. Speed of
ports may not be set the same. Big
difference in baudrates.
Device controller or driver circuits may be d e f e c t i v e .
Cables may not be plugged snuggly
into port or may be broken.
Printer
Possibly out of paper.
Printer l i d , if raised,may inhibit
further printing.
Computer or terminal port to which
printer is attached may be incorrectly configured as DCE or DTE;
verify this. Printer ribbon may
be d e f e c t i v e or worn o u t ; replace
it .
Computer port driving the printer
may not be e n a b l e d , check address
of t he port.
(220)
-11-
Chapter 10
Garbled or lost
data
Terminal,
printer or
computer
Port speeds may not be consistent.
Cable could be faulty.
Character l e n g t h could be wrong.
Flow control may not be occurring.
Parity may be improperly set. Cable
too long or too high c a p a c i t y for
selected Baudrate --> max. length
15 ra!
Communicat ion
line cannot be
established or
ma int ained
Terminal ,
p r i n t e r or
computer
Power may be off on the device.
This would disable DTR (pin 20)
which should not allow the connection to be made. Device or port
must be in on-line mode to keep
DTR on and maintain the line.
Duplex should be the same at each
end.
C o m p u t e r at the far end may
a u t o m a t i c a l l y disconnect if it
determines that the call is being
made by an invalid user.
Cable between the devices may be
faulty or not p r o p e r l y wired.
Improper spacing
Double spacing
Terminal or
printer
Line feeds; see if c o m p u t e r outputs a line feed with each
carriage return. If so, option
for zero line feeds at the device
or option the computer to only
o u t p u t a carriage return.
No spacing
Terminal or
printer
Computer is only o u t p u t t i n g a
carriage return, while the d e v i c e
is not adding a line feed. Option
either the computer or the device
to add a line feed with each
carriage return.
If you look with an oscilloscope to the t r a n s m i t t e d or
received signals, don't expect very fast leading or trailing edges.
10.4
PARALLEL INTERFACE CENTRONICS
This type of interface is mostly used to connect p r i n t e r s to a
PC.
Synchronization
is done by STROBE pulses supplied by
the
t r a n s m i t t i n g d e v i c e , in our case
the PC.
Handshaking takes place
through ACKNLG or BUSY signals. Data and all
interface control
signals are compatible with TTL
level.
Both the rise and fall
times of each signal must be less than 0.2 us. As to the wiring
for the i n t e r f a c e , be sure to use a twisted pair cable for each
signal and
never fail to complete connection on the Return side.
To prevent noise e f f e c t i v e l y , these cables should be shielded and
connected
to the chassis of the host computer and
tha p r i n t e r
(221)
Chapter 10
-12-
respectively.
Interface cables should be kept as short as possible
to avoid p r o b l e m s (max. length 4ra).
Most of
the input and o u t p u t drivers are
very
sensitive
against connecting devices together while power is a p p l i e d . They
may be damaged.
Below you will find the pin assignment and
description
signals, seen from the p r i n t e r side, in our case an EPSON.
Signal
Pin No.
1
2-9
of
Re t urn
Pin No.
Signal
Direction
19
STROBE
In
Used to strobe (Latch)
data in. A pulse > 0.5 us
at receiving terminal is
required.
DATA 1-8
In
Eight TTL-compatible data
lines. Each has its own
signal ground return for
use w i t h twisted pair
cables.
20-27
Description
10
28
ACKNLG
Out
O u t p u t pulsed low for
approx. 12 us and indicates t h a t d a t a has been
received and that the
p r i n t e r is ready to accept
more data.
11
29
BUSY
Out
"HIGH" indicates that the
p r i n t e r cannot r e c e i v e
data. The signal becomes
"HIGH" in the following
cases: during d a t a e n t r y ,
d u r i n g p r i n t i n g operation,
in OFF-LINE s t a t e , during
printer error status.
12
30
PE
Out
A "HIGH" signals that the
p r i n t e r is out of paper.
13
SLCT OUT
Out
A "HIGH" signals that the
printer is in the selected
s t a t e , in most p r i n t e r s
pulled up to +5V through
3k3.
14
A U T O FEED XT
Out
When this input is low,
the paper is automatically fed one line after
print ing.
15
NC
16
0V
(222)
No t used.
In
Logic ground level.
Chapter 10
-1317
GND
18
NC
Out
Printer chassis ground.
Normally isolated from
logic ground.
Not used.
GND
In
Twisted-pair return signal
ground level.
31
INIT
In
When this input is "LOW"
the p r i n t e r controller is
reset to its i n i t i a l state
and the p r i n t buffer is
cleared. A pulse of > 50us
is r e q u i r e d .
32
ERROR
Out
This o u t p u t goes "LOW"
when the printer is in
PAPER END s t a t e , OFF-LINE
s t a t e , or ERROR state.
33
GND
In
Same as pin 19 to 30
34
NC
35
ON
Out
Pulled up to +5V through
3k
to i n d i c a t e the +5V
supply.
36
SLCT
In
Data entry to printer is
only possible when this
signal is "LOW".
19-30
Not used.
If
you are going to make your own cable to connect a p r i n t e r
You will find instead of a
to an I B M - c o m p a t i b l e PC, be careful.
36-pin c o n n e c t o r , a 25-pin one, similar
milar to the one used for RS-232.
will help you to find the right
The c i r c u i t
diagram
(Fig. 10.5)
signals on the PC b o a r d . At the same t i m e , it will show you how a
p a r a l l e l interface is b u i l t
up.
The
8-bit
d a t a b u s is connected
over t r i-s t a t e-dr iver c i r c u i t s d i r e c t to the connector. To transmit
d a t a U l o , U4 , U 1 1 and U16
are
used.
To receive d a t a U9, U 1 6 , U8
and Ul are used.
The
following
timing
diagram
(Fig. 10.5) shows you
the
relation b e t w e e n STROBE, DATA, ACKNLG
and BUSY SIGNAL. You can
check
it w i t h a normal dual
beam oscilloscope.
As mentioned
b e f o r e , it is
very
important that
rise and
fall times of each
signal must be less than 0.2 us.
BUSY
x 05 M î(Mm)
Fig.
10.5: Parallel interface timing
(223)
n
•o
rt
fD
r-l
/ / / / / / / / / / S
Fig.
10.6:
/ / / 7 / / / / / / / / / / / / S / / / / / / / /
IBM p a r a l l e l i n t e r f a c e
/~
Chapter 10
-15-
The
In Table 10.2 are some useful hints for troubleshooting,
device which is connected will be in our case a printer.
TABLE 10.2
Partial list of problems with a Centronics parallel
interface
Symptom
Cause/Remedy
No data is be ing
printed
Check to be sure power is on.
Ensure the device is in on-line mode.
Check all signals according to the above
specifications. Computer port driving the
p r i n t e r may not be enabled; check the address
of the board. Refer to PC manual. Printer lid
if raised, may inhibit f u r t h e r printing.
Printer ribbon may be d e f e c t i v e or worn,
replace it.
Garbled data
Cable could be faulty.
Timing of STROBE signal incorrect, too early or
too late.
Cable too long --> max. leng t h 4m.
Communication line
cannot be established or ma intained
For
manua1.
10.5
Power may be off on the p r i n t e r .
Cable between the devices may be faulty or not
properly wired.
detailed
printer d e s c r i p t i o n , refer
to
the
printer
IEC 625 (IEEE-488, GPIB) INTERFACE
The
general purpose interface bus (GPIB) is a link or network
by which system units communicate with each other. It is widely
used for automation in measurement and control applications.
The
bus allows the connection of up
to
15 devices to a system. Each
system p a r t i c i p a n t performs at least one of three roles:
CONTROLLER,
TALKER, or
LISTENER
A CONTROLLER manages the bus communication p r i m a r i l y
by
directing or
commanding which device (TALKER) is to send d a t a to
other devices, or to receive data
from other devices (LISTENER)
during an
operational sequence. A controller can be
interrupted
and it can command devices to interact d i r e c t l y among themselves.
The GPIB consists of 16
sets according to function:
lines
which
are
grouped into three
(225)
Chapter 10
-16-
8 lines used for data (bit parallel, byte serial, data
t ransfer )
3 lines used for control (provide a data transfer handshake
c o m p a t i b i l i t y with both slow and fast devices)
5 lines used for general m a n a g e m e n t (allows initialization,
i n t e r r u p t s , and special controls)
All i n s t r u m e n t s are connected in parallel to the bus of such a
system via s p e c i a l cables.
The pin assignment
be low :
Pin No.
and
description
of
signals is presented
|
|
|
|>
j
|
|
Lines are also used
to transmit commands.
When ATTENTION (ATN)
= 1, d a t a on the
lines i n t e r p r e t e d as
c o m m a n d . When ATN=0,
d a t a are i n t e r p r e t e d
Signal
Description
1
2
3
4
13
14
15
DIO
DIO
DIO
DIO
DIO
DIO
DIO
Data
Data
Data
Data
Data
Data
Data
16
DIO 8
Data bit 8 (highest) _|
17
REN
Remote Enable: will be a c t i v a t e d when system
is act ive
5
EOI
End or I d e n t i f y : E O I = 1 , ATN=0 -> last b y t e of
a datablock
E O I = 1 , A T N = 1 -> parallel poll
9
IFC
I n t e r f a c e Clear:
Resets the whole system
10
SRQ
Service R e q u e s t :
has same p r i o r i t y for all
instruments in a system.
Is
active whenever an instrument
r e q u e s t s a service. Controller will interrupt and s t a r t
with a serial poll to d e t e r mine and satisfy the requestor.
11
ATN
Attention:
1
2
3
4
5
6
7
bit 1 (lowest)
bit 2
bit 3
bit 4
bit 5
bit 6
bit 7
ATN = 0 Data is t r a n s m i t t e d
ATN
= 1 Commands t r a n s m i t t e d
6
DAV
Data V a l i d :
7
NRFD
Not Ready For Data:
(226)
as d a t a .
i n d i c a t e s the data on the bus is
valid
is sent from the instruments to indicate that
they are not ready to
accept data
Chapter 10
-17-
No Data Accepted
NDAC
is sent from the instruments
that valid data on the databus have not been overtaken
yet .
SHIELD
12
GND
18-23
24
LOGIC
GND
DID 1 ... 8
rJor
DAV
6OME
[i!T~
NRFD
'ReftD/
«t l
ffi'i
1i ! i 1
SOME
f 1 1 11
i t 1 t
1 1 1 l
NDAC
Fig.
MO r*/-sfc^
TOOK- T>rtr*
i
OLfER l
,*
ii
i
?> t
i i
i i
—> *
i OOh
!f
10.7: Timing diagram for handshake
As an example, we will take the Canberra count er/1imer 2071
with the installed GPIB-option (see Fig. 10.8).
Before you can
operate the sealer in a GPIB system, you have to configure the
interface through eight DIP-switches .
The first five switches from
the top are
used to set the
units' GPIB address. The address is the sum of all switches in the
ON position. DON'T select an address already used in your system.
Also, address 31 is illegal; it is reserved for the GPIB command
UNTALK.
The sixth switch is TALK ONLY.
At this point you have to decide whether your sealer works
with or without a controller. The TALK ONLY mode is used to
transfer data from a single module to a single peripheral listener
device, such as a GPIB-prin t er.
If the TALK ONLY switch is o f f ,
the controller must assert REN (REMOTE ENABLE) during the entire
communieat ion.
The seventh switch is RECYCLE. In the ON position, the unit
In the OFF
will clear after readout and start counting again.
position, the unit is in the single cycle mode.
(227)
C h a p t e r 10
-18-
The
e i g h t h switch is FF/CR. In the FF position, the message
unit d e l i m i t e r is sent
out as an ASCII Form Feed. In the CR
position,
it is sent out as a Carriage R e t u r n . The choice of
the
d e l i m i t e r d e p e n d s upon the p e r i p h e r a l device b e i n g used.
Let
us
controller :
take
a
closer
look
at
the
interaction
with
a
A
t y p i c a l c o u n t i n g sequence would begin w i t h
the operator
pressing s t a r t on the Counter.
A f t e r reaching p r e s e t , the Counter
will g e n e r a t e a SRQ (Service R e q u e s t ) via ECOL (END COLLECT) from
count er/1imer.
The
controller
would
t h e n , via
Serial Poll,
determine
the device requiring
readout.
During
this polling
s e q u e n c e , the c o n t r o l l e r will assign each of the devices to be
polled
a Talk a d d r e s s , and
t h e n the device will put out
a
STATUS-byte.
In our c a s e , the
Counter
is the r e q u e s t e r and would
o u t p u t the S t a t u s 42H and remove its service r e q u e s t .
not have b e e n the r e q u e s t i n g
device,
If it
would
it would o u t p u t a S t a t u s 00.
The
controller
c o m p l e t e s the s e r i a l poll mode by
doing a Serial
Poll D i s a b l e (SPD). If the C o u n t e r
is the only possible source of
the
Service
R e q u e s t , the c o n t r o l l e r can
respond
immediately
by
r e a d i n g the d a t a from
the m o d u l e , which also clears its Service
R e q u e s t . To read d a t a from the C o u n t e r , the c o n t r o l l e r r e s p o n d s to
the S e r v i c e R e q u e s t t h r o u g h the Ready
For Data (RFD) line via the
Acceptor Handshake.
Read Clock E n a b l e (RCE)
is a c t i v e once MY TALK
A D D R E S S and Read Clock (RCL) is
sent
to the c o u n t e r to clock the
bytes
from the c o u n t e r s during S3 (output sequencer). After each
LSB (Least S i g n i f i c a n t Byte), the o u t p u t s e q u e n c e r a c t i v a t e s SA to
gate
the message
unit d e l i m i t e r to the bus
(separates counter
o u t p u t s ) . A f t e r S4
of
last
counter
S5
became a c t i v e and gates
ASCII Line
Feed and EOI to the bus (End Message). A f t e r w a r d s LB
(Last B y t e ) is a c t i v e and t e r m i n a t e s the TALK MODE.
Following you w i l l find a Block Diagram (Fig. 10.8), a Circuit
D i a g r a m (Fig. 10.9) and a d e s c r i p t i o n of the signals (Table 10.3).
Troubleshooting
in such a system is l i m i t e d because of
its
c o m p l e x i t y . F i r s t , in many
i n t e r f a c e s you will find large scale
i n t e g r a t e d c i r c u i t s w h i c h s u p p o r t you with all signals used for the
GPIB. Most of t h e m are set up and p r o g r a m m e d via software. Second,
without
a
logic analyzer, you are in great
difficulty.
Timing
r e l a t i o n s among the signals are
essential, and
they can only be
a n a l y z e d by using such an i n s t r u m e n t .
So w h a t to do if your GPIB system doesn't work, and you
have a logic analyzer? Let's go s t e p by step:
1.
Look to see
if power
c o n n e c t e d to the GPIB.
2.
Check
if the interface cables are
p l u g g e d in c o r r e c t l y .
3.
Check the addresses to which each device in the system is s e t ;
d o n ' t use the addresses twice.
(228)
is
applied
to
in
all
good
of
don't
your devices
condition
and
CONNECTOR
TO
S C f l L E R / T l n E Pî
CLOCK
^-j^
GENERATOR
HS/B7
—————
,
-12V
20
+12V
19
GND
18
RMTRS
17
CND
16
RCL
15
STP
14
ST
5
Q pn
13
i
IP
BCD 2
11
D r* n u
t ci
p pp
o
Q / Q
ENflBLE
(CD
CLp<
•-
RESET
*u!
CIRCUIT
<IFC
R1B/R21/
flE3
+-<Q
" SET
HBRT
- -
ABRT
S3 "C
*-<j| s,
RCYL
MV TALK ADDRESS LATCH
A8/AZ3/AZ3/AZS/AZT/AZ«
q
Q
7
C
ECOL
5
pr c
u
ECOL
3
+ 5V
2
+ 5V
1
——TON
*— £^VdÜÜÜ
P —————
OUTPUT D A T A
SEQUENCER
—————————*• ——
R8/All/A18/A2a/AZ7
GPIB
CONNECTOR
n
•y
ta
•O
(T
Fig.
N>
VD
1 0 . 8 : Block d i a g r a m of
counter 2071
GPIB interface
for
Canberra
(5
n
ET
»l
T3
n
n
n
i
NS
O
__
Fig. 10.9: Circuit diagram
of GPIB interfacing for Canberra
c o u n t e r 2071
-21-
TABLE 10.3:
Chapter 10
GPIB option functional description
The
following twelve
functions, i d e n t i f i a b l e on
the GPIB
interface s c h e m a t i c , are described
in terms of the function inputs
and o u t p u t s with brief d e s c r i p t i o n s of the o u t p u t functions.
Oumuti
CK
SAG'
CLOCK
«SFT
RESET
OAV
OATA VA1O
BUS GENERATED
ATM
Ompu«
ATTENTION
BUS GENERATED
READY FOR DATA
ACTIVE WHEN READY TO RECEIVE THE
NEXT BYTE FROM THE BUS
ACTIVE AFTER DECODED BYTE HAS BEEN
ACCEPTED BY THE MODULE
ACTIVE FOR ONE CLOCK PERIOD DUB KG
STABLE AND VALID DATA STROBES ALL
MODULE DECODERS AND LATCHES
«re
OAC
FROM RESET LOGIC
SERIAL POIL
RESTONS*
RSFT
CLOCK
T£
RESET
DATA ACCEPT
READY FOR DATA
TALK ENABLE
TED
TALK ENABLE
S3
«LAV
STATE}
OAC
RFO
FROM NTERNAL CLOCK
FROM RESET LOGIC
BUS GENERATED
BUS GENERATED
FROM TALK DELAY GENERATOR
FROM TALK DELAY GENERATOR
B>TE CLOCK
•I Output DfjU Sequencer end Geimg
inputs
TALK ENABLE
TAKE IT
SERIAL POLL
ROY
SERIAL POLL MODE FROM SERIAL POLL MODE LATCH
BYTE CLOCK
FROM SOURCE HANDSHAKE THING
READY
FROM SOURCE HANDSHAKE TMING
RCYl
RECYCLE
CR/FF
ASCII OS OH FF
FROM OUTPUT DATA SEQUENCER
GENERATED TO SYNCHRONIZE NTERNAL
CBOJITS ON EACH OUTPUT BYTE
NACTIVE DURNG THE PER CO WHEN
STATE AND BYTE CHANGES ARE OCCUR
ON THE RbNG EDGE Of NEXT CK AFTER
DIGITS
BCD«
BCD«
Output!
GENERATES DATA VALID TO THE BUS
ACTIVE NOCATED BYTE RECEIVED ON
THE BUS GENERATED BY DAC AND
READ CLOCK
TERMNATE TALK MODE
CHANGES TO NEXT L SB ON THE FALLING
END OR
EDGEOFRC1
IDENTIFY
RESTARTS
31 Te* Orb* Generator
CK
ATN
MTA
Outputs
16
TED
aocK
TALK ENABLE
ACTIVE FOR TALK STATE
SYNCHRONIZES BUS TO NTERNAL
tnput»
FROM MY TALK ADDRESS LATCH
CLOCK FOR CORRECT TWMG
MY ADDRESS
BYC
BYTE CLOCK
FROM MY ADDRESS DECODER
BUS GENERATED |10|
FROM ACCEPTOR HANDSHAKE TMNC
HVIT
HAVE IT
FKOM SOURCE HANDSHAKE T MMG
FROM SOURCE HANDSHAKE TMIMG
IB
LAST BYTE
ABRT
TON
ABORT
ECO.
ENDOFOOUECT
OVAL
Output!
DATA; «
TALK ONLY MOOt
FROM OUTPUT DATA SEQUENCER
FROM RESET CIRCUIT
SKHJ
FROM COUNTER BOARD
BCE
READ CLOCK
OUTPUT TO COUNTER BOARD
MTA
ENABLE
MY TALK ADDRESS
TO ENABLE RCL
ACTIVE AFTER BUS COMMAND
MTA MACTIVE AFTER DTA OH UNT BUS
COMMANDS CLEARED ON LB AND HVfT
il M, Auixn» Dtcaarr
tnputi
Dl D5
SX) 1
DATA 1 5
ADDRESS
SX i SWITCHES
DATA VALID
FROM ACCEPTOR HANDSHAKE TMrgG
REM
REMOTE
Dl 07
Oulpull
DATA I 7
FROM REMOTE LATCH
BUS GENERATED
BUS ADDRESS SELECT SWITCHES
I to It
NOTE AII«»«ctiM on ttmvmi
DEVICE CLEAR
CK
CLOCK
RSET
MA
RESET
MY ADDRESS
DVAL
DATA VALIO
D6. D7 DATA I 7
Output!
REM
REMOTE
MY ADDRESS
FROM NTERNAL CLOCK
FROM RESET CIRCUIT
FROM MY ADDRESS DECODER
FROM ACCEPTOR HANDSHAKE T WING
BUS GENERATED 07 O6
[10OR01]
ACTIVE AFTER HAVING RECEIVED MY
LISTEN OR MY TALK ADDRESS FROM THE
BUS NACTIVE AFTER RSET
it) RnetCmxl
Input!
POWER ON
NTERFAOE CLEAR
TALK ONLY
REMOTE ENABLE
ACTIVE AT POWER ON
BUS GENERATED
mOMSJO-6
BUS GENERA TED
Outputl
ABRT
ABORT
RSET
RESET
31}«
Output!
MA
WHEN ENABLED SENDS A STOP PW SC 'O
THC COUNTER ON A OCL COMMAND
FROM THE BUS 07 01 10010100]
toi fiairiule Leicri
Input»
fCi
TON
REN
BUS GENE RATED
BNARY WEIGHTED
DVAL
DO.
DATA VALID
O7 M
(ENDMESSAGEl
IS SELECTED
BUS GENERATED
M» T«» A«KMMLAlcri
mpuu
MA
DRIVES BUS COMMAND LNE EOl
PULSE SENT TO THE COUNTER BOARD
DURNG STATE 5 If RECYCLE
FROM NTERNAL CLOCK
ATTENTION
MY TALK ADDRESS
TALK ENABLE
DELAY (Alj 2i
SENT MOST SIGNIFICANT DIGIT FIRST
OUTPUT GATES ASCII L»JE FEED AND EOl
TO THE BUS IEND MESSAGE I
ACTIVE AT THE END Of OUTPUT TO
SENT TO COUNTER TO CLOCK TK BYTES
FROM THE COUNTERS IN S3 DATA
Inputi
DURING DATA OUTPUT CONTAIN THE
COUNT DATA SYNOOONIZED TO B»C
ACTIVE AFTER S« Of LAST COUNTfO
VAL O TO THE END OF BYC FORCED BY
TE, TED OH ROY TO PREVENT BYC
RCl
ACTIVE OURNG AN LSB FROM THE
COUNT ER BOARD
ACTIVE HIGH WHEN THE FIRST COUNTER
S BENG SENT AND LOW FOR OTHtN
COUNTERS CHANGES ON THE F ALL ING
ACTIVE FOR ALL STATES E»CEP1 SPM S*
OR Si GATES BCD DATA WITH ASCII
HEADER 13« HEJI) If HOY AND TE
TO THE BUS
ACTIVE AFTER LSB TO GATE MESSAGE
UNIT DELMITER TO THE BUS (SEPARATES
COUNTER OUTPUTS!
S3
IALLOWNG DATA TO SETTLEI AND RFD
NACTIVE AFTER OAC
HAVE IT
SELECTS EITHER CARRIAGE RETURN OR
FORM FEED AS A MESSAGE UNIT
FROM THE COUNTER BOARD
CODED
GOES ACTIVE AFTER A DELAY TMC
HV1T
RECYCLE MODE SELECTED SJO 7
EDOEOFLSB
BCD'
BCD:
BYC GOES INACTIVE
TAKE IT
FROM TALK DELAY GENERATOR
FROM SOURCE HANDSHAKE T MMG
FROM SERVICE REQUEST LOGC
RESPONSE
LEAST SKXIF
CANT BYTE
CTR AendBnol
RING GOES NACTn/1 ON DAC ACTIVE
TKIT
SRO TO GATE SERIAL POLL
DEL MITER SJO*
Outputi
BYC
ACT WE AFTER ECC UNTIL TL N
REMOTE ONLY
ACTIVE IF SPM WOB TO TE AND
RESPONSE TO THE BUS
7l Source rtendehefce trrMng
CK
SERVICE REQUEST
fROM NTERNAL CLOCK
ACT IVE AT POWER ON OR DURNG If C
CLEARS MY TALK ADDRESS AND SERIAL
POL L MODE LATCHES
ACTIVE AT POWER ON OR DuHiNG HEN
RESETSALL HANDSHAKE CIRCuiTSAfrfJ
THE REMOTE LATCH if TON SELECTED
ACTIVE FOR 101'D5 I- ISXMTOS20SI
MONITORS REN TO PREVENT BUS CON
FLCT WITH AN ACTIVE CONTROLLER
61 &••«•< POM Mod» L»lef
mputi
O< 0?'
DVAL
ABHT
Oulpull
SPM
1?) lote<n»l Oort
DATA 1 7
BUS GENERATED
DATA VALID
ABORT
FROM ACCEPTOR HANDSHAKE TMING
FROM HtaCT CIRCUIT
SERIAL XXL
MODE
ACTIVE AFTER SPE COMMAND
FROM THE BUS D7 07
THE CLOCK CIRCUIT GENERATES THE TMNG FOR PROPER OPERATION C*
NTERNAL AND EXTERNAL CIRCUIT REQUIREMENTS
Pol«*« Inter»« — GREATER THAN 3 *MC
t Interval - GREATER THAN I uiec
001100 AND Dl
01 NACTIVE AFTER SPD COMMAND- 07
CO
OOliaOANDDl 0
7) S*<y«c Rcovjnt Log«
MpuB
REM
SPM
REMOTE
SERIAL POLI
FROM REMOTE LATCH
FROM SERIAL POLL MODE LATCH
MODE
TE
TALK ENABLE
ECOL
END Of COLLECT
FROM TALK DELAY GENERATOR
FROM COUNTER BOAF<0 FOR ANY
STOP COLLECT
(231)
Chapter 10
-22-
4.
If e v e r y t h i n g was correct, disconnect all devices and try
with a single one.
it
5.
If you don't have success, try another cable, another unit.
6.
If after all these tests your controller still does not work,
open it and check if the board is installed properly.
7.
Measure the supply voltage of the board.
8.
Take an oscilloscope and check the signals (TTL-level).
9.
If no
signals after the GPIB-transceivers a p p e a r , check if
signals appear before
the
transceivers - if n o t , your
controller is faulty.
For further diagnostic tips, see Table 10.4.
TABLE 10.4:
List of problems t y p i c a l for a GPIB interface
Symp t om
Cause/Remedy
Device receives wrong commands, transmits
wrong results or status information,
cannot be addressed.
One or more d a t a l i n e s ,
NRFD, NDAC i n t e r r u p t e d
Controller doesn't receive Service Request
Continuously receives Service R e q u e s t
Device cannot be programmed
Device cannot receive commands
No remote control
No parallel poll possible
Cancels receiving after first received
data block
Bus blocked
SRQ
SRQ
ATN
ATN
REN
EOI
EOI
10.6
or tied too low (bad
bus transceiver)
interrupted
tied too low
interrupted
tied too low
interrupted
interrupted
tied too low
DAV interrupted
I/O PORTS
Most microcomputers
incorporate
some
form of parallel
input/output
(PIO) facility.
While an increasing number of
microprocessors provide this as a built-in f a c i l i t y , parallel I/O
invariably takes the form of one, or more, LSI devices known as a
peripheral interface adaptor (PIA). Such devices generally provide
separate 8-bit ports, in which 8-bit lines can be configured, under
software control, as input or o u t p u t .
The interface from the PIA
to the CPU usually consists of
eight data lines, two or more address_1 ines, and difjE_e_rent control
lines like Chip Select (CS),Read (RD), and Write (WR). The data
lines are. of course, bidirectional whereas the address and control
(232)
Chapter 10
-23-
lines are unidirectional and form a subset of the system. The PIA
thus appears as a number of specific memory or I/O addresses which
may be selected by appropriate software instruction^. The_PIA also
utilizes the CPU control bus when, for example, a RD and WR signal
is used to determine the direction of the data flow from/to the
PIA.
In a d d i t i o n , bidirectional buffers are used to interface the
peripheral lines to the PIA. These
buffers are generally
TTL-compatible and
provide
limited
current
drive
capability,
typically in the order of 1mA.
As a typical example for a PIA we will take the 8255, which is
see
also
called
programmable
peripheral interface
(PPI)
Fig. 10.10.
PA3C 1PA2C
PAlC
PAO:
ÏÏ5C
C5C
ovC
A1C
AOC
PC7C
PC6C
PC5C
PC4C
PCOC
PC1H
PC2C
PC3C
PBOC
PB1C
PB2C 20
Fig. 10.10:
^
40
I]PA4
D PAS
DPA6
3PA7
3WR
3 RESET
Doo
DD1
U D2
D 03
DD4
DOS
Doe
D 07
3 »5V
DPB7
DPB6
DPBS
3PB4
21 DPB3
Pin-out of 8255 programmable
peripheral interface
As an example of the use of PIA devices, we will consider the
operation of a keyboard decoding arrangement, see Fig. 10.11. On
most computers, the keyboard consists of a matrix of 60 or more
switches, with the possible addition of further switches reserved
for specific functions.
The key matrix is arranged in eight
columns and sixteen rows. Ports A and B are configured as inputs
while port C is configured as an o u t p u t .
Note that only half of
port C is utilized and that the four output lines are taken to a
four-to-sixteen line decoder.
This device effectively scans the
keyboard rows, addressing each in turn as the binary count on port
C is cycled through its sixteen states under software control.
This process is repeated every 10ms and an appropriate interrupt is
generated when a key is pressed.
This interrupt is done by a
return signal appearing on a column line.
Note that special
function keys, such as "SHIFT" and "CONTROL" do not form part of
the matrix. These higher priority keys are treated separately as
direct inputs to port A.
(233)
C h a p t e r 10
-24-
n n n °n- ^"^^ * !hi|< ^yi
== =
r
i
-^
^ in
xi_ .______
:—. F -*-ov
0
•—T—-p——°""°—=—i
PA3
PA2
PA;
PAO
07-
06050403020100-
PB 7
PB6
PBS
07
06
05
04
03
02
01
DO
AlAO-
i—— ——o
o————————*
-5V
3k 3 « a
—0-
—<h
P92 —<h
PB 1 —<h
PB4
P83
8255
RÊSËT-
I I ! . S V
Functi
1
I1
PBO
RESET
ÇS
PC3
PC2
PCI
PCO
Al
AO
16 * 8 keyboard matrix
220n
HH
«5V0V-
YO
Y15
74159 decoder
DC 9 A
EN
• ov
Fig. 10.11:
K e y b o a r d coding arrangement using a 8255 PPI
Now
let us
assume t h a t there is something wrong with your
keyboard i n t e r f a c e ; how do you
start
t r o u b l e s h o o t i n g . First, try
to find out if only one of a group of keys is not working or if the
whole k e y b o a r d
is not working.
This may be seen by pressing
related keys
in
the keyboard. If your whole k e y b o a r d
is not
working, check the supply voltage.
Take an oscilloscope and have a look if scan pulses appear on
PCO
to PC3. If they do, look at the d e c o d e r o u t p u t to see if this
is working. Pulses should appear on lines 0 to 15. Next check if
the pulses are c o n n e c t e d t h r o u g h to the i n v e r t e r s and PBO to PB7 by
pressing the r e l a t e d keys. Check the special function keys at PAO
to PA3.
If you find that up to now e v e r y t h i n g works correctly, you can
assume t h a t your controller is d e f e c t i v e . But before changing it,
make some more tests on the bus side of the controller.
If
you find t h a t on the p e r i p h e r a l device the keyboard works
properly and that writing to the PIA is correct, then there may
be
something wrong with the read control line.
If RD
appears as
w e l l , we may think
in two a l t e r n a t i v e s : first
the PIA is b a d ,
second it was wrongly initialized so that the ports are not set
p r o p e r l y . M a y b e your software is causing the trouble. So w r i t e a
(234)
-25-
Chapter 10
small test programme which initializes the PIA and so that you can
test each PIA line separately.
10.7
STEPPER-MOTOR DRIVE
Frequently, an interface application will require that the PC
controls the motion of an object.
A device that is often used to
power or move a shaft in precise increments, directions and speeds
is a stepper motor.
Here we consider two approaches to this
problem. The first uses the stepper motor, a device whose angle of
rotation is known reliably from
the pulses which have been sent to
it.
Once c a l i b r a t e d and initialized, no feedback of the rotor's
position is necessary, unless the speed demanded is too high or the
torque required
is too great. Running a motor this way w i t h o u t
feedback is called OPEN LOOP. The
second m e t h o d uses f e e d b a c k , is
a CLOSED LOOP approach, and is called a servo system.
The servo
system can respond more quickly and accurately than the open-loop
stepper motor system and is relatively
insensitive
to hardware
variations. However, it requires position sensors as well as more
complicated drive electronics to ensure stability.
Next we have to distinguish between two d i f f e r e n t ways of
driving the motor. The first m e t h o d is to implement the stepping
sequence by hardware. We have only to send o u t p u t strobes, and a
signal for direction pulses for the coils of the motor are
generated by the hardware.
The hardware implementation has the
advantage of relieving the CPU from dedicated timing loops; also it
is safe even if the CPU crashes.
The CPU only has to know when to
send
the next pulse. This is mostly done on an i n t e r r u p t driven
basis .
The second method
to drive stepper motors is completely
handled
from the CPU by I/O Ports (PIA) and c u r r e n t drivers.
The
CPU has to provide, via
software,
the correct phases for
appropriate lengths of time.
A typical example for
the
system is shown in Fig. 10.12.
second
method
using an OPEN LOOP
Troubleshooting in such a system is very simple. You can only
check if power is applied to the drivers and if pulses are coming
out of the PIA and passed through the drivers. If there are no
output pulses provided by the PIA, refer to S e c t i o n 10.6.
(235)
Chapter
10
-26-
-O A
-O B
CPU
.
output <
ports
Ik2
>f
-OC
Stepper
motor
-O D
lt<2
Open-collector
driver array
Current
limited
+24V supply
Fig.
10.12:
I Common
-*——o „
Open loop transistor driving circuit for 4-phase
stepper motor
In the other case, a CLOSED LOOP system, things become more
complicated.
A simple CLOSED LOOP system is represented in Fig.
10.13.
/n/.«* fo
• Mo/or Coi<s
fror*
Portion Encoder
Fig.
10.13:
Closed loop stepper motor driver with position
encoder
There are two possible faults.
The first is the driving
c i r c u i t , as described before. The second is the position encoder
(236)
-27-
Chapter 10
which indicates the momentary
position
of your driven system. It
strongly depends on the encoder which kinds of signal are fed back
to your controlling device.
If you find out during troubleshooting that the driving circuit
works c o r r e c t l y but the position is
incorrect, you have to check
the
feedback
loop. Depending on the position of the encoder
in
use, check if the o u t p u t signals are correct.
10.8
CONCLUSION
Dealing with interfaces, especially if they don't work, can be
a tough job. N e v e r t h e l e s s , you have to do it; a few additional tips
for troubleshooting are listed below.
1.
Never
start
manuals; a lot
operation.
your
of
job before you have read the operating
result
from
a wrong setup and
problems
2.
Before you start
troubleshooting on boards, study the circuit
diagrams; if they confuse you, try to draw functional
block
diagrams for it makes your job easier.
3.
Write
down everything you are doing; maybe the next day
won't remember what you d i d .
4.
Follow the signals from the destination to the source.
5.
Make
timing
t ogether .
6.
If possible try to implement your own simple test software to
perform
step-by-step
testing.
It makes everything
more
transparent.
diagrams
to
learn
how
signals
are
you
related
(237)
Chapter 11
DEDICATED INSTRUMENTS
-1-
Chapter 11
DEDICATED INSTRUMENTS
11
Previous chapters are devoted to the instruments that are used
NIM modules and M C A . Two
in nuclear
research
laboratories:
in
other
applications
are
examples of
the i n s t r u m e n t s used
described below.
SURVEY METER
11.1
11.1.1
Fields of A p p l i c a t i o n
Ionizing radiation dose-rate meters, or simply survey meters,
are the most important measuring instruments in r a d i a t i o n safety.
We have no sensory organ to g u i d e us on the i n t e n s i t y of ionizing
radiations present
in our environment due to n a t u r a l or a r t i f i c i a l
sources. W i t h o u t proper measuring systems one m i g h t be exposed to
h e a l t h d e t e r i o r a t i n g r a d i a t i o n levels.
Survey m e t e r s are used in radioactive-ore p r o s p e c t i n g as well.
If the instrument is not
functioning according to s p e c i f i c a t i o n ,
the ore may not be located, and the n a t u r a l resource would remain
hidden.
It is important to realize that
if the survey meter is not
functioning p r o p e r l y , we might be harmed by r a d i a t i o n or loose ore
fields. The p r o b l e m is that
sometimes the f a u l t
develops very
slowly, w i t h o u t dramatic signs. The repair staff
and
the user
should know how to check
the p e r f o r m a n c e of
the survey m e t e r to
overcome the hazards to h e a l t h and p r o p e r t y .
11.1.2
C o n t r o l s , Turning On and Quick Functional Checks
On most survey meters a single multi-function rotary switch is
used to :
- turn on and turn off the instrument, battery checking
is the first p o s i t i o n after the s w i t c h e d - o f f state;
- c o n t r o l the measuring range selection.
Some designs have ZERO-SETTING and CALIBRATION controls as well.
xl
xlO
xlO2 xlO 3
Q3
TEST
ON
PROBE
C/S
Fig.
1 1 . 1 : Front panel of a t y p i c a l r a d i a t i o n m o n i t o r
(241)
Chapter 11
-2-
In ore p r o s p e c t i n g , the use of energy-selective measuring
channels is r a t h e r common.
The
front
panel of a t y p i c a l survey
meter with a s c i n t i l l a t i o n d e t e c t o r is shown in Fig. 11.2.
ISOTOPE SELECTOR
ENERGY
WINDOW
no o
MANUAL
CRM
RANGE
o
MODE SWITCH
O
Fig.
11.2:
PRESET
ON
TEST
OFF
Front panel of a t y p i c a l survey-meter
Controls
for
the energy and the i n t e n s i t y range s e t t i n g are
common with the single channel analyzers.
The r a d i a t i o n intensity
can be read from either a moving-coil meter or a d i g i t a l display.
The
first step of the quick f u n c t i o n a l test is the same
for
both design v a r i a n t s :
the instrument
should
be t u r n e d into the
b a t t e r y checking position, The indication should be: BATTERY OK,
or similar.
If this is not so, one should use the STOCK FAULT LIST
given in section 11.5.
If the BATTERY OK c o n d i t i o n was i n d i c a t e d , the second step can
be started by following the calibration i n s t r u c t i o n s of the manual:
adjusting
the controls to the
a p p r o p r i a t e ranges after
the
i n s t r u m e n t was zeroed. Then the c a l i b r a t i n g r a d i o a c t i v e source of
the unit should be put before
the d e t e c t o r at a given distance.
The
readings
should match with those given in
the calibration
c e r t i f i c a t e of the unit.
If this
is not so, one should use the
STOCK FAULT LIST in section 11.5.
W h a t should you do if you have
data on e x p e c t e d readings?
no c a l i b r a t i o n i n s t r u c t i o n s or
If
the
instrument
arrived
without
any
such data
i n s t r u c t i o n s , you
should
inform the s u p p l i e r and ask
replacement under the warranty.
and
for
If the ins t r u m e n t has a l r e a d y been w i t h you for a long t i m e ,
you mus t set up your own testing fac ility . For this you need
a s m a l l ac t i v i t y , Cs-137 or Sr-90 r a d i o a c t i v e source. You
mus t have i n f o r m a t i o n on the present a c t i v i t y or the source ,
wh ich shou Id p r e f e r a b l y be
b e t w e e n 15-150kBq, or according to
the Radia tion Safety R e g u l a t i o n s of your area, If you know
t h e a c t i v i ty of the source and
its
d i s t a n c e to t h e d e t e c t o r ,
the dose- rate can be calculated with acceptable accuracy for
such types of quick tests.
(242)
Chapter 11
-33.
The dose-rate readings should match the calculated values with
less than +/- 50% difference.
4.
This checking, however, is a very
rough
functional one,
indicating only that the instrument is o p e r a t i n g , so could not
substitute
the calibration of the instrument in your National
Metrological I n s t i t u t e or e q u i v a l e n t .
5.
If your
instrument was
recently c a l i b r a t e d in
the N a t i o n a l
M e t r o l o g i c a l I n s t i t u t e , it is a good p r a c t i c e to take readings
in standardized geometry w i t h your
own reference source, and
to use this data in future quick functional tests.
If you can
cover more
intensity
ranges,
the
better.
Your
"work
investment" will pay off in reduced t r o u b l e s h o o t i n g time.
11.1.3
Operating Principles of Systems with Various Detectors
All
survey meters have an ionizing r a d i a t i o n d e t e c t o r
and
c i r c u i t s to convert
the d e t e c t o r - s i g n a l - c a r r i e d
i n f o r m a t i o n into
dose-rate
information and a display unit. The survey m e t e r s are
b a t t e r y - o p e r a t e d , p o r t a b l e units.
With a few e x c e p t i o n s , they
contain DC/DC converters for the d e t e c t o r s u p p l y , as well as
for
the low vol tage.
DETECTOR
HIGH VOLTAGE
REGULATED SUPPLY
Fig. 11.3:
11.1.3.1
SIGNAL
PROCESSING
DISPLAY
LOW VOLTAGE
REGULATED
SUPPLY
BATTERY
Block diagram of a survey meter
Dose-rate meter with ionization chamber d e t e c t o r
(Fig. 11.4)
In this system a DC/DC converter supplies a few h u n d r e d
volts
for
the ionization chamber.
In most
cases,
the low v o l t a g e
electrode of the chamber is connected to a d i f f e r e n t i a l a m p l i f i e r
with FET inputs. The
phase-inverted
output
of this amplifier is
fed back to the common point of the chamber and the input
of
the
amplifier through a very high ohm value, specially designed and
manufactured resistor. Such a circuit configuration can convert
very l i t t l e current
into voltage.
The
differential amplifier
improves the temperature s t a b i l i t y characteristics of the system.
By changing the value of the feedback resistor, the current-tovoltage conversion rate changes as well; the smaller the resistor
value, the smaller will be the voltage o u t p u t of the amplifier for
the same current. Sometimes the o u t p u t of the a m p l i f i e r drives the
(243)
Chapter 11
-4-
moving-coil meter d i r e c t l y ; in some other designs an a d d i t i o n a l
bridge
circuit
serves the same purpose. Linear
and
logarithmic
scale v e r s i o n s are a v a i l a b l e to d i s p l a y
the dose-rate. The ZEROSETTING control p o t e n t i o m e t e r acts on the non-inverting
input of
the d i f f e r e n t i a l a m p l i f i e r .
In some designs a low pass f i l t e r
smooths
the m o v e m e n t of the moving coil m e t e r , and other circuits
p r o t e c t it from o v e r l o a d i n g .
Gn. RESISTOR
Fig. 11.4:
11.1.3.2
Diagram of a survey meter with ionization chamber
Survey m e t e r w i t h Geiger-Mueller tube (Fig.
11.5)
A DC/ÜC c o n v e r t e r s u p p l i e s the Geiger-Mueller tube with
its
operating v o l t a g e .
On the
load resistor of the tube, voltage
pulses
are
developed
for each
ionizing
i n t e r a c t i o n if
the
a p p r o p r i a t e "dead-time" has
elapsed
since the p r e v i o u s d e t e c t i o n ,
l e n g t h , and
The
pulses need to be s t a n d a r d i z e d in a m p l i t u d e and
m o n o s t a b l e c i r c u i t s are used for
this
purpose.
In some designs,
the
pulse l e n g t h d e t e r m i n i n g components (R or C) are m o d i f i e d
on
d i f f e r e n t dose-rate ranges, so
the signals after integration can
directly
drive
the moving-coil meter resulting
in a
linear
d o s e - r a t e scale. In some systems after
the i n t e g r a t i n g a m p l i f i e r ,
a
l o g a r i t h m i c a m p l i f i e r is e m p l o y e d to give a three - four
decade
d i s p l a y span.
In order
to improve the shock
resistance of the
system in new designs, they often use d i g i t a l counters with
timers
instead of the moving-coil meter.
INTEGRATOR
GM-TUBE
PULSE
NORMALIZATION
DETECTOR
BIAS SUPPLY
Fig.
(244)
11.5:
LOW VOLTAGE
SUPPLY
Main components of
a G.-M.
BATTERY
counter
-511.1.3.3
Chapter 11
Survey meter with scintillation d e t e c t o r s
A DC/DC converter supplies the phot omul tipl1er with the
necessary voltage. The current o u t p u t of the phot omul tipl1er is a
function of the i n t e r a c t i n g p a r t i c l e energy. In very wide energy
range, the c u r r e n t pulses ar e integrated
and converted to voltage
signals
to d r i v e the m o v i n g - c o i l meters. Such systems are
often
combined with
l o g a r i t h m i c a m p l i f i e r s to cover many magnitudes in
dose-rates .
In ore
p r o s p e c t i n g , clinical and
another
popular version consists of a
analyzer with either a counting rate
sometimes with a d i g i t a l r a t e m e t e r .
11.1.3.4
industrial a p p l i c a t i o n s ,
complete single channel
meter or a sealer/timer,
Survey meter with semiconductor detectors
A DC/DC converter supplies the d e t e c t o r voltage. The signals,
a f t e r linear a m p l i f i c a t i o n , are passed to various pulse-shaping and
modifying circuits.
In
most
versions
multi-range counting
rate-meters develop
the driving signal for the moving-coil
type
display m e t e r .
In
some advanced
neutron
dose-rate
meters, the energy
d e p e n d e n t Quality Factor weighing is done with a microprocessor and
an ana log-1o-dig i ta 1 converter system.
11.1.4
11.1.4.1
Diagnostic Procedures
Health safety hazards
In all survey m e t e r s DC/DC converters are used to supply
the
detector voltage. The charge on the filtering capacitors might not
be
lethal
in passing
through
the person touching
it,
but
u n i n t e n t i o n a l muscle c o n t r a c t i o n s m i g h t cause serious damage.
The diodes in high voltage power supplies are of good q u a l i t y ;
the reverse currents are
rather
low,
and due to this, even after
tens of m i n u t e s , enough charge might remain to give a painful
"kick". All instruments arriving for repair should be carefully
checked
for the "bleeder resistors" presence, having the role of
a u t o m a t i c a l l y discharging the high voltage capacitors.
Survey m e t e r s are
often used
in areas where they might be
exposed
to radiological contaminations. It is very important
to
check all units entering the workshop for this, and all repair work
should be done only on radiologically safe instruments.
11.1.4.2
High value components safety
It is very important to inform the repair staff on the proper
handling and testing of the v a l u a b l e , hard-to-replace components.
(245)
Chapter 11
-6-
The most often destroyed
meters are the following:
components
during repair of
survey
- FETs, due to electrical discharge;
- phot omul tipliers, due to exposure to daylight
is on .
11.1.4.3
when HV
Preparation of the survey meter for diagnostic
procedures
The
instrument should be checked for nuclear safety, and then
for electrical safety; it should enter the repair workshop only if
both conditions were properly tested and d o c u m e n t e d . One can save
q u i t e a lot of problems in the case of a lethal a c c i d e n t if it can
be proved that such precautions were taken.
The
instrument should be properly cleaned before the repair
work starts because if d i r t
can move
from one
place to another
during
the
repair of an
ionization chamber
system,
such
i n s t a b i l i t i e s m i g h t develop which are rather hard to trace later.
Under t r o p i c a l conditions, it is a good general practice to
"dry out" the units before s t a r t i n g the repairs. This can be done
very e f f i c i e n t l y by a t t a c h i n g a de-humidifier
to a box, wardrobe,
etc.
with closely
f i t t i n g doors. A f t e r 48 hours, most of the
h u m i d i t y would be removed from the instruments.
If you have to open the i n s t r u m e n t , you are kindly r e q u e s t e d
to make note of the following suggestions:
1.
Remove b a t t e r i e s from the instrument before starting.
2.
Try
to locate the minimum number of screws, etc.
gain access to the inside of the instrument.
3.
Mark the screw, e t c . l o c a t i o n s
sequence as you remove them.
4.
Put all removed screws in a box or tray in the same sequence
as the marked areas. Never leave screws just on the table.
5.
If you have to remove many screws
proper markings, make sketches.
with
necessary to
a washable marker pen in
and there is no space to do
It is a justified question, "Why should one put so much energy
into such a s i m p l e , routine activity?". It could happen that you
will be able to continue this very
repair only months from now; it
could be that you will remember all screw positions, their
lengths
in d i f f e r e n t locations,
e t c . , but
what happens if you don't?
It
could happen
that somebody else m i g h t finish the job because you
were p r o m o t e d or
something.
You will
find it worthwhile in the
long run to follow the given suggestions.
(246)
Chapter 11
-711.1.4.4
Recommended test instruments
It is a statistically proven fact that 35% of the faults in
survey meters could be located with visual inspection, smelling and
touching. Please try to rely on your senses before you start using
sophisticated instruments.
The
first item needed in survey meter repairs is
the
radioactive test source. This should be m a t c h e d to the nature of
the survey meter.
For example, if you have to repair a neutron
dose-rate m e t e r , you must have access to a neutron source during
some phase of your work, but please remember
that all nuclear
detector signals can also be simulated by electrical or electronic
means.
It
is important that users of radioactive sources should be
trained on proper handling and
storage of these. The simple rules
are the following:
1.
If
the source is not in use, it should be in its
Try to reduce exposure to yourself and to others.
container.
2.
Never touch the source with your hands.
the less than 37 kBq activity source.
3.
If you use a radioactive source, do not
eating in that room.
4.
You should strictly follow local Radiation Safety rules; wear
the film badge or other personal dose-meter, etc.
This even applies for
allow
smoking
or
The
next
instrument
that you will need
is a m u l t i m e t e r ,
preferably with a minimum 20 kohm/volt input resistance with proper
connecting cables.
If you have an adjustable low voltage power s u p p l y , it could
be advantageous to use this instead of b a t t e r i e s , but this is not
essential .
It is a good practice to measure the current uptake , so you
should have an ammeter, preferably a fused one.
To
check the operation of the DC/DC converters
from the Geiger-Muel1er
tubes,
scintillation
oscilloscope will be necessary with a minimum 3 MHz
50 mV/division sensitivity. A standard probe with a
input resistance
is enough for most of the tests;
range extender might ease the work.
or the pulses
d e t e c t o r s , an
b a n d w i d t h and
minimum 1 MOhm
however, 1:10
With the exception of
the
ionization
chamber system,
electrical pulses carry the information. You can simulate these
with pulses from a generator, having the means to adjust the rate,
width and amplitude.
So you must have a pulse generator with
maximum + /- 10 volts of adjustable amplitude, from 5 microsec to 10
msec w i d t h , and a r e p e t i t i o n rate of between 10 Hz and 10 kHz.
(247)
Chapter 11
-8-
For the calibration of the counting-rate meters one should
have a sealer with a reasonably accurate time base.
The mains
frequency in most countries is not suitable for this purpose.
In repair of ionization chamber systems, a 10 kOhm multi-turn
p o t e n t i o m e t e r with micro-dial can
be a useful
repair
aid to
simulate the d e t e c t o r , as shown in Fig. 11.6.
MULTI-TURN POTENTIOMETER USED IN TESTING
Fig. 11.6:
The technique of electronically simulating a
radiation detector
In testing DC/DC converter
transformer
shorts, the ohm meter
could give information only on very crude faults. A more
approach is when the ohm meter
is connected
into the
circuit
of a transistor (minimum beta - 45) in series
primary of the transformer under
testing. The secondary
sens it ive
collector
with
the
should be
in the base circuit to form a "blocking oscillator", as shown in
Fig. 11.7. Note
the correct coil connection directions symbolized
with the dots.
TRANSFORMER
UNDER TEST
>. 45
Fig. 11.7:
11.1.4.5
jQ.1 OHM-METER
A simple tester for DC/DC converters
Identification of the key points, signal simulation
DC voltage measurements, or pulse shape observations in the
key points, aid the repair efficiency, the quick diagnosis of the
fault.
(248)
-9-
Chapter 11
If the battery of the survey meter is bad, no functions could
be expected from it. So the first key test point is the battery
output.
In most systems there is a possibility to do this checking
with the built-in b a t t e r y tester.
If the DC/DC converter
is f a u l t y , the d e t e c t o r will not
operate properly.
The second key test point is collector of the
transistor d r i v i n g the step-up transformers primary coil.
The third key
test point
is the DC o u t p u t
converter.
Special care should be taken with
m e a s u r e m e n t , because the internal resistance of
rather high.
of
the DC/DC
this voltage
this point is
The fourth key point should give information on whether the
detector
is functioning or not.
The best point to check for
this
is the o u t p u t of the first
active
component. In the case of the
ionization c h a m b e r , a DC level change should be present
at this
point if a radioactive source - 15 to 150 kBq a c t i v i t y - is moved
slowly
to the detector and away from it. The pulse rate should
change as a function of the source to detector distance in other
systems.
The f i f t h key test point should give information on the o u t p u t
signal of the display driver, and the sixth should be the presence
of the display itself.
There is a possibility to introduce simulated signals or
voltage levels to each key test point with proper care and this way
one can test the functions of the circuit after that point.
For example, instead of the batteries, one can give power from
a regulated power supply.
The
same applies for the DC/DC
converters case: if there is no detector voltage, from outside one
should give the necessary voltage after the original supply was
disconnected
from the detector. From a pulse generator, detector
signals can be simulated to drive further circuit sections.
11.1.4.6
Identification of key points if circuit diagram is not
available
It is easy to find the battery connectors, this is the first
key point for testing. For the second key point, one should search
near the transformer. The third test point could be located by
searching for a filter capacitor with a few hundred volts rating.
The fourth test point should be near the signal entry; one should
search for active components outputs.
One may save much time during future repairs if all identified
points are documented, even if only a rough sketch.
It is
important to save such documents and
to make a note in the
instruments log-book that such drawing already exists.
(249)
Chapter 11
-10-
11.1.4.7
Expected normal voltages, currents and pulse shapes at
key points
On the first test point, the voltage should correspond to the
number of cells in the unit. If the batteries are new and their
short circuit current is over 1 amper, there should be not more
than 10% drop if the instrument is switched on. Higher current
u p t a k e is o f t e n the sign of non-functioning DC/DC converter.
At the second test point, a periodical signal should
present; the a m p l i t u d e depends on the type of the converter.
The
third
test point is the DC o u t p u t of the converter.
be
In
most instruments, voltage muliplier circuits are used. If there is
no DC o u t p u t on the last stage of the multiplier, it is worthwhile
to test the o t h e r stages, because often only the last diode fails.
Typical DC/DC converter o u t p u t voltages are the following:
- ionization chambers
200 -
- Geiger-Mueller tubes
350 - 1200 volts
- s c i n t i l l a t i o n detectors
600 - 1000 volts
- semiconductor detectors
When
the
internal
60 -
resistance
of
600 volts
400 volts
the converters is
100 kOhm - 1 MOhm range, the readings on a low
internal
in the
impedance
v o l t m e t e r will be correspondingly lower, so care should be taken in
their evaluation.
On the fourth test p o i n t , a DC level change is expected if the
intensity
of the r a d i a t i o n changes.
In all other
systems pulses
should be present
on
this output
if the d e t e c t o r and the first
stage operates in response to the radiation.
The signals from a Geiger-Mueller tube are many microseconds
long, while the s c i n t i l l a t o r signals are only some microseconds in
width.
volts.
The
If
meter,
amplitudes
the
a
DC
fifth
level
range
from
a
few
test point is the output
change
should
be
the
tens of millivolts to
of a
counting
response
when
rate
the
r a d i o a c t i v e source to detector distance is changed. The deflection
of the moving-coil meter should change in the same way.
If a d i g i t a l display is used in the survey meter, the checking
should follow the instructions given in the sealer section of this
manual .
11.1.4.8
Diagnostic tests in the M.I.P. 10 polyradiamet re
The
M.I.P.
10 is a p o r t a b l e , battery- or mains-operated
survey m e t e r , e q u i p p e d with various types of nuclear radiation
d e t e c t o r s . It can be used as a dose-rate monitor, if calibrated.
(250)
-11(i)
Chapter 11
Controls and display
The picture of the front panel is shown in Fig. 11.8. A main
switch, battery test, loud speaker on-off, and four counting
rate-raeter range selector push b u t t o n s , form the controls of the
instrument.
The moving coil type display has two scales: one
for
the b a t t e r y c o n d i t i o n indication and the other for the CRM readout.
Fig. 11.8: Front panel of a t y p i c a l survey meter
(ii)
Connections
The d e t e c t o r socket
is on the front panel; line voltage,
recorder and sealer o u t p u t s are on the rear side with the mains
f use .
(iii)
Battery operation
Eight
1.5 volt batteries can be loaded
into the b a t t e r y
compartment. In an optional variant, a rechargeable Ni-Cd supply
is available w i t h a drip charging feature. The wiring diagrams of
the two variants are shown in Fig. 11.9.
The
first key test point is the battery v o l t a g e between pin 1
and 11. This can be tested with the built-in TEST p u s h - b u t t o n .
If
new
batteries were put into the instrument and the TEST gives no
indication to the moving-coil m e t e r , the Sie switch has to be
tested
for
continuity. Sometimes corroded b a t t e r y
compartment
contacts create problems; they must be cleaned or replaced as
needed. By regular bi-monthly b a t t e r y c o m p a r t m e n t check-ups, such
failures could be eliminated.
In case of the Ni-Cd battery option, one might expect to find
an indication on the display if
the batteries are already charged,
After a long unused period they might be discharged. If the DS1
LED lights up after the mains is connected, it proves that the
cord, connector, fuse and the transformer are fune t ioning.
(251)
n
s»
y
6)
~O
rt
n
l-l
IM«
IM
«•
II
T»r H,
mvtf
OfTUM iUUHUUTCUlS C>-M it CHUtCCUI
NJ
l
Fig.
11.9:
W i r i n g d i a g r a m of a survey meter
-13-
Chapter 11
If the fuse is blown o u t , one
should
suspect f i r s t the Ni-Cd
batteries
for
short c i r c u i t and the CR201 r e c t i f i e r .
The
test
should be repeated w i t h the 14-pin socket d e t a c h e d if they prove to
be good, because t h e n the short circuit is f u r t h e r away
from
the
battery.
It is good to know that the Ni-Cd accumulators have a limited
life,
which might
be
reduced
by n e g l e c t i n g
the
periodical
chargings. A Ni-Cd cell s t i l l working after four years is of an
exceptional, non-standard quality.
(iv)
The first DC/DC converter
The v o l t a g e from the b a t t e r i e s or from the a c c u m u l a t o r changes
d u r i n g use. To overcome t h i s , a regulated power supply feeds the
c i r c u i t s w i t h +/- 12 volts.
A self-star t ing power oscillator (Q101, Q102) drives the T101
transformer.
If the e m i t t e r current of these t r a n s i s t o r s exceeds a
c e r t a i n l i m i t , the Q105 transistor pulls to ground the base of the
series control power t r a n s i s t o r (Q1U3). The central tap of
the
t r a n s f o r m e r is c o n n e c t e d to the e m i t t e r of Q103 while its c o l l e c t o r
is on the p o s i t i v e pole of the b a t t e r y . The base p o t e n t i a l of t h i s
emi11 er -föl1 owe r is driven by Q104. The base of Q104 is connected
to the r e c t i f i e d o u t p u t signal of the transformer t h r o u g h the R 1 3 1
p o t e n t i o m e t e r and the r e f e r e n c e d i o d e CR103 to form an a d j u s t a b l e ,
feedback-controlled, regulated voltage circuit.
If
the o u t p u t voltage is not +/- 12 v o l t s , the f a u l t
it should be d e t e r m i n e d and e l i m i n a t e d .
causing
Let
us suppose t h a t the s y m p t o m is no v o l t a g e on the +/12
v o l t s rail. F i r s t the d e t e c t o r should be d e t a c h e d from the c e n t r a l
unit.
If the v o l t a g e appears, then the p r o b l e m is in the removed
section.
If the v o l t a g e does not a p p e a r , a quick way to l o c a t e the
fault
could
be done by
separating
the
central
tap of
the
t r a n s f o r m e r from the e m i t t e r of Q103, and w i t h an o h m - m e t e r check
for s h o r t e d co1lee t or-emi11er c o n d i t i o n in Q101 and Q102. If t h e r e
is no short c i r c u i t one should
apply
4 to 8 v o l t s through a fused
ammeter
for a s h o r t time. If the o s c i l l a t o r s t a r t s o p e r a t i o n and
the c u r r e n t is less than 50 mA, a volt m e t e r should be a t t a c h e d to
the
collector
of Q104. This point should
be near
to b a t t e r y
v o l t a g e if the R 1 3 1 p o t e n t i o m e t e r
is
grounding its base and it
should
start c o n d u c t i n g if the p o t e n t i o m e t e r is put into the other
end p o s i t i o n .
If no
change could
be o b s e r v e d , all components
should be c h e c k e d one by one and the bad ones must be r e p l a c e d .
It
might
h a p p e n t h a t the o s c i l l a t o r will, not
start
because
e i t h e r CR104 or CR105 has a short c i r c u i t ; this could be checked by
d i s c o n n e c t i n g t h e m from the circuit.
Another
reason could be a short c i r c u i t in the transformer.
This can be checked for with the previously described "blocking
oscillator" t e s t .
(253)
C h a p t e r 11
Q103
-14can
be
replaced
with two
transistors
in
Darlington
connection.
(v)
The Geiger-Mue11er probe
The
circuit
diagram
is
illustrated
in F i g .
11.10.
The
d e t e c t o r v o l t a g e is d e v e l o p e d in the DC/DC converter on the left. A
blocking
oscillator composed
of Q105
and Q106
generates the
d e t e c t o r v o l t a g e , which is d o u b l e d w i t h the use of CR105 and CR106
and
the c a p a c i t o r C103. The high v o l t a g e is f i l t e r e d by C104 and
C105. A f r a c t i o n of the h i g h - v o l t a g e is fed back to the Q 1 0 1 , Q102
composed
d i f f e r e n t i a l a m p l i f i e r . There are two non-common circuit
elements: the CR101 and
the CR103; they are
current
sources
composed of two transistors in D a r l i n g t o n connection. A f t e r
Q103,
emitter
follower o u t p u t
of
the
long-tailed
pair
is
further
a m p l i f i e d by Q104.
If there is no detector v o l t a g e , f i r s t the blocking o s c i l l a t o r
should be t e s t e d for operation. If the base of Q102 is driven from
outside with a variable voltage
from a helical p o t e n t i o m e t e r s
w i p e r , the operation of
the a m p l i f i e r stages could
be easily
checked.
The
secondary
winding of such
transformers is
rather
vulnerable if it is exposed to humid
c l i m a t e . They often develop
short
c i r c u i t s within the secondary. Local repair is possible
if
p r o p e r interlayer i n s u l a t i o n could be secured combined w i t h v a c u u m
imprégna t ion.
If
R 1 1 0 , the 22 MOhm value
feedback
resistor, fails
to
c o n d u c t , a d a n g e r o u s s i t u a t i o n m i g h t develop. The missing feedback
voltage forces the c i r c u i t to increase the o u t p u t , so a higher than
normal voltage will soon
be
established
on C105
and
the
Geiger-iMueller tube will "behave" accordingly.
The
reverse
leakage current of the CR106 can be 30-100 mA
only, so a dangerous charge m i g h t remain on the C105 capacitor for
a
long
time after the instrument was switched o f f ; the 0.01 J
energy kick can hurt.
Care should be taken to check
the presence and good condition
of the feedback resistor before starting the repairs.
The Geiger-MueHer tube is in the center; on its right side a
pulse shaping c i r c u i t can be found. If the Geiger-Mueller tube is
t r i g g e r e d , a positive pulse will appear on R204, limited to b a t t e r y
voltage
amplitude
by CR201 clamping diode.
The
pulse
is
d i f f e r e n t i a t e d by C201-R204 before it enters Z201, a monostable
m u l t i v i b r a t o r , generating e q u a l a m p l i t u d e and length pulses. A f t e r
f u r t h e r a m p l i f i c a t i o n in Q202, the signal leaves the sonde through
e m i t t e r follower Q201.
The
connecting
cable is rather critical
in
portable
instruments. It should be checked thoroughly for continuity and
wear.
It is b e t t e r to replace the worn cable before a fault
develops.
(254)
-15-
Fig.
11.lü:
C i r c u i t d i a g r a m of a G.-M.
Chapter l l
probe
(255)
C h a p t e r 11
-16-
The Geiger-Mue11er tube signals should never be checked on the
t u b e itself because the c a p a c i t i v e load of the test probe increases
the peak c u r r e n t and reduces the e x p e c t e d l i f e span of the tube. A
suitable point is on C201. If
the tube is not functioning, the
sonde can be t e s t e d f u r t h e r w i t h a pulse g e n e r a t o r from C201.
(v i)
The s c i n t i l l a t i o n d e t e c t o r p r o b e
The
circuit
diagram
of
this probe
is
illustrated
in
Fig. 1 1 . 1 1 . The
h i g h v o l t a g e DC/DC c o n v e r t e r is on the u p p e r half
of the d r a w i n g . An
audio
frequency oscillator c o m p o s e d of three
NOR
c i r c u i t s drives the base of Q102. The step-up t r a n s f o r m e r Tl
is in the c o l l e c t o r c i r c u i t of
Q102,
w h i c h is fed by the e m i t t e r
follower Q101.
On the secondary side of the step-up
transformer a ten-stage
voltage m u l t i p l i e r operates d e l i v e r i n g a p p r o x i m a t e l y + 1000 v o l t s ,
if 10 v o l t s are present on the base of the Q101 e m i t t e r follower.
The
feedback
loop is closed t h r o u g h the 100 MOhm value R201
resistor d r i v i n g
the inverting
input
of
the Z201 o p e r a t i o n a l
a m p l i f i e r , which
is c o n n e c t e d to the base of
the Q101 e m i t t e r
foilower .
Dangerously high v o l t a g e can
develop
in
the circuit if the
100 MOhm R201 resistor is broken. It is easy to test the circuit
by b r e a k i n g the B - B c o n n e c t i n g line and i n t r o d u c i n g a v a r i a b l e DC
voltage
to R108. This way
the o s c i l l a t o r and the power-stage
and
the v o l t a g e m u l t i p l i e r can be t e s t e d s e p a r a t e l y .
The o p e r a t i o n a l a m p l i f i e r can
be
tested if the feedback loop
is opened
at A - A and a v a r i a b l e DC v o l t a g e is injected
to the
j u n c t i o n of R202 and
R206
t h r o u g h a 100
kOhm resistor. If the
voltage input
is c h a n g e d , the o u t p u t should s u d d e n l y change in
the
r e v e r s e d i r e c t i o n when the o p e r a t i o n a l a m p l i f i e r is good.
The
output
signal from the p h o t o - m u l t i p l i e r is
fed
through
junction G to the a m p l i f i e r
section consisting of a b u f f e r 1C
(Z203), a level d i s c r i m i n a t o r 1C (Z202), and the o u t p u t
amplifier
composed of Q201 at TP1 on the e m i t t e r of Q204 .
By the a d j u s t m e n t of the
potentiometer
R215,
the t r i g g e r i n g
level could be set.
Testing of
the s c i n t i l l a t i o n d e t e c t o r is done w i t h a small
15 - 150 kBq a c t i v i t y r a d i o a c t i v e source, p r e f e r a b l y a Cs-137 one.
If
the d e t e c t o r o p e r a t e s , a few m i c r o s e c o n d s long, a few hundred
m i l l i v o l t and p o s i t i v e a m p l i t u d e signals should be observable on
TP1 while the source is a few c e n t i m e t e r s from the d e t e c t o r and the
d e t e c t o r v o l t a g e , e t c . is on.
(256)
T!
(-"•
»701
09
nom
•«v
(»1 • ~
.
»202
I?M
J
„M
Sr
SS
(1*1
Hh Hl—l
u»»
.rCl
n
(U
•o
(D
"l
Chapter 11
If
there
-18-
are
no
o u t p u t signals,
the
detector
should
be
d i s c o n n e c t e d from C211 at G point and a signal from a pulse
generator
should
be
injected
to test the circuit.
If
the
a m p l i f i e r s are good, the p h o t o - m u l t i p l i e r s dynode resistor chain
should be tested for c o n t i n u i t y , preferably with the high-voltage
"on" and by s t e p p i n g from dynode to dynode.
In some versions the photo-multiplier is not in a l i g h t - t i g h t
case a f t e r
the cover of
the probe
is removed.
Under such
conditions
the p h o t o - m u l t i p l i e r should be removed from its socket
before the resistor
chain t e s t i n g , otherwise
the photo-cathode
might be seriously damaged. For further instructions please look
at the section on Detectors in this manual.
(vii)
The r a t e m e t e r and the alarm circuit
These p a r t s of
the c i r c u i t are
shown in Fig. 11.12.
The
signals from the probes are
first
passed
into a S c h m i d t - t r i g g e r
(Z100) to remove the effects of the cable. Both the signals from
the Geiger-Mueller
tube and
the
s c i n t i l l a t i o n detector are
standardized
to e q u a l length and a m p l i t u d e in the probes so
the
counting-rate m e t e r is a simple integrator (Z101) with varying time
constants in the feedback loop in various ranges (R10A-C106,
R105-C107, R106-C108, R107-C109). The o u t p u t signal of Z102 drives
the m o v i n g - c o i l t y p e display m e t e r (Ml) through resistors R 1 1 3 and
R129 serving for full-scale a d j u s t m e n t .
The alarm c i r c u i t starts to operate above a level which can be
set w i t h the Rl p o t e n t i o m e t e r . Checking can be done by injecting a
v a r i a b l e v o l t a g e through a 10 kOhm resistor to the inverting
input
of Z102; if the o p e r a t i o n a l a m p l i f i e r o u t p u t does not change, it
must be r e p l a c e d .
The second
half of
the Z100
generates the alarm signal
f r e q u e n c y and Q106 is the power stage driving the loud-speaker LSI.
11.1.5
Stock F a u l t s in Various Systems and Repairs
Sympt om
Cause
Ac t ion
New dry b a t t e r i e s
Test : no
ind icat ion
c o r r o d e d contacts
clean, if needed replace
faulty switch
clean, if needed replace
meter or R 1 1 3 bad
repair or replace
short c i r c u i t
determine location, repair
short c i r c u i t
determine location, repair
R129 misadjusted
readjust R129
corroded contacts
clean, if needed replace
Test:
(258)
low voltage
Chapter 11
-19-
X
3
3 -n
c.
»N
G
c
r»i
r-
à
Z
0
so
icî
âQ
la-,
s
fc
1
]f
t
1
Ê
xl
< 22W09*4lo^?9S
I
î
rLt.
5;
S
C
Ii
S ï
6oo("OOoo<
< <p
< < on
_
« M . ^ * «
Fig.
11.12:
H n
M r
Ratemeter and alarm circuit
(259)
Chapter 11
Ni-Cd accumulator
-20charger fuse bad
determine w h y , replace
cell bad
replace
rectifier bad
replace
short circuit
determine location, repair
oscillator bad
replace bad component
regulator bad
replace bad component
low voltage
No +/- 12 volts
Q105
More than 12 v o l t s
Less than 12 v o l t s
Geiger-Mueller
sonde, No 12
volts
No d e t e c t o r supply
short, C 1 1 5 bad replace bad component
transformer bad
repair
r e c t i f i e r bad
replace
R131 broken
replace
CR103 broken
replace
Q103
replace
short
R131 misadjusted
readjus t
short circuit
determine location, repair
broken cable
repair, rep lace
bad connector
repair, replace
short circuit
locate, repair
blocking osc. bad
locate and replace
short CR105
replace component
broken C103, CR106
Rill
replace component
short in transformer repair, replace
Detector supply
high
No s ignal w i t h
source
(260)
broken coil
repair, replace
bad regulator
locate, replace
broken R l l O
replace
R10A misadjusted
readjust
bad M301 tube
test, replace
bad Z201
test , replace
-21Signal too short
bad C201,C202,R205
Chapter 11
replace bad components
bad cable, connector replace, repair
Pulse rate higher
than reference
value with standard source and
geome t ry
Pulse rate lower
than reference
radiological contamination
decontaminate
too high d e t e c t o r
voltage
repair DC/DC converter
faulty tube
replace
fault in main unit
locate, repair
too low detector
voltage
repair DC/DC converter
increased "deadtime" Fault in
ma in unit
replace G.-M. tube
locate, repair
with standard
source and
geometry
Scintillation sonde broken cable
no low v o l t a g e
bad connector ,
contact
No d e t e c t o r voltage oscillator bad
Detector voltage
high
Detector voltage
low
repair, replace
repair
locate, repair
Tl bad
repair or replace
Q101
or Q102 bad
replace
Z201
bad
replace
R401
or CR201 bad
replace
broken R201
replace
bad Z201
replace
R401
readjust
badly adjusted
short in Tl
replace or repair
diode short in
locate, rep lace
voltage m u l t i p l i e r
CR102-111
capacitor short in
locate, replace
voltage m u l t i p l i e r
(261)
Chapter 11
No scintillation
detector o u t p u t
signal with standard source in
reference position (TP 1)
Pulse rate higher
than normal in
reference condition
-22dynode resistor
chain element
broken
locate, replace
photo-multiplier bad check, replace
Z203 or Q204 bad
replace
C211
replace
broken
radiological contaminât ion
decontaminate
noise pick-up from
DC/DC converter
locate bad capacitor,
(C102, C212, C208, C209)
replace
noisy photomulti-
rep lace
plier
corona discharge on
high voltage
locate, repair
component
humidity caused high dry system
leakage current
increased background locate, remove open source
if possible
in workshop
Pulse rate lower
than normal in
reference condition
high-voltage too
low
Readjust or locate cause
and repair
deteriorated scintillator
replace
R222 broken
C301-303 broken
replace
no signal from sonde repair or replace
Main unit, counting rate meter
cable is bad
not functioning
repair or replace
in reference mode connector is bad
Z100 bad
replace
Z101, range switches locate, repair or replace
R104-106, R124-127
or C106-109 bad
Counting rate meter bad adjustment or
calibration bad
faulty R-C components in integrain single range
tor loop
(262)
locate, replace
-23Alarm system not
fune t ioning
Chapter 11
bad Z202
replace
bad Z100 2/2 or
Clll, R115
replace
bad Q106 or loudspeaker
11.1.6
Quality Control of the Repaired Instrument
The
r e p a i r e d instrument should be tested with
the reference
source of the workshop in a standardized geometry, that is, the
same distance between the detector and source and
same relative
position. A simple approach is
to mark on plywood the outline of
the detector and the location of the source with its i d e n t i f i c a t i o n
n u m b e r . During all measurements this set-up should be used. It is
very
important
to record
the results and
to compare
them
to
previous tests.
It
is good practice to record the power uptake of
the unit
during t e s t s , which helps later on with diagnosis. An increasing
current
uptake compared to previous measurements could be the sign
of an insulation de t erioration' due
to moisture a c c u m u l a t i o n in the
DC/DC converter
transformer. In an early
stage often a simple
drying m i g h t save the instrument from a total breakdown.
During
the reference measurements all other r a d i o a c t i v e
test
sources should be removed from the standard geometry area to secure
a normal radiation background level. It is a good practice to do
such measurements a minimum of
1,5 meters above
the f l o o r ,
preferably
on
a plywood p l a t f o r m hanging from the ceiling
in a
d e d i c a t e d area of the workshop.
11.1.6.2
The
Dose-rate calibration
dose-rate
calibration
of
the
checked if the activity
following relationship:
of
test
the survey meters could be
source is known, using the
dose-rate/hour [R/h] = K * A [Curie]/R2 [m]
where K, the dose-rate constant of the used test source
A, the activity of the test source in Curies
R, the source detector distance.
Such
check-up tests could not
substitute
the periodical
calibration of the survey meter by the National Metrological
Institute. The expected accuracy of such check-ups is better than
+/- 20%.
11.1.6.3
areas
Marginal testing
It is important to know whether the battery test green and red
are valid or not. Sometimes the series resistor of the
(263)
Chapter 11
-24-
raoving coil meter changes its value and b a t t e r i e s might be thought
to be good even when they are already bad.
To
test
the validity of the indication, the survey meter
should be connected
to a variable power supply and the voltage
should
be reduced u n t i l the measured intensity drops by 10%. Then
the supply voltage should be increased with one volt and the series
resistor of the moving coil meter should be adjusted to the border
of the green and red areas.
This test should be done once yearly
BEFORE the calibration in the National Metrological Institute.
11.1.6.4
S t a b i l i t y test ing
All
instruments should be tested for stable
operation after
repairs, for a minimum of six hours.
This test should be carried
out with a regulated power supply, delivering the nominal voltage
expectable from the dry b a t t e r i e s ,
for example 8*1,5 volts in the
case of the M.I.P. 10 survey m e t e r .
The
intensity should be adjusted to give
the meter. Readings should be taken
in every
graph should be plotted from the results. If
with more than + /- 20% during
the day, the
and repaired.
an 80% deflection on
half hour time and a
the readings change
cause should be traced
Care should be taken with the s c i n t i l l a t i o n d e t e c t o r s during
such measurements; they should not be exposed to d i r e c t sunlight or
to the cold
air
stream
from
an
air-conditioner.
It
is good
p r a c t i c e to monitor the t e m p e r a t u r e on the spot of the measurement.
11.1.7
11.1.7.1
Preventive Maintenance
Recommended
periodical check-ups
The b a t t e r i e s and
the c o n d i t i o n
of
the battery compartment
contacts
should be checked monthly. The
instrument
should
be
tested under reference conditions a minimum of twice yearly.
11.1.7.2
Preservation technologies
Scintillation detector probes should not be exposed to over 50
degrees
Centigrade because the photocathode might deteriorate.
Never leave the scintillation d e t e c t o r ,
for example, in a car with
closed
windows.
The mechanical construction might
survive
high
accelerations, but this should be avoided if possible.
The high voltage DC/DC converters
try
to
are vulnerable to moisture;
store the instruments in dry areas when not
in
use.
If
silica-gel cartridges are in the instrument, their condition should
be checked with a periodicity based on observations.
(264)
-2511.1.7.3
Chapter 11
Log keeping
Keep a log-book and introduce all findings and
activities
(repair, b a t t e r y r e p l a c e m e n t , c a l i b r a t i o n , etc.) into it. Evaluate
the entries yearly and try to improve your work by making use
of
the observations.
11.1.8
Instrument Selection, Parts Replacement
Try to standardize the survey m e t e r s , r e d u c e the brands.
t y p e s , the easier it is to organize the spare supplies
the r e p a i r .
fewer
The
and
Check the h i g h voltage DC/DC converter
transformers; if they
are not the hermetically sealed t y p e , order one replacement and for
each ten units one more a d d i t i o n a l l y .
If you have good workshop
facilities, you m i g h t try to make
the
transformer. If you do not have data on the number of
turns,
measure the weight of the coils and
the diameter of the wire; from
these
two pieces of information one can e s t i m a t e the turn ratios.
The insulation is about 10% of the available area, and the copper
could not be more than 40% there.
If
you have
have to
to replace
replace aa ddiiooddee in
in the
the vvoollttaaggee mmuullttiipplliieerr
and
you do not have
ive the same type, it is better to replace all with
witl the
same type.
11.2
11.2.1
RECTILINEAR SCANNER
Field of A p p l i c a t i o n
Medical
rectilinear
scanners are used to
map-up
the
d i s t r i b u t i o n of radioactive tracers that were i n t r o d u c e d into the
body
for
diagnostic purposes. The o u t p u t
information from
the
scanner is the s c i n t i g r a m , a one-by-one image of the investigated
organ w i t h c o l o u r - c o d e d r e p r e s e n t a t i o n of the local
radioisotope
concentration.
The
areas
with
highest
radioactive
isotope
c o n c e n t r a t i o n s are displayed w i t h red iso-in t ens it y areas, in most
system 8 - 1 2 d i f f e r e n t
intensity
levels could b e v i s u a l i z e d b y
this
imaging
technology. The p i c t u r e s are used to e v a l u a t e the
e x t e n t of the organ tissues p a r t i c i p a t i n g in the normal metabolism
and
the locations of the already damaged areas.
The
scintigrams
are
used
in d i a g n o s i s , in
therapy
p l a n n i n g , and
in
therapy
évaluât ion.
This
imaging technology is non-invasive and very
important
because it can p r o v i d e information on such soft tissue organs which
could not be easily accessed by x-ray or ultra-sound.
11.2.2
Operating Principle
In
the medical scanners, one or more nuclear
d e t e c t o r s are
moved on a meander p a t h above and
sometimes under the investigated
(265)
Chapter 11
-26-
organ. The
nuclear d e t e c t o r
or d e t e c t o r s are
in special lead
shields w i t h apertures in the d i r e c t i o n of the isotope d i s t r i b u t i o n
p r o v i d i n g high d i r e c t i o n a l s e n s i t i v i t y (see Fig. 11.13).
Single-channel pulse-height analyzers are connected to the
d e t e c t o r s , w i t h the windows on
the photopeak of the radioactive
i s o t o p e , w h i c h was i n t r o d u c e d into the body for investigation.
C o u n t i n g rate m e t e r s are
driving
the display system w h i c h
moves
t o g e t h e r with the d e t e c t o r above the scintigrara paper.
The
measured c o u n t i n g rate drives the intensity-to-colour i n f o r m a t i o n
converter,
consisting of a m u l t i - c o l o u r e d
typewriter
ribbon
p o s i t i o n e d by a servo-system under an e l e c t r o m a g n e t controlled
hammer
called
the "tapper".
In m u l t i - d e t e c t o r
systems, each
d e t e c t o r has a separate display u n i t .
As the m e a s u r e d i n t e n s i t y
changes, various coloured portions
of
the t a p e w i l l be put under the t a p p e r by the servo. Since
the
d e t e c t o r and the t a p p e r move t o g e t h e r , if the t a p p e r hits on the
r i b b o n , the colour mark on the s c i n t i g r a m will correspond to the
measured intensity.
As the d e t e c t o r moves above the s e l e c t e d area of interest, it
c o l l e c t s the r a d i a t i o n intensity d a t a along its p a t h d i s p l a y e d
in
colour c o d e , w i t h a d i s p l a y
d e t e r m i n e d by the t i m e - c o n s t a n t of the
counting r a t e m e t e r .
The
fidelity
of
the
image will be a function of
the
s t a t i s t i c s . If the d e t e c t e d number of pulses per square c e n t i m e t e r
are
l o w , the r e l a t e d s t a t i s t i c a l f l u c t u a t i o n will be h i g h , so a
longer time c o n s t a n t
is needed
to smooth
it o u t . For a good
q u a l i t y i m a g e , 600 p u l s e / s q u a r e cm m u s t be o b t a i n e d , t h e r e f o r e
the
d e t e c t o r speed s h o u l d
be
adjusted
to meet t h i s r e q u i r e m e n t . The
scanning speed should be fairly s t a b l e , easily a d j u s t a b l e w i t h i n 30
- 500 c m / m i n range. The d e t e c t o r in the lead s h i e l d is over 60 kgs
in most
designs so a p o w e r f u l motor system is needed to move
it
"line-wise" and "step-wise". The "line" d i r e c t i o n is p e r p e n d i c u l a r
to
the s p i n a 1 - c h ö r d , in some systems they call it the X axis, and
the s t e p d i r e c t i o n as the Y axis.
Scanning is a series of i n f o r m a t i o n gathering processes, which
is r a t h e r
slow. Taking one view often requires over 20 m i n u t e s ,
but less t i m e is needed to c o m p l e t e
the s c i n t i g r a m if the scanned
area is smaller. The p r o g r a m m i n g of the scanning area is done with
the LIMIT SWITCHes on the X and Y axis. A f t e r c o m p l e t i o n of each
line
detected
by the LIMIT SWITCH, a step-wise
d e t e c t o r motion
takes p l a c e and the line scanning d i r e c t i o n is reversed.
It
is i m p o r t a n t to know t h a t if the a c t i v i t y a d m i n i s t e r e d
to
the p a t i e n t is g r e a t e r , the scanning time
is shorter. Another
important
f e a t u r e of the t e c h n i q u e is that if the p a t i e n t moves
during the i m a g i n g , the p i c t u r e will be b l u r r e d and then it should
be r e p e a t e d . If the channel is not on the p h o t o - p e a k , the image
will have poor g e o m e t r i c a l resolution with low d i a g n o s t i c value.
(266)
-27-
Fig. 11.13:
Chapter 11
Main mechanical elements of a medical scanner
(267)
Chapter 11
11.2.3
-28-
C o n t r o l s , Turning-On and Quick Checks
The
nuclear
channels are controlled by the ISOTOPE SELECTOR
(see F i g . 11.14); this switch or p u s h - b u t t o n adjusts the energy and
window s e t t i n g s o p t i m a l for the isotope used.
If
they are
not
p e r i o d i c a l l y checked for a c c u r a c y , poor p i c t u r e s m i g h t result. In
most s y s t e m s , MANUAL ENERGY and WINDOW controls are also a v a i l a b l e .
If the manual and the preset
a d j u s t m e n t s give d i f f e r e n t r e s u l t s ,
it's time for r e a d j u s t m e n t and c a l i b r a t i o n .
ISOTOPE SELECTOR
r
1 ! 1
^ —-^
RECOMMENDED
S4-T 4<?
r".. .......
/^
WINDOW
H. V.
©
<E
Fig.
11.14
ON
ô
COUNTS/SEC
. PRESET
MODE
A
SELECTOR ^
MANUAL
ENERGY
LOUDSPEAKER
^
^
s
.
1
I
1
1
1
OFF
1
1
COUNTING-RATE METER
RANGE SELECTOR
Nuclear channel controls
The
c o u n t i n g - r a t e m e t e r range a d j u s t m e n t is conventional.
If
a r a d i o a c t i v e source is
introduced
into the field of view of the
detector
and the corresponding p r e s e t or manual energy and window
s e l e c t i o n was done, the d i s p l a y m e t e r should
i n d i c a t e changing
values as the source is moved under the d e t e c t o r .
Fig.
An e x a m p l e of the front panel of a scanner control is shown in
11.15.
1000
j^rif^fe^^^^
«fl£
GOOD
E-t W 2 S W Q
W D W O U bj
o
« u
o; UhJK3
M
>
SPEED
[cm/min]
X 0
COUNTS/cm*
INFORMATION DENSITY
COLOR DISPLAY
1 1 1 1
»
STEP SIZE
MANUAL
CO
POSITIONING
Fig.
(268)
11.15:
SCANNING
Scanner controls
lOj
^-/
COLOR ADJUSTMENT
OFF
MATMC
nALNt
>
-29-
C h a p t e r 11
The a c t i v i t y-1o-colour servo should follow the movement of the
display m e t e r as the source is moved under the d e t e c t o r .
The next test
is concerned with the X-Y
movements of the
detector.
First the LIMIT or LIMIT SWITCHes should be set for a
short, few c e n t i m e t e r long
line l e n g t h , with full-size step-wise
notion and the scanning should be s t a r t e d with the START switch in
the lowest SPEED range. The scanner should start m e a n d e r i n g within
the preset borders.
The
speed should be adjusted for higher and higher speeds up
to the m a x i m u m . Take care, because if the limit switch is b a d , the
detector might
try to run away, breaking p a r t s of the mechanical
s y s t e m ; this is why the b o r d e r
control
checking is recommended on
the lowest speed range.
If
the system is already moving, a small source, a so-called
"phantom-organ", should be put under
the detector and imaging
should
be a t t e m p t e d .
Here one
should
follow the recommended
s e t t i n g - u p p r o c e d u r e described
in
the manual. First p o s i t i o n the
d e t e c t o r above the source and adjust the CALIBRATION control to get
the
display meter pointer
to
reach
the red colour zone,
representing
the highest a c t i v i t y zone on
the scintigram.
The
speed should be set
to get
the recommended
600
count s/square
centimeter
information density.
An image of the source should
result
from
this a c t i v i t y
similar
to those which were taken
previously under r e f e r e n c e conditions.
The
d e t e c t o r and source distance is critical in imaging; the
best resolution could be
obtained
from
the "focal plane" of the
m u l t i h o l e d i a p h r a g m before the sein t i llator. The optimal
imaging
d i s t a n c e is given on the d i a p h r a g m .
Each scanner has some means for the detector positioning. This
function
is used during the setting-up adjustment. By looking on
the counting-rate meter or
listening
to a variable
frequency
loudspeaker
sound, controlled
by the r a d i a t i o n i n t e n s i t y , the
operator
should
search
for
the
area with maximum
isotope
concentration.
This manual mode can be used to test
the safety
switches on the border of
the maximum scannable area.
If the
detector
is not stopped there a u t o m a t i c a l l y , the drive mechanism
can be harmed.
Fig.
A block
11.16.
11.2.4
11.2.4.1
schematic
of
a typical scanner
can
be
found
in
Troubleshooting and Stock Faults
Power supplies
In most systems separate
low-voltage regulated power supplies
are used for the nuclear channel and for the detector motors.
A
DC/DC converter feeds the scintillation detector, which receives
power from the nuclear channels voltage regulator.
The
troubleshooting procedure is similar
to all scintillation detector
high-voltage power supplies.
(269)
Chapter 11
-30-
£l
EH OS CO i
I.UQO !H
H HM l
-_|
Cn Q Z1
los z os Si
:o w o x|
të Sa al
.u n « 2l
1
cn
z
0!
8
§
OS
U
X
iJ
0.
S
0<
S
cn
«
M
S"
ÖJ
s 1|
§|
o< <
>H
Ö
EH
Z
O
u
n
O
O
O. <
a
u
PS
H
o
o
u
D
Fig. 11.16:
(270)
Block schematic of a typical scanner
-31-
C h a p t e r 11
The r e g u l a t e d voltage s u p p l i e s of the X-Y m o t o r s are r a t h e r
simple;
their
rated
output
is b e t w e e n
150-300 VA.
They
are
d e s i g n e d to deliver ample power to s t a r t moving the d e t e c t o r by the
s t e p p i n g motors. The speed r e g u l a t i o n is a c c o m p l i s h e d by c h a n g i n g
the f r e q u e n c y of the steps. The
r o t a t i o n d i r e c t i o n is changed by
a l t e r i n g the phase r e l a t i o n s in the c o n t r o l coils of the m o t o r s .
The
high
power s e m i - c o n d u c t o r s m i g h t fail more o f t e n if
the
v e n t i l l a t i o n of
the h e a t s i n k s are
obstructed
or
high voltage
t r a n s i e n t s are present on the s u p p l y lines. This o f t e n h a p p e n s
in
h o s p i t a l s w h e r e the c a b l e s are h e a v i l y
overloaded
and the wire
c o n n e c t i o n s are c o r r o d e d . An AC v o l t a g e s t a b i l i z e r can improve the
s i t u a t i o n for the scanner.
There is only one simple rule in power diode r e p l a c e m e n t , the
symmetry
should be conserved in the r e c t i f i e r , t h a t is if the same
diode which failed is not available, both diodes should be replaced
w i t h p r o p e r l y rated ones.
11.2.4.2
Scintillation detector
All
t r o u b l e s h o o t i n g r e c o m m e n d a t i o n s given for
scintillation
d e t e c t o r s in
this m a n u a l are
valid
for
the d e t e c t o r s of the
scanners as well. But t h e r e are certain stock f a u l t s which are not
common in o t h e r a p p l i c a t i o n s .
D e t e c t o r cable. They
are exposed
to c y l i c a l b e n d i n g s and
movements after
accomplishment
of
each
scanning
l i n e ; this can
amount
to
1 U . O O U cycles/week and two m i l l i o n
cycles/year.
Most
cables are worn out in 3-4 years. The cable d e t e r i o r a t i o n is slow;
sometimes
noises appear w i t h o u t any
apparent
reason, only
in
c e r t a i n areas of the s c i n t i g r a m
even w i t h o u t any isotope.
If such
e f f e c t a p p e a r s , it is a sure sign t h a t the cable is d e t e r i o r a t i n g .
C o n t a m i n a t i o n of the d e t e c t o r .
Under c l i n i c a l c o n d i t i o n s it
happen
that
the d e t e c t o r surface is
contaminated
by
the
p a t i e n t . This can be p r e v e n t e d
by enclosing
the d e t e c t o r in an
easily r e p l a c e a b l e p l a s t i c sheet or b a g .
could
Scintillation
detector
stability.
If
the gain of
the
p ho t omul t i p lier changes during the i m a g i n g , it could happen t h a t
a
f r a c t i o n of
the photopeak
i n f o r m a t i o n w i l l be
lost.
On the
scintigram
it m i g h t
look as if certain organ areas had
lower
m e t a b o l i c f u n c t i o n s . It is easy
to test the system by a t t a c h i n g a
small r a d i o a c t i v e
source to the d e t e c t o r and keeping the scanner
record the measured
intensity
in
this
fixed
g e o m e t r y . A good
scanner should not
drift in one hour more than
ten percent
in
recorded intensity.
11.2.4.3
P u l s e - h e i g h t analyzer
All
recommendations given for p u l s e - h e i g h t
analyzer t r o u b l e s h o o t i n g are v a l i d for the scanners as well; there is, h o w e v e r , one
a d d i t i o n a l uncommon stock f a u l t .
(271)
Chapter 11
-32-
Isotope selector d r i f t . Most scanners have certain preset
channels for the most o f t e n used isotopes.
The adjusting e l e m e n t s
might
change their value due to e n v i r o n m e n t a l e f f e c t s .
It
is
important to check the accuracy of
the preset channel positions
periodically by comparing the manually a d j u s t e d p h o t o p e a k counting
rates
to the preset channel r e s u l t s .
If they d i f f e r by more
than
10%,
readjustment is i n d i c a t e d .
11.2.4.4
Counting-rate meter
The
c o u n t i n g - r a t e m e t e r s d r i v e the in t ens ity-to-colour
code
c o n v e r t e r in the display system.
The
linearity of the conversion
is critical and since it can lead to false medical diagnosis, it
has to be checked p e r i o d i c a l l y .
The
procedure
is r a t h e r s i m p l e .
One should measure the f r e q u e n c y of a pulse generator which drives
the p u l s e - h e i g h t
analyzer.
A graph should
be p l o t t e d on input
frequency vs.
d i s p l a y e d value r e l a t i o n . A c t i o n should be taken if
the i n t e g r a l non-linearity exceeds + /- 5%.
A pulse generator is an essential
of c o u n t i n g - r a t e meter faults.
11.2.4.5
instrument in the d i a g n o s i s
Colour a d j u s t m e n t p o t e n t i o m e t e r
The
most
often used c o n t r o l e l e m e n t of the scanner
is
the
p o t e n t i o m e t e r between the c o u n t i n g - r a t e m e t e r s o u t p u t and the input
of
the
in t ens i ty-1o-colour c o n v e r t e r servo c i r c u i t . It must
be
adjusted so that the maximum a c t i v i t y
area should appear in red
colour
on
the s c i n t i g r a m .
An early sign of wear of
this
p o t e n t i o m e t e r is if the colors are
changing while the c o u n t - r a t e
remains c o n s t a n t during t e s t s w i t h a source fixed to the d e t e c t o r .
11.2.4.6
In tensity-1o-colour code converter servo
The c i r c u i t diagram and
m e c h a n i c a l c o n s t r u c t i o n of a t y p i c a l
converter
is shown in Fig. 1 1 . 1 7 .
In its s i m p l e s t version the
p o s i t i o n of the coloured
typewriter
r i b b o n under
the t a p p e r is
sensed
by a p o t e n t i o m e t e r , m e c h a n i c a l l y a t t a c h e d to a DC m o t o r .
The n o n - i n v e r t i n g input of the
operational
a m p l i f i e r is connected
to
the source of
the i n t e n s i t y i n f o r m a t i o n , t h a t
is,
to
the
counting-rate meter
output
through
the
colour
adjustment
potentiometer.
The
inverting
input is connected to the moving
contact of the p o t e n t i o m e t e r , while one
end is on ground and the
other
on a reference DC v o l t a g e level. The DC motor is c o n n e c t e d
w i t h such p o l a r i t y to give n e g a t i v e
f e e d b a c k , t h a t is, the v o l t a g e
from the p o t e n t i o m e t e r compensates the input v o l t a g e and the motor
r o t a t e s u n t i l the error signal is m i n i m i z e d .
The most c r i t i c a l part is
the p o t e n t i o m e t e r .
They wear out
after
10 m i l l i o n t u r n s , so their expected life is 3-4 years in a
nuclear medicine unit with 5.000 imagings per year.
(272)
-33-
Chapter 11
COUNTING RATE METER OUTPUT
COLOR ADJUSTMENT
DC SERVOMOTOR
REFERENCE
VOLTAGE
COLOR RIBBON
POSITION
TRANSDUCER
COLOR RIBBON
CARRYING CAGE
NUT ON SCREW
1
J
'
J
W/////JP
SCREW
1
/
rur
1
l
1
X,
Pj/j/rf
! $ /-w
\
®
GEAR BOX
DC SERVO
MOTOR
/
COLOR RIBBON POSITION
TRANSDUCER
COLOR RIB
PRINTING EDGE
Fig.
11.17:
Circuit diagram and mechanical c o n s t r u c t i o n of
a t y p i c a l converter
(273)
C h a p t e r 11
-34-
Wear c o u l d be one reason for
the component d e t e r i o r a t i o n , but
under-used
p o t e n t i o m e t e r s can
d e v e l o p noises under
tropical
c o n d i t i o n s as well.
However, they
are r e d u c e d to a normal l e v e l
a f t e r a few hours of o p e r a t i o n .
11.2.4.7
C o n t r o l logic
The
movement
of
the d e t e c t o r is u n d e r the control of
the
scanners logic s y s t e m . Its main f u n c t i o n s are the sensing of the
p o s i t i o n of the scanning area d e t e r m i n i n g b o a r d e r limit
switches,
the p o s i t i o n of the b o r d e r
l i m i t p r o x i m i t y switches and the safety
limit s w i t c h e s , and to c o n t r o l the s t e p p i n g m o t o r s a c c o r d i n g l y .
Some s i t u a t i o n s and a c t i o n s :
s c a n n i n g line-wise border switch a c t i v a t e d :
step-wise m o t i o n s t a r t s , a f t e r this new line scanning
s t a r t s w i t h reversed d i r e c t i o n .
step-wise border s w i t c h a c t i v a t e d :
s c a n n i n g is f i n i s h e d , all motors are s t o p p e d ,
b o r d e r limit p r o x i m i t y switch a c t i v a t e d :
m o t o r torque is r e d u c e d , breaking a c t i o n s t a r t s before
scanning d i r e c t i o n is r e v e r s e d , this a c t i o n is i m p o r t a n t
o t h e r w i s e the m e c h a n i c a l system is exposed to heavy
strain.
safety limit switch activated:
f u r t h e r m o t i o n in same d i r e c t i o n is i n h i b i t e d .
Under
the manual d e t e c t o r p o s i t i o n i n g m o d e , only the safety
limit s w i t c h e s are a c t i v e in p r o t e c t i n g the m e c h a n i c a l system.
11.2.4.8
S t e p p i n g motor c o n t r o l c i r c u i t s
The
block
diagram of
the circuit
is
illustrated
in
Fig. 1 1 . 1 8 . The step
frequency
selection
is done by the SPEED
control
switch.
In a phase s p l i t t e r , d o u b l e phase
signals are
g e n e r a t e d from the step
frequency
signals to feed the r o t a t i o n
d i r e c t i o n c o n t r o l circuit together w i t h the line-wise border
limit
switch information.
Whenever a border
limit
switch
is a c t i v a t e d , the b i - s t a b l e
m u l t i v i b r a t o r , sensing
it, changes the phase of
the triggering
pulse to the t r i a c which d r i v e s the r o t a t i o n d i r e c t i o n control coil
of the motor .
The motor torque is always reduced to zero before the rotation
"firing angle" under the
d i r e c t i o n is reversed by m o d u l a t i n g
the
control of the limit p r o x i m i t y switch.
(274)
-35-
C h a p t e r 11
The step-wise m o t i o n is under the c o n t r o l of a preset c o u n t e r ,
p r o g r a m m e d by the s t e p - l e n g t h s w i t c h .
In manual mode all r o t a t i o n d i r e c t i o n orders are c o n t r o l l e d by
the manual s w i t c h e s and the m o t o r s move only while they are
pushed
and only the s a f e t y logic can i n h i b i t them.
All r e p a i r a c t i v i t i e s should
start with
the c h e c k i n g of the
s a f e t y s w i t c h e s , because if t h e y are s h o r t e d , the m e c h a n i c a l system
w i l l not stop on the b o r d e r s , and
if the torque limiters are b a d ,
expensive
d a m a g e s can be e x p e c t e d . For this reason, it is a good
p r a c t i c e to put the d e t e c t o r
into the c e n t e r of the scanning area
d u r i n g the i n i t i a l p a r t of the testing.
If
t r i a c s should be r e p l a c e d , the rule is to change them
in
pairs. Use
fuses to p r o t e c t
them during
first
t r i a l s after
rep l a c e m e n t .
11.2.4.9
The
A l i g n m e n t of the m e c h a n i c a l system
d e t e c t o r s are rather heavy and the m e c h a n i c a l system
o p e r a t e p r o p e r l y only if the beam
can
s u p p o r t i n g the heavy lead s h i e l d
is a c c u r a t e l y
l e v e l e d . The m e c h a n i c a l system wears out q u i c k l y
w i t h o u t this p r e c a u t i o n .
It is
important to check the leveling of
t h e scanner a f t e r t r a n s p o r t a t i o n .
11.2.5
Q u a l i t y C o n t r o l of the Repaired Instrument
Recommendations
of the IAEA TECDOC-317 on " Q u a l i t y Control of
N u c l e a r M e d i c a l I n s t r u m e n t s " s h o u l d b e followed.
11.2.6
Preventive Maintenance
Recommendations
given
to
the
Nuclear
L a b o r a t o r i e s should be f o l l o w e d , t h a t is:
- instruments
like d u s t ,
Medical
should be p r o t e c t e d from e n v i r o n m e n t a l
moisture
condensation,
excessive
Pilot
effects
line v o l t a g e
f l u c t u a t i o n s , and t r a n s i e n t s ;
- i n s t r u m e n t s should be c o v e r e d a f t e r u s e ;
- rarely used i n s t r u m e n t s s h o u l d be turned on p e r i o d i c a l l y and
m e c h a n i c a l l y o p e r a t e d to p r e v e n t r u s t i n g , and to dry o u t ;
- l u b r i c a t i o n and safety c h e c k - u p s
manua1 ;
should be a c c o r d i n g to the
- only t r a i n e d operators s h o u l d use the i n s t r u m e n t ;
- a log
book should be kept on use and
r e p a i r s , reference
tests and t h e i r r e s u l t s should be a t t a c h e d to the book.
(275)
Chapter 12
RADIATION DETECTORS
-112
Chapter 12
RADIATION DETECTORS
This c h a p t e r presents a d e s c r i p t i o n of d i f f i c u l t i e s with
the
d e t e c t o r s most frequently used
in advanced nuclear spectrometry.
Although a detector
is d i r e c t l y , and
sometimes
inseparably,
c o n n e c t e d to electronics, discussions on the behaviour and faults
of electronic parts is avoided; for this the reader should refer to
Chapters 5 and 11. F u r t h e r m o r e , a t t e n t i o n is d e v o t e d entirely to
the
high
resolution d e t e c t o r s ; simple d e t e c t o r s ,
such
as
Geiger-Mueller or
scintillation, are
described
in Chapter 11.
A c c o r d i n g l y , the chapter s t a r t s with the h i g h - r e s o l u t i o n x-ray and
gamma-ray detectors.
12. 1
HIGH RESOLUTION X-RAY AND GAMMA DETECTORS
Today, we
find
resolution detectors:
on
the market the following
Coaxial pure germanium d e t e c t o r , p - t y p e
Planar pure germanium d e t e c t o r
Coaxial pure germanium d e t e c t o r , n-type
Si(Li) d e t e c t o r
Pure Si(Li) d e t e c t o r
Surface barrier detector
types of
high
for gamma
x-rays
x-rays and gamma
x-rays
x-rays
alpha and beta
The Ge(Li) d e t e c t o r , which started the revolution in the field
of high
resolution spec troscopy, is not p r o d u c e d anymore by the
respected manufacturer of semiconductor detectors.
12.1.1
Selection of a High Resolution D e t e c t o r
When ordering a germanium detector for gamma rays, one must
spec ify :
1.
2.
3.
Resolution (an average d e t e c t o r will have resolution 1.8 keV).
E f f i c i e n c y (varies from 8 to 40%).
Type of cryostat, i.e. the size of the dewar, and the position
of the dipstick.
NOTE ;
The price of a Ge detector steeply increases with the
size (efficiency) and with improved resolution.
NOTE ;
A d e t e c t o r with a v e r t i c a l d i p s t i c k is the most common
type. Other shapes of the dipstick are for special
applications.
(279)
-2-
Chapter 12
For a pure planar germanium d e t e c t o r , we must define
1.
2.
3.
4.
Thickness of the d e t e c t o r
Area of the d e t e c t o r , in square mm
R e s o l u t i o n , in keV
Size of the dewar and type of the d i p s t i c k
|
NOTE : Planar Ge d e t e c t o r s are used in measurement of x-rays
|
|
w i t h energy above 20 keV. Due to intense escape
|
i
p e a k s , they cause d i f f i c u l t i e s in i n t e r p r e t a t i o n of
|
|
c o m p l e x spectra.
|
I________________________________________________________________I
When buying a Si(Li) d e t e c t o r , we must determine:
1.
2.
3.
4.
Resolution of the d e t e c t o r , in keV (an average
d e t e c t o r has
180 eV)
A r e a , in square mm (the usual
sizes are 30 and 80 sqmm)
Type of c r y o s t a t and d i p s t i c k
Type of p r e a m p l i f i e r (optical or resistive feedback)
12.1.2
Installation and Testing of a High Resolution Detector
The high r e s o l u t i o n solid s t a t e d e t e c t o r s are shipped together
w i t h the dewar, but w i t h o u t l i q u i d n i t r o g e n , except if the customer
specifies otherwise.
A pure germanium d e t e c t o r and a modern Si(Li)
d e t e c t o r can be
stored
for
years
at
room
temperature, without
loosing their c h a r a c t e r i s t i c s .
B e f o r e put into o p e r a t i o n , the dewar
of such a d e t e c t o r must
be
filled
with
l i q u i d n i t r o g e n , and the d e t e c t o r
cooled
for
a
period of 4 to 6 hours.
The manual
of the d e t e c t o r s p e c i f i e s the
minimum
time
for cooling, and it is a d v i s a b l e to adhere
to
the
r e c o m m e n d a t i o n s of the m a n u f a c t u r e r s .
With
each
solid
state d e t e c t o r , the customer
receives
a
certificate describing
the p r o p e r t i e s of
the d e t e c t o r , and the
c o n d i t i o n s under which these c h a r a c t e r i s t i c s have been d e t e r m i n e d .
An example of the c e r t i f i c a t e (in this case, for a Si(Li) d e t e c t o r )
is
given
in F i g . 12.1. When d e t e r m i n i n g the features of
the
d e t e c t o r , one
must
try to r e p r o d u c e
as
close
as possible the
measuring c o n d i t i o n s given in the c e r t i f i c a t e .
A general
observation is that the m a n u f a c t u r e r s are
rather
c o n s e r v a t i v e with the s p e c i f i c a t i o n s presented in the certificate.
With
careful adjustment
of
e l e c t r o n i c s and
correct
measuring
p r o c e d u r e s , the value given by
the m a n u f a c t u r e r
can as a rule be
easily
achieved
and surpassed.
Before you start to write
angry
letters
to
the company
that
supplied
the
detector
about
d i s s a t i s f a c t o r y performance of their d e t e c t o r , be sure
about
your
electronics
s e t u p , grounding
and
overall
conditions
in
your
laboratory. For
example, high
humidity
in
the
laboratory can
easily
deteriorate
the
resolution of a Ge
5000V+ as the bias voltage.
(280)
detector
that
needs
-3-
Chapter
12
Section 6
DIÏTKCTOR Sl'KCIFICATIONS AND PKR FOR M A NCK D A T A
(KKAK KNVKI.OPK)
6.1
SPECIFICATIONS
Serial Number
b 86151
Tne purcluso specifications and LliereLore the warranted pcrfoninrtce of tlus detector are as fol
Model
___7229P-7500-1320
Rel. FJficiency __
13
Resolution
2.0
keV (B-.1M)
keV (FVIN)
at 1.33 MeV
keV (1-V.1M)
keV (FV.TM)
Peak/Ccnipton
at 122 keV
: 1
Cryostat Description
Vertical d i p s t i c k , type 7500
6.2 PHYSICAL/PERFORMANCE DATA
Date
Actual performance of this detector when tested is given below.
Geometry Coaxial one open end, closed end facing window.
Dianeter
48
nm
Length
35
nm
Active area facing window
18.1
5
an1
nm
Depletion Voltage
(+)4000
Vdc.
Recomrended Bias Voltage
(+)4500
Vdc.
Distance from window
ELECTRICAL CHARACTERISTICS
.01
Leakage Current at Reccmnended Bias
Preamplifier Test Point Voltage at Recoimended Bias
nA
- 2.3
Vdc.
RESOLUTION AND EFFICIENCY *
Isotone
Enercv(keV)
[•WN(kcV)
FWIM(keV)
r V
Co
122
.836
,, ftf)
GJ
1332
1.79
3.34
Peak/Comuton
42.4
Efficiencv(%)
13.0
:1
All measurements performed at 4 microseconds sliaping time
Fig.
12.1: The certificate for a Si(Li) x-ray d e t e c t o r
(281)
C h a p t e r 12
12.1.3
-4G e r m a n i u m Gamma-Ray Detector
The
International
Elect r o t e c h n i c a l
Commission
published
d e t a i l e d i n s t r u c t i o n s for t e s t i n g of s e m i c o n d u c t o r d e t e c t o r s .
For
gamma-ray
d e t e c t o r s , the IEC
Publication
656
(1979), "Test
Procedures
for H i g h - P u r i t y G e r m a n i u m D e t e c t o r s
for X and Gamma
R a d i a t i o n " can be ordered from
the B u r e a u Central de la Commission
E l e c t r o t e c h n i q u e I n t e r n a t i o n a l e , 1, rue de V a r e m b e , Genève, Suisse.
12.1.3.1
D e t e r m i n a t i o n of resolution
R e s o l u t i o n of a gamma-ray s e m i c o n d u c t o r d e t e c t o r is defined as
the fu 1 1 - w i d t h - a t - h a ï f - m a x i m u m (FWHM) of the peak t h a t corresponds
to the g a m m a - r a y s from Co-60 at the energy of 1.3MeV.
The m e a s u r e m e n t is s i m p l e : place a s u i t a b l e Co-60 c a l i b r a t i o n
source at a d i s t a n c e of 25 mm
from
the face of the d e t e c t o r . The
c o u n t i n g r a t e of the d e t e c t o r should be at least 1000 counts/s. The
r e s o l u t i o n of
a
detector
depends
on
the
counting rate; with a
higher
counting
r a t e , t h e r e s o l u t i o n decreases.
The
commercial
c o m p a n i e s m a k e t h e i r m e a s u r e m e n t s for the c e r t i f i c a t e at a counting
r a t e very close to the nominal 1000 counts/s.
Using
a spectroscopy
amplifier with carefully
adjusted
pole-zero c a n c e l l a t i o n and a good m u l t i c h a n n e l analyzer, c o l l e c t a
s p e c t r u m of Co-60.
In the c e n t r a l channel of the 1.3 MeV peak, at
l e a s t 10,000 c o u n t s s h o u l d be c o l l e c t e d .
With
a m o d e r n m u l t i c h a n n e l analyzer, the FWHM can
be
d e t e r m i n e d by the f i r m w a r e of
the i n s t r u m e n t . O t h e r w i s e , you can
do
it a p p r o x i m a t e l y on the m o n i t o r of the M C A , or
precisely
by
p l o t t i n g the p a r t
of
the s p e c t r u m w i t h
the 1.3 MeV
peak, and
d e t e r m i n i n g t h e F W H M from t h e p l o t .
Put all the c o n d i t i o n s of the measurement and the results in a
log b o o k , t o g e t h e r w i t h the copy of the c e r t i f i c a t e .
12.1.3.2
Efficiency determination
E f f i c i e n c y is a very i m p o r t a n t
p r o p e r t y of a high resolution
gamma ray d e t e c t o r . The Ge d e t e c t o r e f f i c i e n c y is defined as
the
net area u n d e r the Co-60 peak at the energy of 1.3 M e V , c o m p a r e d to
the net area u n d e r the same peak measured w i t h a 3x3
inch Nal
d e t e c t o r . The e f f i c i e n c y is given in %.
Place a Co-60 c a l i b r a t i o n source at a d i s t a n c e of 25 cm from
the surface
of the 3x3 inch Nal d e t e c t o r , take the s p e c t r u m and
d e t e r m i n e the area under the 1.3 MeV
peak. R e p e a t the measurement
with
the Ge d e t e c t o r under i d e n t i c a l g e o m e t r y . The r a t i o of
the
i n t e n s i t i e s g i v e s you the e f f i c i e n c y :
eff
(282)
=
I(Ge)
I(Nal)
x
100
-5-
C h a p t e r 12
Some o t h e r very i m p o r t a n t i n f o r m a t i o n t h a t tells us a b o u t the
q u a l i t y of the d e t e c t o r , is the d e p e n d e n c e of the efficiency on the
energy of the d e t e c t e d gamma rays.
A n u m b e r of m e t h o d s have been
designed
for
a c c u r a t e m e a s u r e m e n t of this p r o p e r t y , and
computer
programs
are
available
for
e v a l u a t i o n of m e a s u r e m e n t s .
The
simplest methods
involve
the use of a set
of
calibrated
gamma
sources.
For d e t a i l s see "A Guide and I n s t r u c t i o n for D e t e r m i n i n g
Gamma-Ray
Emission
R a t e s w i t h Germanium
Detector
Systems",
by
K. D e b e r t i n , May 1985.
12.1.4
Si(Li) and Planar Ge D e t e c t o r
They
are
for high r e s o l u t i o n x-ray s p e c t r o m e t r y .
From
the
outside, there
is only one
essential observable
difference,
compared
to the gamma-ray Ge d e t e c t o r s : on the top of the d e t e c t o r
cap we find
a b e r i l l i u m window.
It r e p r e s e n t s
a barrier that
separates
t h e vacuum
inside the detector
from
the atmospheric
pressure
o u t s i d e , and
does not
permit
light
to f a l l on the
detector.
It does not stop x-rays a p p r e c i a b l y , e x c e p t if they
are
of very low e n e r g y , s a y , below 4 keV.
A newly
received
x-ray
detector
should
be
t e s t e d for its
performance
i m m e d i a t e l y upon arrival. A f t e r cooling it w i t h
the
l i q u i d n i t r o g e n for several hours, the t e s t s can start.
12.1.4.1
Connections and i n i t i a l t e s t s
The x-ray solid state d e t e c t o r s use two t y p e s of p r e a m p l i f i e r :
w i t h r e s i s t i v e f e e d b a c k , or w i t h
o p t i c a l feedback. Most of the
c o n t e m p o r a r y Si(Li) d e t e c t o r s use the o p t i c a l feedback because t h i s
p e r m i t s t h e m to o b t a i n b e t t e r
resolution.
The
one
honourable
e x c e p t i o n are
the d e t e c t o r s p r o d u c e d by ORTEC, using r e s i s t i v e
feedback.
Let
us
consider
the case of a preamplifier with o p t i c a l
feedback.
A f t e r connecting
the high voltage
(SHV c o n n e c t o r s ,
usually
a
RG59
cable;
observe the p o l a r i t y ,
in
most
cases
a
negative voltage
is a p p l i e d ! ) , use
a BNC cable to d i s p l a y the
output
(sometimes called
E N E R G Y ) from
the p r e a m p l i f i e r
in
an
oscilloscope. Before the h i g h v o l t a g e is a p p l i e d , the o s c i l l o s c o p e
(try
to use the a m p l i f i c a t i o n on the v e r t i c a l scale t h a t gives 5
V / c m , and a t i m e c o n s t a n t
of
the sweep
of 100ms) will show t h a t
t h e r e is some n e g a t i v e v o l t a g e (about 1 2 V ) on the o u t p u t . This you
can see if you s w i t c h the oscilloscope from DC to G R O U N D o p e r a t i o n .
Slowly
s t a r t to increase the v o l t a g e , say a b o u t 100 V per
second.
If you s t o p for a m o m e n t at 300 V, you will see on the oscilloscope
the
picture
as
shown
in Fig. 12.2. A f t e r a few
seconds, the
s a w t o o t h shape of the signal w i l l become s t r e t c h e d out (Fig. 12.3),
u n t i l you see
only a s t r a i g h t line. N o w ,
the v o l t a g e can
be
increased to the p r e s c r i b e d v a l u e .
(283)
C h a p t e r 12
-6-
Fig.
12.2: Scope shows sawtooth shapes when high v o l t a g e is
applied to an o p t i c a l feedback p r e a m p l i f i e r
Fig.
12.3: In a short t ime, the shape changes to a low
frequency saw
It is time to put a r a d i o a c t i v e calibration source on the
d e t e c t o r . It is p r e f e r a b l e to use a Fe-59 source, e m i t t i n g x-rays
of 5,9 keV. If you do not have a c a l i b r a t i o n source, use an
e x c i t a t i o n source
in a s u i t a b l e holder, and place a sample
that
contains mainly iron, in the position of the s a m p l e , on top of the
holder.
If
the counting rate is h i g h , the sawtooth w i l l appear again.
Increase the sweep frequency and the v e r t i c a l a m p l i f i c a t i o n of the
oscilloscope, and you will observe the steps on the increasing line
of the sweep, Fig 12.4.
(284)
-7-
Fig.
12.4:
C h a p t e r 12
Shape of the p r e a m p l i f i e r o u t p u t w i t h x-rays
coming into the d e t e c t o r
The d e t e c t o r and the p r e a m p l i f i e r are working.
12.1.4.2
Résolut ion
After
connecting the p r e a m p l i f i e r to the main a m p l i f i e r , and
t h i s to the m u l t i c h a n n e l analyzer, adjust the a m p l i f i c a t i o n of the
main a m p l i f i e r
so
t h a t the 5.9 keV peak will be
registered
in
channel 300 or there about. Use
long shaping
time (try to m a t c h
the s p e c i f i c a t i o n on the c e r t i f i c a t e ! ) , say 16 m i c r o s e c o n d s , and
the a m p l i f i c a t i o n a b o u t 300.
Make the energy c a l i b r a t i o n of
the MCA using two c a l i b r a t i o n
x-ray
sources, or two d i f f e r e n t pure m e t a l s on top of the holder
w i t h the e x c i t a t i o n source.
One
of
the two should be the Fe-59
source.
Collect
sufficient
counts in the peak to o b t a i n good
s t a t i s t i c s . Determine the FWHM of the k-alpha (the largest) of the
Fe-59 r a d i o i s o t o p e peaks. The FWHM value of the 5.9 keV peak, in
eV, is the resolution of the d e t e c t o r , as given in the c e r t i f i c a t e .
If no Fe-55 c a l i b r a t i o n source is available, you can use an
e x c i t a t i o n source, and an iron sample. A l t h o u g h the resolution is
determined
at
5.9 keV (the Mn K-alpha line), the value
obtained
with the iron target (6.4keV for the K-alpha line) will be a good
approxima t ion.
12.1.4.3
Efficiency
Determining the efficiency of the x-ray d e t e c t o r as the
function of x-ray energy is a tedious and long procedure (see for
example
the IEC Publication 759, "Standard
Test
Procedures
for
Semicondict or X-Ray Energy
Spectrometer". Around 10 keV, a Si(Li)
detector
efficiency
is
100%,
falling
to zero at 2 keV, and
decreasing less radically on the high energy side.
(285)
Chapter 12
12.1.5
-8-
Maintenance of High Resolution D e t e c t o r s
In
addition
to all the established routines for
the proper
h a n d l i n g of nuclear i n s t r u m e n t s , some specific p r e c a u t i o n s should
be a p p l i e d
to the solid s t a t e d e t e c t o r s
that
o p e r a t e at
low
t e m p e r a tures .
All m o d e r n , high resolution solid
state d e t e c t o r s can be
stored
at
room t e m p e r a t u r e .
In your l a b o r a t o r y , you might
still
have an old Ge(Li) d e t e c t o r , or a Si(Li), m a n u f a c t u r e d before 1982.
These d e t e c t o r s have to be kept at a low t e m p e r a t u r e at all times.
How
do we know when the level of the liquid n i t r o g e n
in
the
dewar is too low? One can buy some level measuring devices (or, as
an e l e c t r o n i c s m a n , you can design one yourself). More simple
is
to put the d e w a r w i t h
the d e t e c t o r on a b a t h r o o m scale, and
d e t e r m i n e how
heavy
the system
is w i t h , and
without
liquid
nitrogen.
In many
l a b o r a t o r i e s , the dewar
is refilled by a rather
dangerous p r o c e d u r e of taking the d e t e c t o r from the dewar, filling
the l i q u i d nitrogen by p o u r i n g
it in
the dewar, and reinstalling
the d e t e c t o r .
If you are a very careful and rather
strong m a n ,
this t e c h n i q u e works. But it is m u c h more a d v i s a b l e , p r a c t i c a l and
safe
to a c q u i r e a transport dewar w i t h a filling a t t a c h m e n t
(see
Fig. 12.5).
RELIEF/DRY
NITROGEN
PRESSURIZATION
INLET
Fig. 12.5: The nitrogen transfer device
The d e t e c t o r cap is connected
to the dipstick with a flange,
see Fig 12.6. An 0-ring takes care of vacuum tightness of the
joint. Be careful not to pour liquid nitrogen on the flange. The
cooling will make the rubbery material of the 0-ring b r i t t l e , and a
vacuum leak will develop. Some manufacturers use metal rings; they
to
are safe as far as the cooling is concerned. but are d i f f i c u l t
replace.
(286)
Chapter 12
-9-
By m i s h a n d l i n g , liquid nitrogen can be poured on the flange.
While this is bad,
it is not necessarily a c a t a s t r o p h y .
Tighten
the screws h o l d i n g the b o t t o m and top flange t o g e t h e r ; you m i g h t be
lucky, and
the 0-ring can be squeezed enough to still hold
the
vac uum.
It is a recommended policy
to keep the d e t e c t o r in l i q u i d
n i t r o g e n all the time. The s h o r t a g e of liquid nitrogen m i g h t force
you to let the d e t e c t o r warm up.
If this h a p p e n s , let it warm up
all
the w a y , i.e. let it spend a day or two at room
temperature
before it is cooled again.
Dflnn
Of lee
Copper
Aluminium
Fig.
12.6:
The inner life of a semiconductor d e t e c t o r .
the flange with an 0-ring.
Note
High resolution detectors are sensitive to mechanical shocks.
Be very
careful when moving them. A special
feature of
these
d e t e c t o r s is microfony. Mechanical vibrations or accustic signals
will be t r a n s f e r r e d to the d e t e c t o r , and u n d e s i r a b l e noise will be
produced.
Different
precautions can be taken to avoid these
effects:
- place the dewar of the detector on a soft, rubbery mat;
- put sound-absorbing m a t e r i a l around the d e t e c t o r c a p ;
- put some fine sand in the b o t t o m of the dewar, to a level
that
the end of the dipstick will be inserted in the sand;
this prevents bubbles
from
forming
on
the surface of the
d i p s t i c k end.
With the Si(Li) d e t e c t o r s , the berillium window is the most
sensitive part. It can be easily broken; high h u m i d i t y will cause
(287)
C h a p t e r 12
-10-
corrosion resulting in the appearance of m i c r o s c o p i c vacuum
leaks.
When not in use, the detector should be p r o t e c t e d with the p l a s t i c
cap.
If s t o r e d for a longer p e r i o d , the d e t e c t o r top should
be
covered w i t h a p l a s t i c
s h e e t , with
some s i l i c a gel to avoid h i g h
humid it y.
12.1.6
T r o u b l e s h o o t i n g of Ge and Si(Li) D e t e c t o r s
The
s e m i c o n d u c t o r h i g h - r e s o l u t i o n d e t e c t o r s are
normally
bought together with
the p r e a m p l i f i e r .
The
p r o p e r t i e s of the
d e t e c t o r (and w i t h them its p r i c e ) are d e f i n e d for the c o m b i n a t i o n
de t e c t or + p r e a m p l ifier + dipst ick.
With
some
detectors,
the
preamplifier
can
be ordered separately as a replacement, to be
i n s t a l l e d locally.
There are several possible combinations
and each s h o u l d be t r e a t e d in a s e p a r a t e way:
and c o n f i g u r a t i o n s ,
(i)
The
ÜRTEC d e t e c t o r s are recognized by
their
shape: the
p r e a m p l i f i e r is i n t e g r a t e d in the
cylinder of the d e t e c t o r
head,
see
Fig.
12.7. This s t r e a m l i n e d
solution
is
p r a c t i c a l for some m e a s u r e m e n t s where the d e t e c t o r must be
inserted
in a measuring
c h a m b e r , and
the
protruding
p r e a m p l i f i e r is an o b s t a c l e .
These d e t e c t o r s are somewhat
more d i f f i c u l t to service and repair.
(ii)
A widely
accepted
configuration
is shown in Fig. 12.8.
H e r e , the p r e a m p l i f i e r is located
in a separate box that is
attached
to the d i p s t i c k . Such a p r e a m p l i f i e r can
contain
the i n p u t s t a g e , w i t h
the FET
at room
t e m p e r a t u r e : this
solution
is sometimes a p p l i e d to the gamma-ray
detectors.
In most cases, the p r o t r u d i n g box does not c o n t a i n the input
stage;
t h i s is somewhere inside the d e t e c t o r c a p , w i t h
the
FET at low t e m p e r a t u r e .
Suppose
t h a t you have
a
gamma-ray
detector,
with a
p r e a m p l i f i e r o u t s i d e the d e t e c t o r cap. Something seems to be wrong
w i t h it. B a s i c a l l y , there are
two
possibe
types of
defects,
mechanical and
e l e c t r o n i c . The d e f e c t s and their
symptoms are
listed in Table 12.1
(288)
-11-
Fig.
12.7: Streamlined d e t e c t o r p r e a m p l i f i e r setup
Chapter 12
Fig.
12.8
Preampl i f ier
outside
TABLE 1 2 . 1 ; Defects and how they are recognized
(a)
(b)
Mechanical defects:
- the vacuum inside the
d e t e c t o r cryostat is bad
moisture or ice accumulated
at the neck of the cryotat
- the detector is broken
no signal at the o u t p u t
when gamma source is close
- the wires inside the
detector heads are broken
no signal with source or
with pulser
Electronic faults
- input FET is defective
no o u t p u t signal, with test
signal on the input
- d e t e r i o r a t e d resolution
either damaged d e t e c t o r or
moisture accumulation in
the p r e a m p l i f i e r
- bad efficiency (x-ray)
dirty Be window or
detector crystal surface
(289)
Chapter 12
-12-
There are
some actions that you can implement to repair a
defective or improperly operating detector. But there are a number
of
faults when the detector must be returned to the factory
for
repair. Let us analyze some typical cases.
(i)
When touching the detector c a p , it feels very cold; moisture
or
ice a c c u m u l a t e at the point closest to the dewar.
This
is a sure sign that the vacuum inside the detector cryostat
is bad. If you have a good vacuum
laboratory, with a
pumping system and a He
leak
d e t e c t o r , you
can find the
leaking spot, and repair it. Then you pump the cryostat for
several hours, at the same
time heating the bottom part of
the cryostat, and the protruding dipstick to 80 or 100
degrees C.
Without
the
proper vacuum e q u i p m e n t , the
d e t e c t o r should be returned to the manufacturer's service
laboratory.
ATTENTION : Do not apply high voltage to a detector which
shows signs of a bad vacuum inside the cryostat.
Some detectors (but not all) have a t e m p e r a t u r e sensitive switch inside the cryostst that prevents
the a p p l i c a t i o n of high voltage to a d e t e c t o r that
is not sufficiently cooled.
(ii)
The most obvious loss of vacuum happens if the Be window on
an x-ray detector
breaks.
Immediately
switch off high
v o l t a g e , cover
the d e t e c t o r with a t i g h t plastic cap, and
remove the detector with
cryostat
from
the dewar. If you
are fast, the d e t e c t o r crystal itself can be saved, and
the
costs of repair will be only
for mounting a new Be window.
W i t h o u t vacuum but with high voltage on, the d e t e c t o r will
heat up, the Li will d r i f t out of the d e t e c t o r , and you will
have to buy a new one.
(iii)
If
the detector is properly cooled, and yet
there
is no
signal at
the p r e a m p l i f i e r
o u t p u t , there are
several
possible reasons:
- you
forgot
detector ;
to put a calibration source in front
of
the high voltage is not p r o v i d e d ; check the opérât ion
the bias supply and the connections
the
of
the low voltage power supply
(usually +/- 24 V) is not in
order; check the o u t p u t s of the preamplifier connector
(mounted on
the main
amplifier
backplane) and
the
connecting leads;
the preamplifier is defective; closely inspect the printed
board in the preamplifier and all connections;
(290)
-13-
Chapter 12
- the inaccessible parts of the detector (the crystal itself
or
cooled FET) are f a u l t y ; in this case, without a well
e q u i p p e d vacuum laboratory, you can do very little.
(iv)
The d e t e c t o r resolution has d e t e r i o r a t e d . The most frequent
reason is
the h u m i d i t y and
the condensed water
in the
p r e a m p l i f i e r . Use a hairdryer to remove moisture. A second
possible cause is
dirt
on
some
components of
the
p r e a m p l i f i e r , or on the connector pins protruding from
the
cooled part of the cryostat.
Clean
t h e m , preferably with
m e t h a n e , but at least w i t h a dry and clean cotton buds. Pay
special a t t e n t i o n to the
high voltage
filter
in
the
p r e a m p l i f i e r , usually made of two highohmic resistors and
two high voltage capacitors.
The
test
input on the preamplifier box is connected
to the
d e t e c t o r crystal via the high voltage supply lead. Using a pulser,
or a pulse g e n e r a t o r , you can inject
the signals through
the
d e t e c t o r diode to the FET. If
there are signals on the o u t p u t of
the p r e a m p l i f i e r , you can conclude that the FET is working, and
that
the high voltage lead
to the detector
crystal is
not
interrupted.
If the x- or gamma-rays are still not d e t e c t a b l e , you
can conclude, w i t h a high p r o b a b i l i t y , that the detector itself is
faulty .
NOTE: Modern high resolution semiconductor d e t e c t o r s are good
and
reliable instruments.
If operated with proper
care, they will
last for many years. Even the old
d e t e c t o r s can be
trusted; fifteen-year-old Ge(Li)
detectors
that
s t i l l operate with
full
original
specifications are not unusual. In most cases, it is a
human error
that
kills a detector.
Handle
these
detectors with care, and you will not need the above
troubleshooting advice.
(291)