Download Sharp 37DM-23H Specifications

CA10 Course Notes
Line Stage
Line signal generation is carried out within IC201, with synchronisation being supplied by the incoming sync
pulse. The line stage generates various supplies, including EHT, focus, G2, A1 and heaters and the line
scan coil drive signal.
Figure 15 : Block Diagram of Line Stage
Line time base generation
IC201 generates the line drive signal internally using an oscillator whose frequency is controlled by various
external components, the output of this oscillator is approximately 15.625kHz. Synchronisation is taken
from the incoming video signal ( tuner or video inputs ) line synchronisation pulse when a signal is being
received, in all other cases the oscillator is left to free run. When free running the on screen display is
locked internally by comparing the sand castle pulse with the graphical information.
When the set is first turned on, using the standby button, the line oscillator runs at twice line speed 31.25kHz. IC 201 then monitors the feed back from the line stage on pin 41 ( SCL ). When this signal is
correct, the oscillator switches to 15.625kHz. The reason for this is so that the capacitors on the secondary
windings of the line output transformer charge up faster to enable the line stage to reach its optimum
operating characteristics quickly. This will prevent any excessive strain being put on the windings of the line
output transformer. It is also necessary as the line drive circuit uses secondary supplies from T601 to
ensure that the output transistor is turned hard on, if the line speed was not doubled then the set would not
have enough energy to start up.
Line drive circuit description
The line drive circuit does not use a conventional coupling transformer, but is instead directly coupled to the
drive IC as shown in the diagram on the next page.
From pin 40 of IC201 ( H OUT ) the line drive signal is fed through the DC blocking capacitor C606 into the
base of Q601. D601 prevents any spikes that may appear on the drive signal from damaging the rest of
the circuit. R604 pulls up the base of Q601 to the +13V supply.
As the line drive starts up at 31.25kHz, there will be no voltages generated by the line output stage to enable
the line driver circuit to operate correctly. Therefore a feed is taken from the +13V and -16V lines off the
chopper stage. In this situation, when Q601 is on the base of Q602 will be taken low ( +13V plus -16V
giving -3V ) and Q602 will not conduct. When its base goes high, Q602 turned off, its collector current will
start to rise, but this rise is limited by a number of factors. First is R605 and the second is the current drawn
by Q602 itself. Both of these conditions mean that the base of Q602 never exceeds 0.65V. As Q602
conducts an emf will be generated in the secondary winding of pins 4, 6 and 10.
Page 16
Sharp Electronics (UK) Limited
CE Technical Support Group