Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 17
Once IC201 has detected a feed back pulse, it will switch the line drive signal frequency to the correct
running rate of 15.625kHz. At this point the supplies from the chopper stage are not used and those
generated by the flyback transformer, T601, will take over.
As the current drawn by the base of Q602 is proportional to that drawn by its collector, it is desirable to have
a low impedance source to drive its base ( hence the use of a coupling transformer in a conventional line
driver circuit ). In this chassis a base current of four to five amps ins not unusual, and therefore a low
impedance source need to be provided, which in this case is provided by the winding 4 to 10 of T601.
Assuming that the line has started up, when Q601 is off the current to turn on Q602 will flow from pin 10 of
T601, through R605, L602 and L1/R606 into the base of Q602. This will turn on the device and current will
flow in the primary winding of T601.
When Q601 is turned on, the current that would normally flow into the base of Q602 is now diverted through
Q601's collector/emitter junction. This will turn off Q602 as its base voltage will drop below 0.65V. In actual
fact the base voltage of Q602 drops to about -10V.
As can be seen from the diagram, pin 6 of T601 also generates the +45V supply used in the east/west
correction circuit, but the voltage on the anode of D602 is negative. This is due to the fact that the waveform
appearing at pin 6 is over 50V in the positive direction, but only about -15V in the negative direction ( mainly
overshoot ). It is this negative going waveform that is rectified to produce the turn off voltage for Q602.
Figure 16 : Line Drive Circuit
R606 and L601 couple the line drive signal generated by Q601 to the base of the output transistor, Q602.
These components are necessary to ensure that the transistor is fully saturated. They also, to some
extent, provide protection to Q601 and IC201 should the output transistor go short circuit and allow
excessive current and/or voltage to appear on its base.