Specifications

CA1 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 60
primary winding of the line output transformer.
A pulse is fed back from pin 10 of T601 to indicate to the line time base generator that the stage has
started up. At this point the drive signal changes frequency from 32.25 kHz ( twice line frequency ) to its
normal running frequency of 15.625 kHz. Also at this time, to enable the output transistor to draw enough
current to maintain its secondary voltages, its base current must increase significantly. As there is not
enough power in the +7V supply to achieve this, so a tap is taken from T601 ( winding 3 to 8 ) which feeds
into the collector of Q602. This will provide a low impedance source, thus ensuring adequate current flow.
It is also important to turn off Q601 quickly to prevent it overheating and failing ( note that on the 37cm
versions of this chassis, the line output transistor is not fitted with a heat sink ). When the line rate
changes it is imperative that this is achieved, but just relying on the -10V supply via R616/9 is not enough,
as this supply can not supply adequate current over the very short time it is required to maintain the
voltage. To provide a lower impedance path, C604 and C622 are fitted. These capacitors allow the
current and therefore voltage to be maintained on the collector of Q604 while Q601 turns off. As this off
time is of a relatively short duration, the charge on the capacitors is not reduced significantly. During the
on time of Q601 the charge on C604 and C622 is replenished via R619 and R616 from the -10V supply.
It is important that Q602 and Q604 do not turn on at the same time as could lead to the failure of the
output transistor due to it working in the linear part of its characteristic. To eliminate this possibility, L601
is fitted to slow down the change of voltage at the base of Q601.