View Sharp 66GS-62 (serv.man17) Technical Bulletin online
When fault finding on the power supply fitted to the GA10 chassis, it is important to note that its ability to regulate
correctly is dependent upon the power factor circuit output. The power factor circuit sits between the mains bridge
rectifier and the resonant power supply. When the set is first turned on, its operation is muted and the supply generated
across C713 will be approximately 320VDC. Once the power supply is operational, the supply across C713 will
increase to 420VDC. The set will operate without the power factor circuit turning on, but it may be intermittent, or start
up, then shut down.
will be about 14.5VDC. Note that all voltages are taken with reference to the negative end of C713. This supply is
taken from pin 7 of the chopper transformer via R718, D737 and C709. Note that the mute circuit that operates on the
power factor circuit is dependent on the base voltage of Q704. If the base voltage rises to 10.7V (its emitter being fixed
at 6.8V by D714), then the mute is released and the power factor circuit will operate.
There is a misprint on the schematic diagram. Between the supply line and the emitter of Q704, a 2.2k
Either reprogram or blank the NVM. This problem may cause the power supply to go into the standby mode, or just
inhibit the line or east/west drive signals. Checking waveforms and voltages may be misleading with this fault scenario.
check R718, D737, C709 and the operation of the mute circuit based around Q704 and Q705.
No power factor corrector or power supply operation. This normally means that the power factor circuit has failed and
the mains input fuse is open circuit. Check and replace D736, D739, and Q701. If Q701 is short circuit, it is likely that
R714 has gone open circuit causing R705 to go faulty and resulting in Q720, Q721 and IC701 failing. In this situation
replace R705, R714, Q720, Q721 and IC701.
start up oscillator will continue to run and series of pulses will be seen at the junction of R781 and C716.
If the power supply does not kick off - start up oscillator running, but Q702 and Q703 not energising the chopper
transformer - the HT will be low (up to 40V) or zero. Q702 or Q703 breaking down internally can cause this, even
though they can measure all right on a meter.
to rise. Q713 has been known to fail short circuit, but other components in this area can cause the same fault.
Unit intermittently will not start up from standby, shuts down or the line collapses. Q606 failing intermittent, it may read
perfectly alright on a component test. This transistor is part of the protection feedback circuit.
There is a protection line that monitors the outputs to the loudspeakers. If this output becomes unbalanced, then the
protection pin on the processor (pin 63 - PROT) will go low, putting the set into standby. This normally happens if the
loudspeakers are left unplugged when servicing the set. Depending upon the component tolerances on the chassis,
this fault can also happen intermittently while the set is running.
There are two possible causes of this problem - D724 can go leaky or short circuits, reducing the +5V supply, or the
processor can fail. There are two different types of microprocessor fitted to this model depending upon the size of the
EEPROM (IC1003). See parts issues Technical Bulletin for more information on this.
When the set is left in standby, it may turn on. This fault can be very intermittent. This problem is caused by failure of
This problem can be caused by one of the HT feedback resistors (R742, R743, R744 and R745) on the secondary of
the power supply. Usually it is R743 that fails and it is best to check this component by substitution as the fault may not
respond to flexing the PWB.
has been compiled to show all the part numbers for items refered to in this document. If the parts noted in the text are
not faulty, carry out fault finding procedures to locate the problematic part.