View Sharp 28HW-53 (serv.man20) Technical Bulletin online
IC702. If the supply on pin one of this IC falls below 5V, the operation of the power supply becomes unreliable (Note
that you will have to connect your meter ground to the negative end of the smoothing block). As can be seen from the
previous circuit description, the supply circuit is fairly complex, however only a few components cause problems. For
low supply voltage to IC702 check R770 (270
up. Normally its output goes low at around 3V.
breaking down under load. It is important that these two resistors are checked by substitution as they often read OK on
a meter, even though they are faulty. Note that they should be metal film resistors and not carbon composition.
encountered are no remote control operation or intermittent control of the volume, either increasing dramatically or
rising slowly with the OSD visible. D729 can cause these problems, but it is more common for noise to enter IC702 and
spurious remote control commands to be outputted to the microprocessor. To prevent this happening fit a 470nF, 16V
SMD capacitor across the input of the optical diac – pins 1 and 3 of IC708 (see picture below). The Sharp part number
for this capacitor is VCKYTV1CF474Z. This capacitor was fitted during production of all GF and HW models.
Intermittent failure of the chopper transistor can be caused by dry joints in the snubber circuit (C710, R715 and D711)
or poor connections on the main reservoir capacitor, C705.
can lead to a power supply that does not work, causes excessive RF interference or is unreliable. Note that this device
has been replaced by an alternative which requires D712 and R717 to be changed - see Technical Bulletin CTV 2002
12 01 for more details.
If there is a mains surge (or lightning strike) Q701 will invariably fail. D712, Q702, Q703 and R716 will also have
suffered damage, as will the rectifier diodes (D701, D702, D703 and D704). It is advisable to change all of these
components to prevent further failures, even if they are not faulty.
IC706 can cause high or low HT. It is a programmable zener diode that provides a stabilised voltage to the opto
coupler. The Sharp part number for this device is RH-IX1704BMZZ.
Erratic operation of the processor (locking out, no control, etc) can be caused by the 3.3V rising to 5V due to one of the
components in the series regulator going faulty - Q710, Q711 or Q712. Check for 3.3V on the emitter of Q712.
isolation transformer be used in conjunction with the set. This will ensure safe connection to the primary side reference.
This waveform will also change in pulse width depending upon the load demanded from the power supply. The greater
the load the longer the on time. The FET will switch on at about 4.5V, this is indicated as a short levelling off period on
the upward ramp.