View Sharp 28HW-53 (serv.man19) Technical Bulletin online
If the negative supply used to turn off the horizontal output transistor is too low, then it will not turn off fully. This will
result in a large voltage across the collector/emitter junction and current being drawn at the same time. The power
generated will have to be dissipated by the transistor itself and therefore it will get very hot. Eventually it will fail. A
cause of this is failure of C607 (low capacity or leaky), causing the negative supply to fall. Note that it is advisable to
change this for a 105
Dry joints in the output stage can also cause this problem – the most common areas affected are the horizontal coil
earth return circuit C613, R613 and associated circuitry and the scan coil connector itself. Sometimes C613 and/or
R613 will go open circuit resulting in no horizontal scan leading to the possibility of the horizontal output transistor going
short or leaky. C613 can also go faulty under load, so it is best to check by substitution. C528, C632 and C615 are
also known to cause the output transistor to fail intermittently.
diode goes short circuit. This can take up to several hours to happen.
erratic operation or premature failure of the device.
Clean and resolder legs on C613. In severe cases arcing from the joint may have caused failure of R610, R613 and
L603 in addition to the transistors Q601 and Q506. If Q506 runs very hot when replaced L603 must be changed.
If D1601 on the focus modulator PWB (76cm models only), goes leaky or short circuit, the result is an over large picture
due to the EHT dropping to about 20kV.
Sometimes it will appear that there is no horizontal drive signal, even though the microprocessor has gone through the
boot sequence. In this situation, it is possible to release the horizontal mute to enable the horizontal stage to start.
There are a number of ways that this can be achieved, one is to short out the base/emitter junction of Q607, and the
other is to temporarily isolate link JL2. If this link is removed, it is vital that it is replaced after completing the repair as
failure to do so will result in premature failure of the line stage.