Product specifications

DA100 (50Hz) and DA50W Chassis – Repair Notes

Page 31 of 80

Faults in the Horizontal Stage

Horizontal Output Transistor Failure

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

C device if it has not already been done.

For intermittent failure of the horizontal output transistor it is recommended that C607, D610 and D611 be

replaced. 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 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.

It has been known for the power supply opto coupler feedback IC, IC705 to cause the HT to rise slowly. This

results in Q601 failing before the avalanche diode goes short circuit.

Note that the horizontal output transistor must be the correct type. Failure to use the correct transistor will

result in erratic operation or premature failure of the device. Use part number Q601 is RH-TX0144BMZZ for

Q601.

Horizontal Drive Problems

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.

Figure 28: Location of Link Wire JL2

Large Picture

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.

Sharp Electronics (UK) Limited - May 2003

Revision 3