MY SERVICE EXPERIENCES
Part-3
As the picture is stretched out, I concentrated to its pincushion circuits. It was right. The IC used to control pincushion distortion [LM 393N] was found faulty. Its pin number [8]&[4] short. Replaced the same.
Still the effect was the same as described earlier. Switched OFF the set, and checked. Now it was a cold check; because, if I attempt a hot check, with power ON, it is sure and certain that the newly replaced horizontal output transistor [2SD2539] will get shorted within seconds. so I did all resistance measurements, after desoldering out the the Pin amp IC stated above from board with Power OFF.
The pin number 8 of the said IC must get [+9V], states the schematic diagram. So I followed the supply track to this IC. There is a coil [L501] which supplies voltage to this IC was found open. This might have happened when the IC shorted. Replaced it with an other (47uH) coil. Switched on the set. Found almost OK, except to some minor adjustments to vertical lines with pattern generator. [A barrel effect with new pincushion IC]. Some times, the technician who tried to make this set right previously, might turned the pincushion adjustment preset for an experiment. There is preset resistor in circuit to adjust it. Adjusted it, and checked OK.
It is working since one hours. No problem at all. All perfect picture. Heat to horizontal output is minimum. good picture clarity as a Trinitron must have.
Conclusion: The set have first been out of control of secondary power supply voltage [135VDC]. It have raised automatically, due to the defect of bypass electrolytic capacitors, and this voltage turned up to an extent that, the microcontroller of the set could't detect this. If it might exceeded 150VDC, the microcontroller will detect it, and will switch OFF the power supply section automatically.
Here the set have worked with high voltage, which in turn forced the pin cusion output IC, to an extent, that it has shorted internally. As this IC has shorted, there is not control over the vertical and horizontal amplitude, which in turn put a high load to flyback output transistor, and it failed.
