TCL plasma TV PDP4226 secondary non-boot failure repair - android - Phones Developers

Photocoupler

During the overhaul of a TCL plasma TV model PDP4226, the first step was to open the back casing and measure the output voltages of each power supply section. At the moment of powering on, the VS voltage gradually increased to around 25V but then suddenly caused the entire power supply to shut down. Based on my experience, this strongly indicated an issue in the VS output circuit. I checked all components involved in forming the VS voltage, including Q5, Q6, D21, and D22, and found no abnormalities. The power supply for IC17, IC18, IC19, Q5, and Q6 was also normal. Using an oscilloscope, the waveforms of Q5 and Q6 appeared to be stable and within expected ranges. Additionally, the preamplifier showed no signs of short-circuiting during normal operation. At this point, the only remaining suspects were the transformers T4 and T5. However, after testing two identical transformers, the problem still persisted. It wasn't until I randomly tested C19 (a 1µF/630V capacitor) that I noticed something unusual. When I tried to charge and discharge it, there was no response, which meant it had lost its capacitance. Replacing C19 resolved the issue completely, and the TV returned to normal operation without any further problems. This case highlights how sometimes a small, seemingly insignificant component like a capacitor can cause major system failures. It's always important to thoroughly check all possible causes, even those that might appear unlikely at first glance.