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 cover and check the output voltages of each power supply section. At the moment of power-on, the VS voltage gradually rose to 25V, but then the entire power supply shut down immediately. Based on my experience, this strongly indicated an issue with the VS output circuit. I checked all the components involved in the VS voltage generation path, including Q5, Q6, D21, and D22, and found no visible damage or abnormalities. The power supply for IC17, IC18, IC19, Q5, and Q6 was also normal. Using an oscilloscope, I verified the waveforms of Q5 and Q6, which appeared to be functioning correctly. 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, even after replacing them with identical units, the problem still persisted. During a random test, I discovered that C19 (1µF/630V) had no charge or discharge behavior, indicating it had lost its capacitance. Replacing this capacitor resolved the issue completely, and the TV functioned normally afterward. This case highlights how a seemingly minor component can cause significant problems if not properly tested.