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Changhong plasma TV 3D42A3700iD soundless map failure repair - android - Phones Developers
I posted a help request earlier, you can check the details: "[Help] Changhong 3D42A3700iD Plasma TV has no image."
The information about plasma TVs is very limited. If there's no significant progress in the query for a long time, I'll take it step by step. Since the VS-ON signal is abnormal and this signal comes from the logic board, there are two possibilities:
1. The VS power supply has a short circuit or overcurrent issue, which causes the protection mechanism to trigger, resulting in an abnormal VS-ON signal.
2. The logic board itself is faulty.
I decided to start with the easier checks first. I measured the load of the VS power supply and found no short circuit. Therefore, the problem must be on the logic board. I thoroughly inspected the board, reinstalled it, but the issue remained. I didn't give up. Without a schematic diagram, I started tracing the circuit, following the VS voltage to the X board. There were only a few filter capacitors on the X board. Then I moved to the Y board, removed it, and checked the power transistors. The issue was found in one of the power transistors—its resistance between the D and S terminals was abnormal. Both forward and reverse measurements showed more than 100 Ω, which wasn’t a breakdown, but something was wrong.
After checking the PCB traces, I noticed two snubber capacitors. Visually, they looked fine, but upon closer inspection, there was a sign of metal splatter under one of them. I decided to remove the capacitor. Sure enough, there was a crack between the two legs of the capacitor. A quick test confirmed that it had failed. The original capacitor was rated at 331/1KV, but I didn’t have a matching one on hand. However, I had three 221/2KV capacitors available. I replaced the original two 331s with these three 221s. Although the capacitance was slightly different, their frequency characteristics and voltage rating were better than the original.
After replacing the capacitor, I checked all other power transistors and found no issues. I closely monitored the VS voltage and VS-ON signal on the TV. Everything remained stable, and neither the voltage nor the signal fluctuated anymore. Finally, the picture returned!
I ran the TV for half an hour and observed that all power rails heated up normally. The replaced capacitor was only slightly warm, which was expected. The repair was complete.
Testimonial: After repairing home appliances for so many years, I felt that "first-time" excitement again. Even though this might seem routine for experienced plasma technicians, it was a big milestone for me. I realized how much I still have to learn in this area, and it motivated me even more.