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Changhong plasma TV 3D42A3700iD soundless map failure repair - android - Phones Developers
I posted a help request earlier, titled: “[Help] Changhong 3D42A3700iD Plasma TV has no image.†The information available about plasma TVs is quite limited. If there’s no significant progress in the search, I’ll take it step by step. Since the VS-ON signal isn’t functioning properly, and this signal comes from the logic board, there are two possibilities:
1. The VS power supply has a short circuit or overcurrent issue, causing the protection mechanism to trigger and make the VS-ON signal abnormal.
2. There’s a problem with the logic board itself.
I decided to start with the easier checks first. I measured the load on the VS power supply and found no short circuit. That pointed to an issue with the logic board. I thoroughly inspected the board, reinstalled it, but the fault remained. Not giving up, I tried to trace the circuit without a schematic. I followed the VS voltage all the way to the X board, which only had a few filter capacitors. Then I moved to the Y board, removed it, and checked the power transistors.
During the inspection, I noticed that the resistance between the D and S terminals of one of the power transistors wasn’t normal—both forward and reverse readings were over 100 Ω, which didn’t indicate a breakdown. I then examined the printed circuit traces and found two peak absorption capacitors. At first glance, they looked fine, but upon closer inspection, I noticed a sign of metal splashing under one of them. I decided to remove the capacitor, and sure enough, there was a crack between its two legs. Testing confirmed that it had failed.
The original capacitor was rated at 331/1KV, but I didn’t have a suitable replacement on hand. However, I did have 221/2KV capacitors available. I used three of them to replace the original two. Although the capacitance value was different, their frequency response and voltage rating were actually better than the original.
After replacing the faulty capacitor, I checked the 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 flickered anymore. Finally, the long-lost picture returned!
I ran the TV for half an hour and observed that the temperature rise across all power rails was normal. The replaced capacitor was only slightly warm, which was expected. From that point on, the repair was complete.
Testimonial: After repairing home appliances for so many years, I felt that “first time†excitement again. Although this might be common among experienced plasma technicians, it was a big step for me. It made me realize how much I still have to learn in this area, and it motivated me to keep improving.