Changhong UD42B6000iD LCD backlight is on the light to extinguish a troubleshooting example - android - Phones Developers

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Every year, during the busy harvest season, business tends to drop significantly. People are too occupied with collecting crops, and when something breaks, there's no time to fix it. One day, I was in the store doing nothing when a customer came in, panicking. He said his LCD TV wouldn’t boot properly—screen turned on for just one second, then went black, but the sound was still working. He asked if we could repair it this year. I said yes. After checking, it was a Changhong 42-inch LED LCD model UD42B6000iD, possibly a 3D TV. The screen would briefly turn on, then go black. The sound played normally, so I suspected the backlight or light bar might be faulty. I quoted the cost, and after the user agreed, I took the TV apart and made some initial measurements.

The power supply board tested fine, and the main voltage of the backlight board was normal. However, I found that the field transistor Q50 was damaged. Checking the surrounding components, I noticed that two 6R2 resistors and one 470Ω resistor were also burned out. Further testing revealed that the over-current protection pin of the main chip GB3000 had a short to ground, which likely caused the chip to fail as well. At that point, I thought the repair was complete, but a few days later, the board was returned again with the same issue. The problem wasn’t with the backlight board, so I needed to dig deeper.

I tested the voltage on the light bar and saw it spiked to about 40V before dropping immediately. To determine whether the light bar or the backlight board was at fault, I measured the voltage across each of the eight channels. Six of them showed around 3.8V when the screen was bright and dropped to 1.8V after protection triggered. However, the top two channels were abnormal—showing 30V when bright and 20V when off. This confirmed that the new backlight board was not the issue, and the problem likely lay with the lamp beads themselves.

After contacting the user, I explained the situation and got approval to disassemble the screen. I removed the panel carefully and found the exact fault. It was a shorted lamp bead, which I fixed by either replacing the seat or using a flying wire method. I used an air gun to heat the back of the seat without touching the front to avoid damaging it. After replacing the faulty component, I reconnected the cable and made sure the insulation was properly stripped and secured.

Finally, I powered the TV back on. The screen lit up perfectly, and everything worked flawlessly. The user was thrilled, and I felt a huge sense of accomplishment. This repair process was challenging but rewarding. A big thanks to my wife for helping me through the whole thing—without her support, it wouldn't have been possible.