TCL LCD TV LE42D8810 power supply board TV4205-ZC02-01 maintenance experience - Home Appliances

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Let me share my experience with you. I'm not a professional LCD TV repair technician, but I do fix inverters and servo drives for industrial machines. A friend of mine works in appliance repair, and he often sends me LCD TV power boards and control boards to help him out. I like to assist, so I usually take them on.

One day, he sent me just the power board. He said that when he first opened it, there were several small burned components—probably due to voltage surges. I thought, "Wow, that's a lot." I joked about needing a warmer place, maybe because it was cold outside. But he explained that the FETs were shorted, and he had already replaced some of the burnt parts. After testing, the TV could boot up, the indicator light was on, but the backlight wasn’t bright. He couldn't figure it out, so he asked me to take a look.

I started by checking the small resistors that were burned. I tested for shorts and confirmed everything looked okay. Then I powered it on and checked the hot components first. The power board used a FAN7930 chip. After verifying no components were overheating, I shorted +5VB and PON to force boot. The output voltages of +12V and +5V were normal, but the high-voltage output was 0V. That’s strange.

I looked up the FAN7930 circuit diagram online. The GND (pin 6) and VCC (pin 8) were both at 0V, and the INV pin was at 2V. According to the data sheet, VCC should be above 10V for normal operation. I checked the circuit again and found that the high-voltage circuit required the +5VSB, PON, and BK/ON pins to be connected together. I had done that during the forced boot, but the high-voltage section still didn’t work. It was confusing—was there a short, or was there another issue?

After rechecking the diagram, I noticed that the VCC pin of the FAN7930 was connected to LEDVCC through a transistor. I measured the VCC voltage and found it was 9V. But after turning off the BK/ON, it dropped to 0V. That meant the high-voltage circuit was actually being activated, but the output was still zero. I wondered if the 9V was too low to trigger the transistor, preventing the circuit from working properly.

So I asked my friend to get a new FAN7930. After replacing it, the VCC voltage jumped to 15V. I turned it on again, and the high-voltage output reached 100V. Success! My friend took it back to the machine, and everything worked perfectly. No more issues!