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

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Let me share my experience: I’m not a professional LCD TV repair technician. My main area is fixing inverters and servo drives for industrial machines. A friend of mine is into appliance repair, so he often brings me LCD TV power boards and control boards to help fix. I usually do it because I like to help out. 😊 😄 This time, he sent me just the power board. He mentioned that when he first opened it, there were a lot of small burned components—probably from overvoltage. I said, “That’s a big problem! Maybe the TV was in a cold place? I need a warmer environment.” He laughed and said that the board had partial short circuits, and the FETs were burnt. He tried to replace the damaged parts himself, but after testing, the TV could power on, 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 burned chip resistors. They looked tiny, but I tested them for shorts before powering up. No issues found. Then I touched the hot components first. The power board used a FAN7930 chip. After confirming 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. I searched online for the circuit diagram of the FAN7930. Pins 6 (GND) and 8 (VCC) were at 0V, and pin 7 (INV) was at 2V. According to the datasheet, VCC should be over 10V. I checked the circuit again and realized the high-voltage circuit had some starting conditions. I noticed that the PONBK/ON signal needed to be connected with +5VSB. I did that, but still, the high voltage didn’t come on. It was confusing—was there a wrong short, or was there another issue? Then I took another look at the schematic. I noticed that the VCC pin of the FAN7930 was connected through a transistor to LEDVCC. I measured the VCC voltage and saw it was 9V. But during the forced boot, it dropped to 0V. That meant the short was correct, and the high-voltage circuit was turned on, but there was still no output. I thought maybe the 9V excitation voltage wasn’t enough to turn on 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 powered it up again, and the high-voltage output reached 100V. The repair was successful! My friend took it back to the TV, and everything worked perfectly. No more issues! I’ve learned a lot from this experience. Even though I’m not an LCD TV expert, every repair helps me grow. I hope this story helps someone else who might face similar problems. Let me know if you have any questions or need further details! 😊