32-inch Haier LCD TV LC32UT7 three no boot failure repair - appliance repair - Huaqiang Electronic Network

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I recently worked on a 32-inch Haier LCD TV model LC32UT7 that had a "three no boot" issue. The user reported that the TV sometimes turned on, but not always. Today, the screen was dim and wouldn’t fully power up. The power indicator wasn’t bright, and I couldn’t test it properly. After checking the screen for damage, I suspected a power board problem. I started by testing the power supply voltages: yellow, green, and brown were present, but when I tried to counter-offer with orange, green, and brown, I realized the power board might be the issue. I told the customer that if the screen was faulty, we couldn’t repair it, as there was no electricity to guarantee anything.

I decided to open the unit and inspect the power board. There were no bulging capacitors or signs of burning. I measured the +5VSB voltage and found it to be zero. However, the large capacitors showed around 275V, which was normal. It looked like the secondary power circuit was the problem. I noticed a six-pin chip LD7535 controlling the power stage. When I checked the voltage at pin 5 (VCC), there was no power, and the ground resistance was only 100 ohms. Tracing the circuit, I found a resistor R5 with 100 ohms, and after removing it, I discovered that Qv3 H2A was shorted. I replaced the transistor, and the power indicator lit up. Pressing the channel button allowed the TV to boot normally—seemingly fixed.

But then, after about half an hour, the TV turned off again. The blue light was on, but the boot light was off. I measured the +5VSB voltage and saw it was 5.2V, which was okay. However, the STB voltage was only 4.2V, and the +12V and +5V would briefly appear before disappearing. This cycle repeated several times. I did some online research and found limited information on this specific model. However, I found that similar power boards from Sanyo, TCL, and others had similar issues. A common fix involved modifying the board, specifically changing certain resistors and components.

One recommended technical modification involved removing a 1K chip resistor at RJ34, replacing a 100K resistor at RA46 with a 1K one, and adjusting RA41 with a 5.1K and 12K resistor. Another method suggested connecting RB60 and RB63 in parallel with 10K resistors, removing RJ34, changing RA46 to 1K, and adding a 100K resistor in parallel with RA32. I tried the first method and it worked temporarily, so I documented it here for future reference.