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 would sometimes power on and sometimes not, and now it's not turning on at all. When I checked, the power indicator was dim, and there was no display. After inspecting the screen and finding no damage, I suspected a power board problem. I started by checking the voltage from the power supply: yellow, green, and brown were present, but when I tried to test the screen, the power light wasn’t bright enough to confirm.
I decided to disassemble the unit and found no bulging capacitors or burn marks on the power board. Testing the +5VSB line showed no voltage, while the large capacitors had around 275V, which was normal. However, this didn't help me identify the exact fault. I asked the customer if they wanted to proceed with repairs, explaining that without proper power, I couldn't guarantee anything. Eventually, I removed the power board for further inspection.
On the secondary power circuit, I found a six-pin chip LD7535 controlling the power. Measuring the pins, I noticed that pin 5 (VCC) had no voltage, and the ground resistance was only 100 ohms. Tracing the circuit, I found resistor R5 was 100 ohms, and then discovered a short in Qv3 H2A. Replacing it fixed the issue temporarily—after powering on, the indicator light came on, and pressing the channel button allowed the TV to boot normally. But after about 30 minutes, it turned off again, and the blue light stayed on while the boot light went out.
Further testing showed that +5VSB was stable at 5.2V, but the STB voltage dropped to 4.2V, and both +12V and +5V would briefly appear before disappearing. This made the repair tricky. I searched online for similar issues with this model but found little information. However, I noticed that the power board is used in Sanyo and TCL TVs as well, and some users reported similar boot issues.
According to some technical guides, the problem might be related to the MP01009 465R1013 board. A recommended fix involves modifying certain resistors. One solution suggests:
1. Removing the 1K resistor at RJ34.
2. Replacing the 100K resistor at RA46 with a 1K resistor.
3. Replacing the original 5.1K and 12K resistors at RA41 with different values.
Another method recommends:
1. Connecting RB60 and RB63 with 10K resistors in parallel.
2. Removing the RJ34 resistor.
3. Changing RA46 from 100K to 1K.
4. Adding a 100K resistor in parallel with RA32.
I tried the first method and it worked for a while, but the second one is said to be more reliable. I'm sharing this for future reference, in case others face the same issue.
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Shenyang Tiantong Electricity Co., Ltd. , https://www.ttradiator.com