Skyworth LCD TV 32E55HM second repair machine multi-fault repair experience - android - Phones Developers

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I received a Skyworth LCD TV model 32E55HM with three no issues — no display, no sound, and the indicator light was dim. After checking, it turned out that the backlight was not working properly. I first thought of replacing the inverter tube, but when I opened the warranty sticker, I realized this unit had already been repaired before. I was worried about potential screen damage, but decided to take the risk and proceed with the repair.

After disassembling the TV, I found that there was no voltage on 5V, 12V, or 24V. Removing the power board revealed that the switch transistor (10N60) had failed — its three pins were open, and diodes D201 and D202 were also damaged. I replaced them with 13N60 transistors, and the power supply started working, but there was noise. I suspected the new transistor didn't match the original parameters. I then replaced it with a new 10N60, but after powering up, there was a loud noise, and one leg of the transistor exploded. I was scared and immediately stopped the test.

I cleaned the motherboard and took some photos for reference. The new transistor showed no short circuits, but after testing ZD202, ZD205, R213, and the fuse, all components were blown. The board was blackened, and even after replacing the switch transistor with 20N60, the voltage came back, but the power supply was still noisy. The red light was on, and sometimes it flashed green. Eventually, the red light cycled between red and green every six seconds. Measuring the voltages, U13 was 2.6V, U10 was 2.6V, and U12 was 3.3V. When U10 dropped below 2.3V, the green light turned on. The CPU was warm but not hot. Using the formula from 8117, I found that U13 was normal, but U10 should have been around 1.2V. Further resistance checks showed that U13 was 20 ohms, while U10 and U12 had only 2 ohms between them. After removing the power transistor, the resistance returned to normal, leading me to conclude that the CPU was faulty.

After replacing the CPU, the TV powered on normally. The red light stayed on, and the green light appeared briefly before turning red again. However, the screen would turn off right after the light came on, then restart repeatedly. I disconnected the backlight cable and heard the boot-up sound. Testing the backlight, I found that two groups of LEDs worked normally, while the third group shut down. Suspecting a faulty backlight strip, I opened it up, but everything looked fine. The next time I tested, the same issue occurred, which was very frustrating.

Further testing revealed that when the power was cut, 5V, 12V, and 24V all dropped. I suspected overvoltage protection or a no-load condition. After disconnecting the overvoltage circuit, the voltages stabilized. Replacing 817 and 431 didn’t help, and even replacing TEA1733P didn’t fix the problem. I was getting desperate and considered buying a new power board, but I wanted to find the root cause. After checking the peripheral circuits of the power chip and finding no issues, I tried replacing the switch transistor again, but it didn’t work. My frustration grew, and I was about to give up.

While reviewing the schematics, I noticed something strange: the current-limiting resistor of the switch transistor was reading 0.18 ohms, which was abnormal. I replaced it with a 0.39-ohm resistor, and suddenly, the TV worked perfectly. All the backlight groups lit up, and the machine was finally fixed. I did a final check, and everything was stable.

However, I noticed that AV2 input had a white image, while AV1 worked fine. Checking the interface, I saw signs of corrosion and blackening, which made me think this TV might have been struck by lightning. When I brought it home, the situation was worse, and the user eventually sold it as scrap. Looking back at all the failed attempts, I felt a mix of relief and frustration.

This repair took several days, and during that time, my wife gave birth to a healthy baby, so I wasn’t able to work on it for a few days. The fault wasn’t complicated, but I missed a small detail due to inexperience. I hope other technicians can learn from my experience and avoid similar mistakes. This was a tough case, but I’m glad I didn’t give up.