Changhong UD42B6000iD LCD backlight is on the light to extinguish a troubleshooting example - android - Phones Developers

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Every year, during the busy season of autumn harvest, business drops significantly. People are too occupied with collecting crops to even think about fixing things. That time, I was caught up in the rush, and the guy got really panicked. He sat in the store doing nothing, then came to me asking if I could fix his LCD TV. The screen didn’t turn on at boot, but the sound was working. He asked if I could repair it this year. I said yes. I went down to check a Changhong 42-inch LED LCD TV, model UD42B6000iD, which is also a 3D TV. When I powered it on, the screen lit up for just one second, then turned black while the sound continued. It looked like the backlight or the light bar might be faulty. But there weren't many possible issues here. I gave an estimate, and after the user agreed, I took the TV apart. The model and block diagram are shown below:


[attach]514704[/attach]

First, I tested the power supply board and found it was working fine. Then I checked the main voltage and resistance of the backlight board. I discovered that the field transistor Q50 had failed. Checking the surrounding components, I found that the overcurrent detection resistors (two 6R2 and one 470Ω) were all damaged. Further testing showed that the over-current protection pin of the main chip GB3000 was shorted to ground, causing a complete short circuit in the peripheral circuit. It seemed like the main chip was also damaged, so the repair was completed.

A few days later, the board was returned, and the issue remained. The screen was still not working properly. I suspected the backlight board might be faulty, but I wasn’t sure. To confirm, I measured the voltage across the light bar. It spiked to around 40 volts and dropped immediately. To determine whether the problem was with the light bar or the backlight board, I directly tested the light bar circuit. There were eight channels, each connected to a field transistor. The main chip GB3000 controlled the conduction of these transistors, and it would shut them off if overvoltage or undervoltage occurred, protecting the current flow and ensuring uniform brightness.

At the moment of power-on, I tested the voltage at each of the eight loops. Most of them were around 3.8V when the screen was bright, dropping to about 1.8V after protection. However, the top two channels were abnormal, reaching up to 30V when bright, and around 20V when dark. This confirmed that the new backlight board wasn’t the issue, and the problem was likely a short-circuited lamp bead. Here’s the image of the backlight:

I contacted the user again, explained the situation clearly, and asked for permission to open the screen. No more delays, here's the image:

The fault point was obvious, but it wasn’t easy to fix. Here’s the picture:

There were two options: either replace the socket or use a flying wire. Either way, I didn’t want to change the entire light bar or cable. Since the screen was already removed, I wanted to avoid further delays. I decided to replace the socket and find a suitable double-column 12-pin socket. I used an air gun to heat the back of the socket, making sure not to heat the front to avoid damaging it. Here’s the image:

This type of socket isn’t easy to solder, but I managed to do it successfully. Here’s the result:

The next step was dealing with the soft cable. I cut the burnt part neatly, and since both sides were too wide, I made sure to align them properly. I used a wallpaper knife to carefully scrape away the outer insulation without damaging the inner wires. Here’s what it looked like:

The original machine had a blue plastic mat at the plug. To ensure good contact between the cable and the socket, I removed it and attached it to the back of the trimmed cable. It was simple—just plug it in. Here’s the image:

Now it was time to see the miracle happen. My heart was pounding, my blood pressure was high, and I turned the power on. The lamp beads lit up perfectly! It was a joy without darkness, and I couldn’t help but laugh. Here’s the image:

Next, the screen was shocked and stunned. I made sure to handle it carefully. If I had to give up, the test machine worked fine, and the picture was normal. Wait for this image, huh, huh?

Thank you for watching. A little embarrassing, but it worked out! The whole process was assisted by my wife, so I’d like to say thank you to my beautiful wife.