32-inch Matsuura assembly machine LED LCD TV screen repair light bar - appliance repair

I received a repair request for a Songpu assembly machine 32-inch LED LCD TV. The user mentioned that there's no picture, the machine is silent, the screen doesn't light up, but it boots normally. He asked why there's no sound. I suspected a video issue. After opening the unit and testing, I found that when powered on, the voltage at the LED interface rose to about 50V, but the screen remained dim. This suggested that the LED lamp beads were damaged and the system had triggered protection. Using an LED test tool, I found that the lamp beads weren’t lighting up, confirming they were broken. I informed the user that the lamp beads needed replacement, though I wasn’t sure if it was worth the effort or if it would be hard to charge him. I ordered new ones, which took about three days to arrive. The repair ended up with a slightly orange-black and brown screen, and I couldn’t guarantee it wouldn’t be damaged during installation. The user agreed to leave the phone after the repair and left.

After lunch, I started dismantling the screen. I had previously replaced the lamps on two 22-inch Samsung LCD monitors, so I had some experience. This was my first time removing a TV screen. I carefully unscrewed the front panel and removed the screen and other components, wrapping them in plastic to keep them dust-free. I examined the backlight strips—each had 10 LEDs, and there were three strips in total, connected in a 10-3 configuration. When I opened the LED cover and saw the tiny lamp beads, I was shocked. They were patched, and I didn’t know how to solder them. I didn’t even have the right tools. I decided to check which LEDs were bad using the LED test tool. I found three defective ones, each in different strips. Since I couldn’t solder them, I looked up the model online, but there was no match. I then searched for similar sizes on Taobao and found two options: 3528 and 2835. I wasn’t sure about the original wattage, so I ordered 2835 0.2W LEDs.

The parts arrived quickly, and I searched for LED patch welding tutorials online. To my surprise, there were very few videos, and most showed LEDs with protruding leads. I had to figure it out on my own. I took out the old LED strip, melted the old solder, placed the new LED, and tried to solder it. The first attempt failed—no light. I kept trying. This time, I learned from my mistakes. The original solder joints were tiny, and the LED covered the joint completely. Soldering was difficult, and the iron could easily deform the LED. I used a knife to scrape the joint and soldered it directly onto the board. Finally, one of them lit up. I repeated the process for the others, and by the second and third attempts, they all worked! I tested the board with the test machine, and it lit up successfully. But then, something strange happened—when I turned it off, the LEDs burned out again. I tried replacing them three times, but nothing changed. Was it the LEDs? Or did the soldering damage them?

I decided to short out the faulty LEDs. I used the LED test tool again and confirmed that the remaining LEDs were working fine. I also tested the circuit board, and everything seemed okay. I reassembled the screen and connected it back to the test machine. The brightness was normal, but the user still reported no sound. I realized that maybe the AV input wasn’t properly connected. I checked the video signal and found that the user might have been pressing random buttons, thinking the TV was working. In reality, the image wasn’t being displayed correctly. I explained this to him, and he understood.

Here are a few questions I’d like to ask:

1. When the first LED strip lit up, the voltage at the interface was 33V. According to the formula I found online, VF=3.2V, 10×3.2=32V. Is 33V considered normal?
2. What effect does having three broken LEDs have on the overall performance (not referring to the board or the LED itself)?
3. When the three LEDs were removed, the voltage dropped to 29V. Using the same calculation, 9×3.2=28.8V. Is 29V acceptable?

I also think it would be helpful if someone could share a video showing proper LED patch soldering techniques. It’s really challenging without visual guidance. If anyone has experience with this, I’d love to hear your thoughts!

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