Panasonic 42-inch LCD TV:
The issue was a black screen with sound, and the TV would enter protection mode after about 10 seconds. I opened the back cover and checked the power board voltages before the protection kicked in.
During the process of removing the back plate, the backlight turned on three times for about half a second each time, then the TV went into protection. My first thought was that there might be an issue with the LED constant current board. After careful inspection, I found no obvious faults. Next, I suspected the LED backlight itself, but disassembling the screen was risky and complicated. I needed a way to test the backlight without fully taking it apart.
I remembered reading online about using a megohmmeter (also known as a shaker) to test LED backlights. This device can output several thousand volts, but its internal resistance is high, so it can’t provide much current. It’s ideal for testing LEDs in series. I decided to give it a try.
The backlight consists of two groups of LEDs. I first identified the positive and negative poles of each group by checking the output from the LED constant current board. Then I used a multimeter to determine the polarity of the shaker’s output. I connected the shaker to the first group of LEDs and rotated it at 100 RPM. The multimeter showed around 60 volts.
I repeated the same test on the second group of LEDs and got around 40 volts. Clearly, the two groups were unbalanced, which suggested one or more LEDs were damaged. This imbalance likely caused the TV to enter protection mode. To fix this, I had to replace the entire light bar. However, I wasn’t a professional repair technician and had never removed a screen before. I was a bit nervous, but the owner was supportive, so I decided to proceed.
I started by removing the surrounding screws and carefully lifting the frame.
I took my time to remove the clips on all four sides and lifted the LCD screen carefully—this required two people to avoid damaging the delicate film cables connected to the boards.
Once the screen was out, I could see the white polarizing film.
I then removed the three layers of polarizing films and the diffuser film. Finally, the LED light bar became visible.
I peeled off the reflective film, which was attached with double-sided tape, and the light bar was right in front of me.
With the light bar exposed, I used a multimeter to check each LED. I found one burned-out LED, which was a leadless chip package and couldn’t be soldered easily. So replacing the entire light bar was the best option. But I couldn’t find the exact model on Taobao. Instead, I bought some generic LED beads and tested them using a 9V battery and a 100-ohm resistor. I measured the voltage across the LED and determined it was a 3V 350mA type.
To be safe, I bought four of them and asked a colleague to help with the soldering.
I directly soldered the LEDs onto the lens, reassembled the screen, and connected everything back up. When I powered the TV on, it worked perfectly again.
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