Maintenance log: 42-inch Panasonic LCD boot black screen after 10 seconds into the protection state - android - Phones Developers

LC03-3.3 SOP8 TVS Static Protection 3.3V

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

Panasonic 42-inch LCD TV:

The issue is a black screen that turns on, with sound, and enters a protection mode after about 10 seconds. I opened the back cover and tested the power board voltages before the protection triggered.

During the time the back plate was being removed, the backlight turned on three times for roughly half a second each, then the system entered protection. My first thought was there might be an issue with the LED constant current board. I carefully disassembled it but found no faults. Next, I suspected the LED backlight itself, but removing the screen would be risky and complicated. How could I check if the backlight was faulty without taking it apart? I remembered reading online about using a megohmmeter (shaker) to test the LED backlight. This device can output several thousand volts but has high internal resistance, making it suitable for testing LEDs in series.
I decided to try this method. I divided the backlight into two groups and identified the positive and negative poles of each group by analyzing 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’s positive and negative terminals to the first group of LEDs and measured the voltage across them while turning the shaker at around 100 RPM. The multimeter showed approximately 60 volts.

I repeated the process for the second group of LEDs and got about 40 volts. Clearly, the two groups were unbalanced, indicating one or more LED beads were damaged. This imbalance likely caused the protection circuit to activate. To fix it, I needed to remove the screen and replace the light bar. I’m not a professional repair technician, and I had never taken apart a screen before. I wasn’t very confident, but I explained the situation to the owner, who gave me full support. With that, I felt more at ease and started the job immediately.
I removed the surrounding screws and took off the frame.

Carefully prying open the clips on all four sides, I lifted the LCD screen. This step required two people to handle it gently to avoid damaging the film cables connected to the boards.
After removing the screen, I saw the white polarizing film.

Next, I removed the three layers of polarizing films and the diffuser.

Finally, the LED light bar appeared.

I then removed the reflective film (attached with double-sided tape) to reveal the light bar.


At this point, I used a multimeter to check each LED bead. If any were burned out, I could spot them by looking through the lens. These were leadless chip packages, so soldering them with a regular iron wasn't feasible. Therefore, replacing the entire light bar was the most efficient solution.

However, I couldn’t find the exact model on Taobao. I ended up replacing individual beads. I bought LED beads rated at 6V/160mA and 3V/350mA. Which one was correct? I tested one with a 9V battery and a 100Ω resistor, then measured the voltage across the bead with a multimeter. It read 3V/350mA.
Since I was worried about my soldering skills at home, I bought four beads and asked a colleague to help.


I directly soldered the beads onto the lens, reassembled the screen, connected everything, and powered it on. The TV worked perfectly again.