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LG CX 77 reboot loop with black screen

Hi everyone,

A few months ago, my 77CX (bought in dec 2020) started behaving strangely: when I tried to power it on, the LED would blink, it would make a noise, reboot, and then it would work. Sometimes it would cycle twice. But once powered on, it would run fine, even for a full afternoon.

Recently, however, it stopped powering on at all. It just keeps cycling: LED blink, noise, reboot. After unplugging and plugging it back in, it does the same thing again and again.

I’ve made a video of the problem so you can hear the sound and see the LED blinking (if the video does not work, you can watch it on my reddit post).

It’s out of warranty, so I’d like to repair it myself. I just hope the panel itself isn’t defective. Any tips on how to diagnose if it’s the power board, main board, or the panel?

Thanks.

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TL;DR: I fixed it, I just had to replace the power board.

It was not smooth.

I ordered the LG OLED Diagnostic JIG so it was easy to check the voltage (because it powers on the panel). I knew for sure the power board was bad.

After swapping the board, I still had a black screen even with the JIG. The voltages were okay, so I worried it might be the panel. I reconnected the main board (instead of the JIG) and still had a black screen, this time without the boot loop. I tried replacing the T-Con and got a strange ghost image, but at least something appeared on screen.

Right before giving up, I realized a cable might not have been seated correctly. When using the JIG (or replacing the main board), you have to disconnect/reconnect a thin cable to the main board that’s held by a small latch. It’s easy not to push it all the way in. I reseated it, put the original T-Con back, and it worked!

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Samuel Bernard crwdns2934231:0crwdne2934231:0
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