Backlight only working (perfectly) on the bottom of 55" LG screen

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LG 55SJ850V

LG 55" 4K UHD EDGE LED TV

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Aron Uno Larsson @aronuno

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发帖于: 2024年8月3日

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Backlight only working (perfectly) on the bottom of 55" LG screen

I just got this TV for free. Challenge is that the backlight only works on the bottom of the screen.

This is a EDGE lit LED screen. And my research based on other forums leads me to think that this screen is only lit up by a strip at the bottom.

So I do not understand how the backlight-led can be perfect at the bottom and up about 18% of the screen, and then go dim. I can see all the pixels perfectly, it only very dim compared to the 18% at the bottom.

Not seeing how changing the lightstrip might solve this, since they are strong and bright at the bottom.

I have done factory reset of the screen and this is happening on the system-graphics of the screen as well so it should not have anything to do with any signal or HDMI-cable.

On images you can see this. The camera made it burn at the bottom, but on the macbook-screen you can see how even the light is.

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jayeff @jayeff 声誉积分: 541.2千 11 413 1.6千 · Answer 1 · 2024-08-03T19:35:28-07:00

Hi @aronuno

By systems-graphics do you mean the Picture test option in Settings > Picture?

Just wondering if perhaps the bottom of the screen is too bright and the rest of the screen is normal and not that the bottom is OK and the rest is duller??

LED's are connected in series connection so it could be a faulty power supply feed to the section of the strip, especially if both sides are fed by the same particular power feed from the power board, for that particular section of the strip.

Here's a link that shows the LED strips. (Select the 4th mini image at the left of the main image of the strip to view the LEDs).

Looking at the image there's 25+ LEDs on each strip (can't see more than the number 25 on the strip itself so maybe 30?) and there appears to be 10 input connections for the strip.

Maybe check the cabling from the strip(s) to the power board to see how many wires there are and if they multiple between both sides or not i.e. wire goes from same terminal on the power board to each of the LED strips and not just the one strip.

This is only a guess of course.

Here's the service manual that may help. p.52/62 shows examples of when to change the power board and p.56 shows the voltage values supplied by the board

Mike Davis @mikedavis29611 声誉积分: 1 · Answer 2 · 2025-08-31T17:20:26-07:00

I had this issue after replacing the led edge strips at the bottom (there are no others).

The old led strips heated up and melted the foam seal at the bottom as well as the thick diffuser sheet edge, so it looks serrated.

I used black gasket caulking, very carefully with a small brush and scraper, to the bottom foam strip, so the led light could not escape the front of the tv frame.

I also mistakenly tried to turn the diffuser around to have a clean edge there. The trouble is that the acrylic diffuser sheet has a small reflective strip at the top, to reflect the light back and light the upper portion of the screen.

I am now looking for a broken 55” tv to steal the diffuser sheets from it. Hopefully I find one compatible or one I can trim down to work.

LG 55SJ850V

Backlight only working (perfectly) on the bottom of 55" LG screen

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