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Multiple symptoms: entire backlight off or partial gray area

TV: LG 55GA7900

My TV shows multiple symptoms.

When it shows problem, few seconds after turn on the TV, either 1/10 of screen shows shadow (LCD still shows image) that looks like partial backlight out or entire backlight goes off (Still I can see lcd is working when I put flashlight).

On the other hand, the TV works just fine sometimes for as long as it gets when it doesn’t show the problem.

I initially thought this could be a power supply board issue because all led bulbs seem to work just fine when the TV works fine.

So I was going to replace the power supply board but I also read an article saying that some TV shuts off when there is bad led bulbs in the back light.

I need help to isolate the problem whether the power supply board is wrong or there is a bad led and here are my questions.

  1. When the TV shuts down to protect it from bad led bulbs, is it just turn the back light off or turn entire TV off?
  2. Can defective led strip be working fine for hours, partially out, or out entirely and the symptom randomly repeats?

Thank you in advance for all of your answers.

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@highart

  1. When the TV shuts down to protect it from bad led bulbs, is it just turn the back light off or turn entire TV off? It does not. Your TV does not check the backlight (only your inverter circuitry) as a POST check
  2. Can defective led strip be working fine for hours, partially out, or out entirely and the symptom randomly repeats?
  3. Yes, if it is loose connections etc.

Measure the output from the power board to the LED’s and se what voltage you get. Sure it is possible that this is an LED issue (common for LG at this time) but you are not guaranteed it is. At least do some more trouble shooting.

Anytime you want to order replacement LED’s verify your panel first. Check the label on it and order the backlight array according to panel manufacturer. It is not uncommon that place like LG etc. change panels during assembly and use a different manufacturer. Using the proper panel ID takes the guesswork out.

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Hi @highart

Based on your description of the problem I believe you have backlight LED failure. This is a common type of failure that will initially show as failure of the some LED’s before the entire backlight shuts off for good.

If you decide to replace the LED strips, I would recommend doing all the strips working or not. The other strips will fail soon and the repair process is not easy for this fix…

For your model it looks like the backlight is edge-lit. Here is a link to the parts you will need:

https://www.shopjimmy.com/lg-6916l-1249a...

Best of luck!

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Thank you for your answer. I may try to replace a led strip.

I checked the website you provided. Based on its size, I guess this TV needs to led strips probably lit from each side edge.

Do you have any idea if I can use 2 exact same led strips (6916L-1249A) for each side or 2 different led strips needed for left and right?

さんによる

From my experience there is usually more than one section to an edge-lit LED configuration, but based on the research I've done it looks like there's only this one long strip for the whole bottom.

If this was my TV, I would take it apart completely before ordering the part to ensure its the right one for this model (the bar itself will have some numbers on it to identify what it is). If would also confirm that the LEDs are what failed considering there is usually some sort of physical burning on the strip.

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This might help: LG 47LN5400 LED Replacement

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J. Rhee さん、ありがとうございました!
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