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No Backlight on LED TV

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Southerner1959:
I'm an electronics engineer, but I have limited experience of modern TVs. So I hope somebody can advise

I have an LG 47LN570V . . . which uses LEDs for the backlight.

When you turn it on, it flashes the LG Logo at full brilliance for about a quarter of a second . . . also when you turn it off. So firstly, I presume this means the LED arrays behind the screen must be OK?

Also, shining a torch on the screen you can see images, menus etc, and audio is working OK.

Despite downloading the Repair Manual, there are no circuit diagrams . . . so been trying to suss it out.

There are two separate feeds on the PSU board to the backlight . . . each with it's own diodes, smoothing caps and series Switching FET. I have replaced the capacitors . . . and measure 75V across each bank.

I have checked the voltage being fed to the Backlights . . . and there is just a quick pulse on switch on, in line with when the screen comes on for a moment.

So I guess it is the control circuitry going wrong, that presumably feeds a signal to the switching power FET, to adjust the intensity of the backlight?

I guess lots of possible faults could be causing this . . . so could I just bypass the FET, and feed the 75V to each LED feed? (it seems avery high voltage to me . . . . but I'm guessing there's a constant current regulator IC in each of the LED strings inside the screen?)

I know there would no longer be any Backlight adjustment . . . but this is my own set, and normally have it set at 100% anyway.

Any advice, or corrections to my assumptions would be much appreciated!

fix2003:
its possible one of the led strips is faulty and shutting the circuit down for all  you will need to strip the screen down test each set see which one is faulty and Replace faulty LED strip

downunder:
Suggest you also check out this Youtube video.

https://m.youtube.com/watch?v=Bf5T2wwtydY

kennyc:
Watched the video . Puts you off even trying to fix the fault . He even has 3 faulty lights at the end of it .Even if you fix it and you give it back to the customer ,charge them  they will be back within a month with the same fault but just another LED gone. Thats if you dont break the screen while taking it apart. Remember as an apprentice years ago with Radio Rentals you did a screen clean on the old TVs and you would put it all back together and some dust would get on the tube and you had to do it all again 

downunder:

Hi Kenny,

I agree, these videos usually provide more questions than answers. I only intended it as a guide to cracking open the panel, which would be too long to describe in detail. I open up maybe 2 a week and it does become quite routine. Ah well, back to rugby training.

Bruce

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