Why have half my LED backlights stopped working?

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LG Television

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Jamie @jpen

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发帖于: 2017年12月29日

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Why have half my LED backlights stopped working?

I have an LG 60SJ850T television. Half of the LED backlights have stopped working. The screen is displaying images but the right side of the screen is dark. I have taken the TV apart and the backlights on the left of the screen are working well but none are coming on at all on the right side. Connections to the boards all look good. No obvious blown capacitors on the PCB. The PCB has an output for the left LEDs and the right LEDs so I wonder whether this component of the PCB is faulty or whether it’s the LEDs themselves.

Any advice or suggestions on further tests to do would be much appreciated.

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Philip Le Riche @pleriche

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It looks like you should be able to measure the voltages on the pins of the two LED connectors. (But keep fingers WELL AWAY from everything on the other side of that dotted line running across the board - that's where live mains lives - and it might be hiding in one of those capacitors even after unplugging!)

If the voltages on the connectors are similar it would indicate a fault with the LEDs themselves but if they're different, a fault with the logic board. If the latter, look on the other side of the logic board to see where the circuit traces go, and hence which components generate them. By even closer inspection of that area and comparing the components supplying the two sockets there's just a chance you'll spot a bad solder joint or suspect component.

If it appears to be the LED string at fault I'm not sure how repairable that will be but if you can access them you may be in with a chance.

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Thanks for the help! I’ll measure the voltages today and see where that takes me.

由 Jamie 完成的 2017年12月29日

So voltmeter testing with the TV powered up and backlights on shows the left sided output pins are receiving 1v where as the left sided outputs are receiving 0.1v. If the back lights are powered off or the cables removed both outputs receive 0.1v. I’ve checked the circuit board and can’t find any blown capacitors or suspicious solder joints. Does the above indicate the board is bad? Is it worth just trying to source a new one?

由 Jamie 完成的 2017年12月30日

Those voltages are strange because there's no way 1v is going to light the LEDs! Forgive me for a silly question, but are you sure you were reading your voltmeter correctly? Was the screen on at the time, even if only showing the OSD or a No Input display? Judging by the silk screen printed key against those sockets I think you should be measuring the voltage between one of the pins marked LED+ and any of the others, or possibly between LED+ and the metal frame.

However, the fact that the voltages are different tends to indicate that the board is at fault rather than the LED string. If you can source a new one that might save you a lot of trouble.

由 Philip Le Riche 完成的 2017年12月31日

Thanks for your reply. No problem with asking. I'm new to TV fixing so no offence at all. Those readings quoted above were taken with the TV powered on and backlights running. After some more reading and your comments I have taken some more readings in different scenarios.

由 Jamie 完成的 2017年12月31日

At the LED+ points with both leads plugged in standby

R1 - 0.1V then power on peak to 1.7V fade to 0.3

R6 - 0.1V then power on peak to 1.5V fade to 0.5

L2 - Fluctuating around 6V will slowly fall from approx 29V when LEDs have just been on) - then with power on peak to 18V before being rock steady at 1V

L6 - Fluctuating around 6V will slowly fall from approx 29V when LEDs have just been on) - then with power on peak to 18V before being rock steady at 1V

As you correctly point out the LEDs need a higher voltage to turn them on but once on seem to be supplied with 1V

To see what the board does if I take the LEDs out of the circuit I unplugged both leads

With both leads unplugged in standby

R1 0V then power on = 1.9V then fade to 0V

R6 0V then power on = 1.7V then fade to 0V

L2 0V then power on = 1.89V then fade to 0V

L6 0V then power on = 1.86V then fade to 0V

由 Jamie 完成的 2017年12月31日

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Thanks for the help! I’ll measure the voltages today and see where that takes me. — Jamie
So voltmeter testing with the TV powered up and backlights on shows the left sided output pins are receiving 1v where as the left sided outputs are receiving 0.1v. If the back lights are powered off or the cables removed both outputs receive 0.1v. I’ve checked the circuit board and can’t find any blown capacitors or suspicious solder joints. Does the above indicate the board is bad? Is it worth just trying to source a new one? — Jamie
Those voltages are strange because there's no way 1v is going to light the LEDs! Forgive me for a silly question, but are you sure you were reading your voltmeter correctly? Was the screen on at the time, even if only showing the OSD or a No Input display? Judging by the silk screen printed key against those sockets I think you should be measuring the voltage between one of the pins marked LED+ and any of the others, or possibly between LED+ and the metal frame.

However, the fact that the voltages are different tends to indicate that the board is at fault rather than the LED string. If you can source a new one that might save you a lot of trouble. — Philip Le Riche
Thanks for your reply. No problem with asking. I'm new to TV fixing so no offence at all. Those readings quoted above were taken with the TV powered on and backlights running. After some more reading and your comments I have taken some more readings in different scenarios. — Jamie
At the LED+ points with both leads plugged in standby

R1 - 0.1V then power on peak to 1.7V fade to 0.3

R6 - 0.1V then power on peak to 1.5V fade to 0.5

L2 - Fluctuating around 6V will slowly fall from approx 29V when LEDs have just been on) - then with power on peak to 18V before being rock steady at 1V

L6 - Fluctuating around 6V will slowly fall from approx 29V when LEDs have just been on) - then with power on peak to 18V before being rock steady at 1V

As you correctly point out the LEDs need a higher voltage to turn them on but once on seem to be supplied with 1V

To see what the board does if I take the LEDs out of the circuit I unplugged both leads

With both leads unplugged in standby

R1 0V then power on = 1.9V then fade to 0V

R6 0V then power on = 1.7V then fade to 0V

L2 0V then power on = 1.89V then fade to 0V

L6 0V then power on = 1.86V then fade to 0V — Jamie

LG Television

Why have half my LED backlights stopped working?

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