LG 39LN5400 no power

[unimatrix93]

Hello everyone, I have an LG 39LN5400 with a no power problem.

I plug in the TV and the power led flashes twice. No screen, no sound. Also no reaction to power button/remote.



I checked the PSU output values and while the output towards the mainboard is OK the output to the LED backlight looks weird.



The output table shows 133.2V for the 39inch version.

I measure 107V on the + and -42V on the - side of the connector.



What should I check next?

Is the backlight faulty and I should dissassemble the screen or is it the PSU?

Thanks in advance!

[nomoresonys]

Re: LG 39LN5400 no power

unplug the connector and measure the pins, see what you get.

[dskall]

Re: LG 39LN5400 no power

What do you get on your pwr-on pin when up press the power button? Are you getting backlights?

[unimatrix93]

Re: LG 39LN5400 no power

What do you get on your pwr-on pin when up press the power button? Are you getting backlights?

I get 0v on the left pin (blue wire). 3V on the middle and the bottom one and the right one drops to 0.5V when I press the power button.

I have no backlight at all.

[unimatrix93]

Re: LG 39LN5400 no power

unplug the connector and measure the pins, see what you get.

I get 109V on the + pin and 0V on the - pin with the backlight disconnected.

[yokoono]

Re: LG 39LN5400 no power

Defective LED strips.

[handsworth]

Re: LG 39LN5400 no power

I've fixed many of these LN series and the problem was a defective main board. Using a led tester the backights were ok. Power supply is probably good if standby light is on and dcv is good on output, Its the return to PS from main board that is faulty. If back lights are bad, the LN series is easy to see dark image with flashlight test. Also I would suspect main board because no remote response. Hope this helps

[dskall]

Re: LG 39LN5400 no power

I get 0v on the left pin (blue wire). 3V on the middle and the bottom one and the right one drops to 0.5V when I press the power button.

I have no backlight at all.

What is voltage on pow-on pin on the power board when in standby and after you push power on.

[yokoono]

Re: LG 39LN5400 no power

I get 109V on the + pin and 0V on the - pin with the backlight disconnected.

There are 36x 3v LEDs in the backlight. 36x3 = 108V. You measured 109V. I understand that the backlight does not turn on. So the power supply works since you have burned out LEDs or imperfect contacts.

[unimatrix93]

Re: LG 39LN5400 no power

I've fixed many of these LN series and the problem was a defective main board. Using a led tester the backights were ok. Power supply is probably good if standby light is on and dcv is good on output, Its the return to PS from main board that is faulty. If back lights are bad, the LN series is easy to see dark image with flashlight test. Also I would suspect main board because no remote response. Hope this helps

Hey thanks for the tips!

How/where should I test the return values from the MB to PSU? I want to check that before taking apart the screen for the LEDs.

[unimatrix93]

Re: LG 39LN5400 no power

What is voltage on pow-on pin on the power board when in standby and after you push power on.

By Pwr-on pin you mean the AC connector's pin or something else? Sorry for the confusion.

[unimatrix93]

Re: LG 39LN5400 no power

There are 36x 3v LEDs in the backlight. 36x3 = 108V. You measured 109V. I understand that the backlight does not turn on. So the power supply works since you have burned out LEDs or imperfect contacts.

But why does the output table says 133V for the 39inch version if it's only 36x3v LEDs?

[yokoono]

Re: LG 39LN5400 no power

That's what it says on the net. Maybe there are more models.
https://shopee.com.br/Kit-Barras-Led...259.9216557141
https://www.ebay.es/itm/402774070948...MAAOSw-MpgKnak

[unimatrix93]

Re: LG 39LN5400 no power

That's what it says on the net. Maybe there are more models.
https://shopee.com.br/Kit-Barras-Led...259.9216557141
https://www.ebay.es/itm/402774070948...MAAOSw-MpgKnak

That's interesting, I proceed to take apart the screen then.

[unimatrix93]

Re: LG 39LN5400 no power

I've fixed many of these LN series and the problem was a defective main board. Using a led tester the backights were ok. Power supply is probably good if standby light is on and dcv is good on output, Its the return to PS from main board that is faulty. If back lights are bad, the LN series is easy to see dark image with flashlight test. Also I would suspect main board because no remote response. Hope this helps

I see. Well I don't have display wathsoever, I checked it with a flashlight, also TCON doesn't get any voltage either.

[unimatrix93]

Re: LG 39LN5400 no power

What is voltage on pow-on pin on the power board when in standby and after you push power on.

Sorry, I found the PWR-ON pin.

It's always 3V, doesn't change even If I press the power button.

[unimatrix93]

Re: LG 39LN5400 no power

So I took apart the screen and the first thing I saw was the brown/burnt like residue around some LEDs:



After using my tester I found multiple dead LED beads.


Now I'm searching for new 2835 3V LEDs beads locally, hope I can find some.

What do you guys think?
Is this the cause of the problem or the MB?

[nomoresonys]

Re: LG 39LN5400 no power

i think so, you should probably replace all of the leds, because they are all the same age and its just a matter of time and probably sooner than later, before more go out/fail.

[dskall]

Re: LG 39LN5400 no power

Sorry, I found the PWR-ON pin.

It's always 3V, doesn't change even If I press the power button.

I would say you have a mainboard problem.

[unimatrix93]

Re: LG 39LN5400 no power

I would say you have a mainboard problem.

I ordered a new mainboard with the same specs, but the LVSD connector and some caps/resistors are placed differently.

I ordered one, because the LG CPU is most probably shorted, it gets blazing hot as soons as I turn on the TV and also the PCB is yellowish under it.

As far as I can see, everything else is the same (CPU, ICs, etc).

What should I do now?

Transfer the LG CPU from the new board to mine, its a BGA chip.

Or transfer the LVSD cable and the caps/resistors to the new board?