LG 55 LED No video

[anupkg1985]

Hi,

I have a LG 55 inch LED tv which has no video. I tested the SMPS board and looks like voltage to backlight LED is missing. I could hear the sound and able to change volume and source so concluded that main board is ok. I'm getting around 0.06V when checked against LED + and - terminals. Also able to get 3.2V on DRV on from mainboard. Any idea whats going wrong?

[neilc6]

Model number? Looks like you have no backlight instead of no video. If you have no backlight, bad LEDs are suspect. Typically you will get a flash on power up but an open LED string will cause the backlights not to flash.

[handsworth]

Typical LG. Do the flashlight test.

[anupkg1985]

Yes, I have video on screen when tested with flash light. It's backlight problem, but I suspect the power supply as there are no voltages on the board even after disconnecting backlight from the board.

[nomoresonys]

With tv powered on VERY CAREFULLY check the voltage between the legs of looks to be C510 the big capacitor, note it says 450 volts, THIS IS DEADLY VOLTAGE so be careful not slip with the probes or touch anything with your hands etc, set meter to 200 or 600 vdc, what voltage do you have? don't do test if you are uncomfortable doing it.

[anupkg1985]

I have 390V on main cap. All standby, 12V and 24V are present. Picture on screen but no backlight. I suspect issue with LED driver circuit but not sure.

[budm]

What is the full model of the TV?
You have two separate LED driver circuit, you can see that you have two boost converters there on the board.
I assume that you verify you do have BL-ON and DIM control signals, correct?

1) Turn TV off, out all connectorS back in place, meter set to 200VDC. Keep the black meter probe on the chassis, Red on one of the L+ of the LED connector.

2) Turn the TV on while watching the meter to see how high it jumps up to and then wait 10 seconds to see how low it goes down to.

3) Repeat step 1 and 2 with the other 3 pins of the LED connector..It is important that you follow the steps.

[neilc6]

Looks like your model is 55LN or 55LA series with backlight failure very common. LGs have shutdown protection of the backlights. https://techhometravel.com/lg-ln-ser...ed-backlights/

[budm]

YEP, will not be surprised if it has blown LED, the test steps I give will tell you if the LED power supply is OK or not.

[nomoresonys]

Originally Posted by budm

YEP, will not be surprised if it has blown LED, the test steps I give will tell you if the LED power supply is OK or not.

+10, Yep, do Buds test, see where you stand.

[anupkg1985]

I did the test suggested by budm and below are the readings -

LED+ Jumps to 128V (high) and then goes to 84.5V (low)
LED- 0V
LED+ Jumps to 210V (high) and then goes to 85.5V (low)
LED- 0V

Looks like issue with the LED strips only, could you confirm if anything else is going wrong?

Model - 55LN5100

[budm]

Originally Posted by anupkg1985

I did the test suggested by budm and below are the readings -

LED+ Jumps to 128V (high) and then goes to 84.5V (low)
LED- 0V
LED+ Jumps to 210V (high) and then goes to 85.5V (low)
LED- 0V

Looks like issue with the LED strips only, could you confirm if anything else is going wrong?

Model - 55LN5100

Time to open up panel and check out the LED strips. I would replace the whole set.

[anupkg1985]

Finally opened the panel this weekend and fortunately nothing wrong with the strips, only one of the joint between two strips was loose. Everything light up as soon as fixed the joint. TV is back on, thanks to every one!

[budm]

Thanks for the follow-up.

[neilc6]

Surprised the LEDs are still good or the lenses havent fallen off. If you want your LEDs to last, turn down the backlight level for all inputs and all smart apps.