55 lg model 55uf6450 no backlights

[tvtanner]

I need a little info. All voltage is ok out of ps.but set is dead other than remote turns on and off. Voltage to backlights read 119 volts on each of the two legs out of ps. Question is it possible one led is out thats causing my problem? Set is only 15 months old. I can unplug lights and theres no effect on voltage reading. I suspect both string of lights are out. Am i correct and can they be replaced?

[ReeceyBurger123]

Re: 55 lg model 55uf6450 no backlights

Led failed.

[dskall]

Re: 55 lg model 55uf6450 no backlights

You have an image on the screen? Yes backlights can be replaced. Hardest part is removing panel without breaking it.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

Thanks- I'm giving it a try. I found two strips of 60 led on each strip. I have 119 volts across each strip. That means 2 volts across each led. Is that correct? I thought it took appx 4 volts for each one. Which is right? They look pretty simple to replace. Where I can I buy these or are they available? These leds are rectangle two leds built together. I guess I have a series -parallel combination. Any help would be appreciated. This set is out of warranty.

[budm]

Re: 55 lg model 55uf6450 no backlights

How many wires per LED strip?
Vf will be based on how much current flowing through the device.
https://www.maximintegrated.com/en/a...ex.mvp/id/3256
Can we see some pictures of the boards? I doubt that the Vf will be that low.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

2 wires per strip coming from power supply. They shine against plastic or glass sheet on one side to light display.

[budm]

Re: 55 lg model 55uf6450 no backlights

I need to see good clear pictures of the board.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

Thanks for the return budm. I think i overlook the very obvius thing i should check first. The voltage to the lights would come up to 119 volts when i turned on set and i thought it was correct not knowing how the lights were wired. The voltage after checking were suppose to be 193 volts which would make the drop across each light appx. 3.2. Which is what it is suppose to be. I ordered power supply panel today. Should work.

[budm]

Re: 55 lg model 55uf6450 no backlights

Is this your power supply board?
http://www.shopjimmy.com/lg-eay64009...ver-board.htm#
LG EAY64009301 Power Supply / LED Driver Board
It uses two LED driver circuit, one per each set of LED string.
That 119V in is the input Voltage to the boost converter to boosted up to light up the LEDs.
The reason you do not have the boosted Voltage can be due to:
1) No BL-ON signal to turn on the LEDs driver circuit, should check that.
2) If BL-ON is present: The DCV at the LED+ pin may be working but it jumps to high due to open circuit in one of the LED string, you should leave the Black probe on the chassis and red probe on either one of the LED + PIN then turn on the TV to see how high the Voltage goes up to, then turn the TV OFF and then move the red probe to -LED and then turn the TV to see what DCV you get, then do the same on the other -LED pin.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

Thanks- i will check the back light on tomorrow. I have checked the on off start up and the voltage does not change when turned on. You are right i do not get the boost voltage at all. Yes that is the correct board i'm working on.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

Back to you Budm- I installed PS and got same results. Shorted backlights on to 3.2 turn on voltage and lights came on. screen looked like million of colored dots. I had t-con panel on hand changed it and same results. one side of screen looks little different than other side. had good brightness but no sign of video or picture while trying hdmi input. I tried remote but I could not tell when I had it on right input. I'm thinking main panel at this point. Any thoughts you might have I would appreciate it.

[budm]

Re: 55 lg model 55uf6450 no backlights

So before you bought the new power supply board, did you check to find out if you do have BL-ON?
BL-ON comes from main board and at this point it looks like the problem is on the main board or something is locking out the main board from sending BL-ON.

[tvtanner]

Re: 55 lg model 55uf6450 no backlights

I had already ordered ps so I waited until it came in and tried it first but I did try the old board and it was same as new board. Nothing much left but main board. Set is only 15 months old so I guess its worth trying. I hope I haven't damaged display by removing it first. It really wasn't that much trouble. I just hope I didn't damage any ribbon cables that caused all those color dots on screen. I double checked and they look ok. Thanks for helping. I will keep you posted.

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

Hey TV Tanner, did you get this set working? I have the exact same model with the exact same symptoms (120v out to led driver but never the boost voltage to 192v) was it an issue with the main board or was it failed/shorted LED backlight strips?

[budm]

Re: 55 lg model 55uf6450 no backlights

Please read my post #9. You do have meter to take measurement, correct?

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

Yes, I have a multimeter. I'll do as you suggest in post #9 and let you know the results.

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

Ok, when measuring the pins shown in the picture attached (with the table showing voltages included) I get 0v, 0v, 8.85v, 8.87v, 0v, and 0v. When I press the power button, I get the following: 75mV, 0v, 13.26v, 13.25v, 0v, and 3.38v.

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

When measuring the led+ and led- (both sets) when the TV is plugged in but not turned on yet, I get about 62v on both of the LED+ pins (and like 0.20V on each led- pin) when I press the power button, led+ jumps to about 90v right away then rises slowly to 121.8v over perhaps 8 seconds or so. I read 121.8v, 2.0v, 121.9v, and 1.98v across the led pins.

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

There is no voltage spike that I can detect with my multimeter on any of the 4 led +/- pins as power is turned on. Also I measured the pin outs to the main board again because I think the 5th pin down should have 13.3v while turned on, right? I measured again carefully and I am definitely NOT getting any voltage on that 5th pin at any time (on or off). Perhaps that's the problem?

[evanroheff21]

Re: 55 lg model 55uf6450 no backlights

Well....... Just realized there's a second row of pins under the first line of 6. D'oh. Measurements on that line from top to bottom are 3.37v, 0v, 13.3v, 13.3v, 13.3v, and 3.38v.