LG 43UJ6300 Backlight problem

[tmhobadcap]

I got this not working tv from a friend. When I try to turn it on, there will be a quick LG logo flash only, and then nothing but sound. When look closely with a flashlight, there is image on the screen.

I have experience in similar situation before with other brand of tv. It is because one or more backlight leds are not working. I have no led tester. I only use a 3V dc to find out which led is not lighting up. And then replacing it can fix the problem.

But for this tv, I tested all leds individually and they all could be lighted up.

I saw the below Youtube video showing how to fix the same problem but for a difference LG model.
https://www.youtube.com/watch?v=4GYdI8f39BU

In the video, he used an led tester. And all leds light up. At about 26:35, he found the bad led. The bad led was also lighted up and the led tester was showing with jumping voltages. Why that one is the bad led even though it can be lighted up? Without the led tester, will there be some other ways to find out the bad led?

[dick_barton]

Re: LG 43UJ6300 Backlight problem

It may not be a bad led. Have you tested to ensure that there is no poor connection either at the led connector/socket or where the led strips connect to each other.

[tmhobadcap]

Re: LG 43UJ6300 Backlight problem

Yes, the connections at the led connector/socket or where the led strips connect to each other have been checked to be good.

[mazmaz]

Re: LG 43UJ6300 Backlight problem

Hi
This is problem of bad back-light led.
you should replace them with new one compleatly or find and replace only failed led(s).

[RDC55]

Re: LG 43UJ6300 Backlight problem

Try taking a video of LED's at power up, then slow motion play to capture bad LED if there is one.

IMHO if one is bad the strips should be replaced with new as failure of others is imminent.

[nomoresonys]

Re: LG 43UJ6300 Backlight problem

Test the power board if it's ok then make sure your getting the backlight on signal from the mainboard, could very well be a bad/weak led, testing will tell, I don't go to the pain of tearing it down til I'm pretty sure it is a backlight issue.

[pacoelcico]

Re: LG 43UJ6300 Backlight problem

Van a ser los led cambia todas las tiras por unas nuevas el problema de cambiar solo los led que esten encortocircuito es que los que estan operativos estan un poco tocados y despues de poco tiempo se terminan quemando tambien y volver a reparar de nuevo

[tmhobadcap]

Re: LG 43UJ6300 Backlight problem

Van a ser los led cambia todas las tiras por unas nuevas el problema de cambiar solo los led que esten encortocircuito es que los que estan operativos estan un poco tocados y despues de poco tiempo se terminan quemando tambien y volver a reparar de nuevo

Sorry, I cannot understand the language you used.

[tmhobadcap]

Re: LG 43UJ6300 Backlight problem

My tv is normal and working now. What I did was just switching the positions of two led strips.

There are totally 8 led strips. 4 strips on the left and 4 strips on the right. Each strip on the left is with 4 leds and each strip on the right is with 5 leds. The upper 2 strips on the left and the upper 2 strips are connected in series to form Group A. And the other 4 strips are connected in series to form Group B. I followed the suggestion of RDC55 to take a video of the leds at power up hoping to find out which led is bad. At first, in the video (Video 1), the leds flashed too quickly and I could not see which was bad. Then I used a better camera to take the video (Video 2) and I could see quite a lot of leds did not flash at 00:05. With the help of the slow motion playback, I took a snap shot (Photo 1) which can show clearly that the left bottom 2 strips and the right bottom strip with 2 leds from the right did not flash.

But as I mentioned before, all strips and leds are tested to be good (can be light up), therefore, I suspect that the left bottom 2 strips are with poor connection at the sockets. I then sprayed the 2 connections with contact cleaner. But power on again just have the same result. Then I switch the positions of the left upper 2 strips with the left lower 2 strips. Power on again just have the same result. Then I switch the positions of the right bottom strip (the one with the 2 led on the right did not flash) with the right upper strip. But this on power up makes all the leds light up constantly (Video 3) which means the backlight problem is gone. I really do not know why. Could somebody kindly explain to me why and what exactly the problem is?

Video 1:
https://youtu.be/T9x9y3Pa3JY

Video 2: https://youtu.be/V-jmCHEDF8w
https://youtu.be/V-jmCHEDF8w

Video 3: https://youtu.be/3Bml5nwy8zs
https://youtu.be/3Bml5nwy8zs

Photo 1:


ps: I made a mistake on the model no. The correct model no. is 43UJ6200. I do not know how to edit the title.

[cheeky2]

Re: LG 43UJ6300 Backlight problem

As Dick Barton has already stated to you check the connections. Looking at them isn't sufficient you need to get a decent meter and measure the continuity between the connectors. If the resistance is high then you need to put a link between the two point with wire and solder them up. Any high resistance generally above 1ohm between the points can cause the leds not to turn on. I have seen this with a meter many times and has caught me out before and puzzled me until realising.

Even with a led tester all leds can power on ok however if you look closely at the voltage output from the led testers you can see a fluctuations in the output voltage where the resistance on the connectors is affecting the output voltage. Also the leds can briefly flicker as the voltage is ramped up slightly to compensate for the higher resistance of the connections of the led strips.
If it was me I would put the strips back where they weren't powering on and isolate the fault on the connector as moving them around may only prolong the early failure later on.
Do you wish to strip the TV back down again and go through it all again? I think not

[tmhobadcap]

Re: LG 43UJ6300 Backlight problem

As Dick Barton has already stated to you check the connections. Looking at them isn't sufficient you need to get a decent meter and measure the continuity between the connectors. If the resistance is high then you need to put a link between the two point with wire and solder them up. Any high resistance generally above 1ohm between the points can cause the leds not to turn on. I have seen this with a meter many times and has caught me out before and puzzled me until realising.

Even with a led tester all leds can power on ok however if you look closely at the voltage output from the led testers you can see a fluctuations in the output voltage where the resistance on the connectors is affecting the output voltage. Also the leds can briefly flicker as the voltage is ramped up slightly to compensate for the higher resistance of the connections of the led strips.
If it was me I would put the strips back where they weren't powering on and isolate the fault on the connector as moving them around may only prolong the early failure later on.
Do you wish to strip the TV back down again and go through it all again? I think not

Thanks for your comments. I would not open the tv again until it does not work again.
Do you mean the tv still have connection problem now? If yes, which connection, the connections at the sockets? Then why now it can work now ?
Also, why the connection problem at the socket can cause only 2 of the leds on the right bottom strip not flash instead of the whole strip not flash?

[cheeky2]

Re: LG 43UJ6300 Backlight problem

Your choice but expect to open it up again in the next couple of months!
What leds light up is really irrelevant the point being is to resolve the underlying issue.
You have moved some strips that have originally a high resistance connection to somewhere where the strips didn't have the issue originally on the connections. Even a slight variation in the resistance will allow them to work which is why they are working.
Surely logic will prevail by swopping them around the only thing you have changed is the connections between the leds. So therefore it has to be the connections between the strips that was at fault originally.