LG 42LS575T intermittent backlight issue

[dynomoman]

Hello,

Having an issue with an LG 42LS575T that has an intermittent backlight problem. This model has a separate LED driver board.
The set seems to run ok for a few hours. Turn off and next day it wont always come on. I get sound with no picture. Shine a torch at the screen and I see the LCD is working ok, just backlight dropped out. I tried freeze and heat treatment but it shows nothing

Has anyone seen this issue before on this set ?

Thank you!

[downunder]

Hi Dyno,

Time to strip the display panel and check the LED strips, after you check that BL-ON (INV-ON) signal is present when it fails, as well as a substantial voltage present on the connector on the LED driver board supplying the LEDs plus 24V supply to the LED driver.

LED backlights are essentially a "bad technology" and they're falling over like flies. One bad LED results in no LEDs at all due to protection circuits. A common cause of grief is that the LED strips often have a joiner near halfway on each strip. These sometimes develop a bad connection or high resistance. You may also find signs of overheating around a bad LED. Remember that the LEDs on each strip are in series.

Could ramble on for hours about LED backlighting, so see how you go.

Bruce

[dynomoman]

Thanks for the info Bruce.

If this set is like my LG 47 inch version the LED s are along one edge. I had to take my main tv a 47 inch LG panel apart due to a bad LED strip which I found was a track had shorted on the strip and went open circuit. A manufacturing defect by the looks  of it.I manage to repair that one ok,  but it's not a job for the faint hearted for sure.

This 42 inch i am working on runs ok for days at a time then fails to light the backlight. I am not 100% it's the LED S but it could be. I will carry out some checks as you suggest. I agree running these LED S so bright with all that heat must really punish them.

I have read on the net that my model of tv has quite a lot of issues with the back light going intermittent . But no info as to why sadly.

Regards
Mike

[downunder]

If repairs are successful, it's a good idea to turn the "backlight" setting in the menu down to 50% so that the LEDs aren't being driven at full bore. Currently working on a Sony that is top and bottom lit by about 280 LEDs. Wouldn't you know it, every one of them lit when checked with a DMM in diode mode. Back to the drawing board.

Bruce

[dynomoman]

Not sure if this is of any use to anyone but I found these on Aliexpress today. Looks like replacement LED strips like the ones in the video on you tube.  Sadly mine are rectangle close together types.

http://www.aliexpress.com/item/2pcs-x-Original-32-inch-LED-Strips-w-Optical-Lens-Fliter-Large-Size-TV-Panel-Backlight/32571400469.html?spm=2114.031010208.3.96.jKLUsX&s=p&ws_ab_test=searchweb201556_1,searchweb201644_5_79_78_77_82_80_62_81,searchweb201560_6,searchweb1451318400_6148

[dynomoman]

Not sure if this is of any use to anyone but I found these on Aliexpress today. Looks like replacement LED strips lick the ones in the video on you tube.  Sadly mine are rectangle close together types.

http://www.aliexpress.com/item/2pcs-x-Original-32-inch-LED-Strips-w-Optical-Lens-Fliter-Large-Size-TV-Panel-Backlight/32571400469.html?spm=2114.031010208.3.96.jKLUsX&s=p&ws_ab_test=searchweb201556_1,searchweb201644_5_79_78_77_82_80_62_81,searchweb201560_6,searchweb1451318400_6148

I found a picture of my LG 47LM760T LED fault Bruce. This set had no backlights at all. Track blew out right in front of the led. I repaired that and it's worked nearly 2 years without any issues. Strange one heh? I think there must of been a jump over between the two tracks? Bad PCB maybe? I cut that piece of burnt board away and linked it with wires. Looks big under the microscope but it was tiny in effect.

[downunder]

Fairly sure I've had that on an LG before, but the board was so badly burnt I didn't even attempt to fix it.

Bruce

[dynomoman]

Ok, well here is my findings on this fault. Interestingly another one of these LG 42LS575T came in with the same backlight issues. It seems this is more of a bad joint issue than an LED backlight failure. I re soldered the power supply and led driver PCB's as they looked a bit dry all over. Both sets now run fine with no intermittent failures of the backlights. I have run them both for a few days to confirm. Originally one set ran for less than 10 minutes before the backlights went out while the other ran for about an hour.

Hopefully this info is useful to others.

Thanks
Mike

[Turnip]

Hi Bruce,

Am I right thinking top and bottom Leds indicate a thick plastic panel behind the screen as used on small screen TV and monitors ? - Don't get so many smashed screens on those, also depth is minimal.

Guess there's a downside though in terms of heat dissipation with maybe 140 Leds in each string, though suspect 140 X 3v is an unusually high feed voltage - Chris.

[downunder]

Hi Chris,

Never really paid much attention to the thickness, but you may be right. Larger screens tend to have plastic posts mounted to keep the plastic sheets away from

the LEDs. Top and bottom lit screens tend to have a hell of a lot more LEDs and usually they're mounted on metal strips for heat dissipation. Makes them

impossible to work on. I'm not much into theorizing....just a colonial after all.

Bruce

[Turnip]

Hi Bruce, thanks for the 'heads up' from our fave colony.

Nice one, but maybe not for telly-bashers if top and bottom Leds can't be replaced individually, and such screens are more difficult to smash having a 10cm solid plastic panel behind them. Recall have met few  'top & bottom's' with a smashed screen, usually Pink/No backlights with CCFL.

Guess not so much on Ebay then - Pity - Chris.

[tv tony]

Hello Bruce, LG UK supplied the strips at one time but have taken them off their site now, typical!