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Louie, Technician
Category: TV
Satisfied Customers: 2557
Experience:  25+ yrs. experience in the field and 5+ yrs. online, down to component level.
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Have an LG 55LA7400-TC TV (edgelit from bottom) that

Customer Question

Have an LG 55LA7400-TC TV (edgelit from bottom) that developed a dark shadow on the RHS of the screen top to bottom.
Initially I put up with it until the screen started going blank (lost video) but TV still had
Used to take about 5 hrs or so of viewing before video loss, but over time got shorter and
shorter until it was just a few minutes.
The TV only has two large PCB's (PSU and Mainboard) everything else required is incorporated into these 2 boards (LED driver & Tcon etc).
I opened the back and saw that the LED Driver board was intergral in the PSU, it's a stereo setup with a left and right side driving half the screen each.
When the TV had normal video (apart from the dark shadow on RHS), the main output voltages to the LED's (4 tracks per side carrying voltages) was...
LHS: 93.6v, 25v, 24.4v, 25.2v.
RHS: 93.6v, 24.2v, 25.1v, 36.8v (side with dark shadow).
When the screen went blank (audio still present) these voltages on both output sides
dropped from 93.6v down to around 70v, and the other 3 voltages were approximately halved.
When I disconnected the LED ribbons (FFC's) from the PSU the lowered voltages all remained low, so I assumed the problem was in the PSU and not the LED edgelit strip.
I ordered a new PSU for the TV and while I was waiting for deivery I bought the service
manual for the TV.
In troubleshooting, the service manual shows "dim light" pictures of the screen, ie shadows on the LHS, RHS, and middle of screen, and also "no light" (blank/black) screen, and the fix is "Exchange LPB(LED driver)".
I should mention here that from time to time when I powered up the TV during this problem period, every now and then there would be a dark shadow on the LHS, or in the middle of the screen as well as the RHS, but I could get rid of these by turning the TV off for 10 seconds and then repowering it. This technique never got rid of the shadow on the RHS.
The new PSU arrived, I duly installed it and expected all to be well again. On powering up, the dark shadow on the RHS of screen immediately reappeared. I suspected there may be a bad connection down at the LED edglit strip, so I completely dismantled the TV.
The edgelit strip has 2 long separate PCB's of LED's glued to it, L & R, with 75 LED's per
All connections seemed to be good and no physical damage apparent.
I powered up the TV in this state (no screen, diffuser etc) so I could observe the LED's.
All LED's fire up, go up and down in brightness as the mainboard brings the TV to life, and as soon as it reaches the ready state, the first 25 LED's on the RHS go out (each and every time tested).
To see if it was a problem in the LED bar or the PSU, I was able to turn the LED bar over
(end over end) and plug the cables from the PSU straight into the LED bar sockets (mapping of the voltage tracks on the FFC told me this would work).
Logic told me if it was the LED strip that was bad, the 25 LED no lit area would now be on
the other side, if the problem was the LED driver in the PSU, the no lit area would stay on
the same RHS.
On power up all LED lights came on, went through their up and down ritual, and on reaching the ready state, the first 25 LED's on BOTH sides went off!!!
I put the LED bar back the correct way and on power up both sides still had 25 LED's out.
Power off and then back up again and I was back to the RHS only being the no worker.
Because this TV has a Dimming function, I decided it may be the Mainboard telling the LPB(LED driver) to turn those lights off.
To test this theory I removed the PSU->Mainboard connector at the Mainboard end and put a jumper from standby to PWR-ON, and another jumper from standby to DRV-ON.
Now all I have hooked up is the PSU and the LED bar. Power on and all the LED's come on and the 25 on the RHS still go off when ready state is reached.
All LED's bench test OK with both a 3v battery pack and a MM diode test.
I only have one theory left now, and that is the CPU has told the LPB(LED driver) via the
dimming function to turn those lights off, and it retains this instruction even with the Mainboard disconnected?
To test this I would need to rebuild the TV but decided to ask you guys for an opinion first.
I've never used the dimming function so I'm sceptical of that theory, although I'm not the
only user of the TV and there's one user who continually presses wrong buttons and needs help to bail out of the state they've created on the TV.
Now that you have the history of symptoms and tests, I am asking for any help or assistance that you may be able to offer.
The TV was manufactured June 2013 so is not that old (6 mths out of warranty - extra 12 mths from the retailer), and is a great TV so I'd like to get her fixed.
Apologies for the length of the post, but I thought a full history may make it easier to
solve the puzzle.
PS, I can send you any pictures or part of the service manual if that helps with diagnosis.
Submitted: 7 months ago.
Category: TV
Expert:  Jack replied 7 months ago.

Hi , I`m Jack , I will be helping you today
Can you please send me a reply when you`re ready ?

Expert:  Jack replied 7 months ago.

Sorry , posted here by mistake , will opt out and repost your question for other techs to look at

Expert:  Jack replied 7 months ago.