LED backlights in LG television...

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Externet
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LED backlights in LG television...

Post by Externet » Thu Sep 10, 2020 7:29 am

Hi fellows.
A LG model 50LN5400 TV has two circuits for backlights. They do not turn on.
There is video by sticking a flashlight onto the screen.
LED strips connector is marked L+, L- and L+, L-
From both L+ to gnd there is 80 V. From L+ to L- nothing. Seems to be in a 'protection state' How to test the LED strips or confirm the driver supplies are healthy ? I have no spare LED strip nor another driver supply :sad: Cannot even find the schematic. :???:
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Re: LED backlights in LG television...

Post by Janitor Tzap » Thu Sep 10, 2020 7:42 am

Try this first:
First thing you do is try to force start it. This requires you to press buttons on the TV at same time which in this case was pressing both the ok & Power button at the same time for about 15 -30 seconds then power off TV and restart if all is well the TV will then power up with picture & sound.
If that doesn't work.
Consider that the Light Engine has failed.
Here's a Parts Supplier, but looks like the part are currently out of stock. :(
https://www.shopjimmy.com/lg-50ln5400-u ... version-2/


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Re: LED backlights in LG television...

Post by Externet » Thu Sep 10, 2020 8:16 am

Thanks, Tzap. Do not understand the above.
I could "force" feed the supply into a LED strip by jumping from GND to L- with a ~100 ohm resistor ?

+80V--------------------------LED+ LED- ----------------Mosfet-----------------gnd

(red is connector)

Edited... Seems this is it: second schematic = third image at :

----> https://electronicshelponline.blogspot. ... d_fragment_
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Re: LED backlights in LG television...

Post by Janitor Tzap » Thu Sep 10, 2020 9:46 am

Okay,
I guess that fix I found from LG Forum wasn't very Clear.
:roll:

Here's the manual I found, and it goes to page 10 "Buttons on TV Set"
https://www.manualslib.com/manual/57436 ... =10#manual

Follow the "Button Press" thing again, and try it.


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Re: LED backlights in LG television...

Post by Externet » Thu Sep 10, 2020 9:49 am

Seems this is it: second schematic = third image at :

----> https://electronicshelponline.blogspot. ... d_fragment_

Q803 and Q805 are off, controlled by the IC shown below. Now I have to check on its data sheet
----> https://pdf1.alldatasheet.com/datasheet ... 9483F.html

how that IC is configured on the TV schematic to detect why is it shutting down... Will take me a while.
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Re: LED backlights in LG television...

Post by Janitor Tzap » Sat Sep 12, 2020 8:20 am

Sorry for not getting back sooner.....

But here is some info that will be helpful.
https://smiseks.wordpress.com/2016/12/0 ... nd-repair/


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Re: LED backlights in LG television...

Post by Externet » Sun Sep 13, 2020 8:19 am

Thank you, Tzap.

After looking at some dismantling videos and closely inspecting how this LG 50LN5400 is put together, decided to NOT do it the way everyone does it.
Good thing ! Handling such a large thin LCD and keeping it safely and lint/fingerprints-free aside until replacement emitters arrive would had been risky.

Instead, cut a PET 2L soda bottle into twenty 2" x 1" strips and with the TV screen down on a matress, rear plastic cover removed, removed a bunch of perimetral tiny screws, inserted the strips at the latches that hold the front and the rear shells together. Disconnected the display ribbons, The entire rear dislodged from the front assembly with no hurdles nor effort, diffusers stayed in their place and sequence untouched, circuit boards still attached to the rear shell; LCD panel untouched !
Able then to test LED emitters; found two kaput. Ordered replacements and will update progress...
The dismantling part I was afraid of goofing with presented not a problem. :grin: Will later find out if LED driver circuitry is healthy...
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Re: LED backlights in LG television...

Post by Janitor Tzap » Sun Sep 13, 2020 1:01 pm

Able then to test LED emitters; found two kaput. Ordered replacements and will update progress...
Do you mean the LED Back Light Strip?
ImageIn the article I linked too....
He states that he found a dead LED on one of the LED Back Light Strips.
Which was causing the safety protection circuit to kick on, cutting the power to the LED Back Light Strips.

Also, he recommended just replacing all the LED Back Light Strips.
Instead of just replacing the burned out LED's.
Because the other LED's on the Back Light Strips have several years of running time on them already.
Thus, it wouldn't be to much longer before they start to burn out as well. :wink:


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Re: LED backlights in LG television...

Post by Externet » Sun Sep 13, 2020 2:03 pm

Thanks.
Yes, just like your image, one emitter in a strip and another in other strip found dead; had to test all 60 one by one as failed open, the whole series section shows dark. Ordered all the dozen strips kit ($36) and will replace all the owner wants. Replacing the $0.10 emitters individually takes a hot air gun I do not have.
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Re: LED backlights in LG television...

Post by Janitor Tzap » Sun Sep 13, 2020 7:55 pm

Cool, 8)

One thing you can do, while your waiting for the new LED Back Light Strips.
Put jumpers across the burned out LED's, on the effected strips.
Then try powering on the Back Light.
If they light up, then you don't have too worry about checking the power supply/Light Engine.
The set just needs those old LED Back Light Strips replaced.


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Re: LED backlights in LG television...

Post by dacflyer » Mon Sep 14, 2020 12:44 pm

sometimes the work is harder than it's worth..
them screens are getting thinner and thinner...super easy to crack..

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Re: LED backlights in LG television...

Post by Janitor Tzap » Mon Sep 14, 2020 6:39 pm

dacflyer wrote:
Mon Sep 14, 2020 12:44 pm
sometimes the work is harder than it's worth..
them screens are getting thinner and thinner...super easy to crack..
Yeah,
Anything 40" or under in screen size is not worth repairing. :(

The only way I might consider repairing something that picture size.
Was if I had several TV's of the same model and manufacturer.
{That I got for free.} :wink:
I could then cannibalize the good parts from the worst TV's.
And use those parts to fix the other TV's, that just need a simple repair.
Then take the repaired TV and sell it.

Though, it could be worth while, investing some money in LED BackLight Strip rebuilding.
Especially when it comes to the larger displays 50" - 80" or larger.
Here's a vid from ShopJimmy that show how to easily replace bad LED's.
https://www.youtube.com/watch?v=EZVQKEp ... 2Q&index=3
Basically, you save the old LED Backlight Strip.
Then when you have some down time.
Repair, or Rebuild the Backlight Strip, then label what it came out of,
and place it in to your stock of good parts.
Then when you have a TV of the same Model & Make come in for repair of the Backlight Strip.
You'll have one in stock, and be able to get the TV back to the customer faster.
And since you are buying the LED's & Lenses in bulk, you can save yourself & the customer some money. :wink:


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Re: LED backlights in LG television...

Post by Externet » Tue Sep 15, 2020 8:03 am

Anything smaller than 40" is not worth repairing ... That size is less fragile; the dismantling part is less risky.

Larger than 40" have a higher price sticker, worth repairing; if dismantling/reassembling goes well.

Smells like the cost of the repair bill is more driven by the price of the item than the effort in the repair process. :???:
Once I had a discussion somewhere else about if replacing brake pads on a Honda should be cheaper than on a RollsRoyce... being both the same effort. :shock:
Or oil change, or patch a tire...

Edited much later... Fixed, owner picked up the TV very happy.
The prying top and bottom shells with a small screwdriver, inserting shims in between at the 20 catches locations was a success. Perhaps a picture explains better:
LG TV.jpg
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