LED backlight on LG television...

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Hi.
Am getting too old. Cannot find the schematic section for the backlight drivers in a 50LN5400 service manual.
----> [ https://servlib.com/lg/tv/50ln5400-ta-50ln542y-tc-chassis-lb36b.html ]

Unit does not turn on backlights; a flashlight against the screen shows video. The connector "+led" and "-led" for the backlights is dual; like two sets of drivers. "+ led" referred to chassis shows 80V at each branch; "+led" referred to "-led" shows no voltage. [P801 at top edge, near left on image]

If I had a known healthy string of LED backlights I could rule out the supply by plugging and confirming; but not.
If I knew the healthy voltages at the backlight connector pins, I could rule out things too.

If the two mosfets are not doing the grounding path at the schematic below; well that would match symptom.

Guidance, please ?

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1599681106821.png1599680694924.png1599681106821.png
 

Dodgydave

Joined Jun 22, 2012
9,688
What a Lousy service manual, ( note to self don't buy LG crap)..

Only circuit I can find is on page 22, top left showing the pwn and dim control.
 

abrsvc

Joined Jun 16, 2018
41
Hi.
Am getting too old. Cannot find the schematic section for the backlight drivers in a 50LN5400 service manual.
----> [ https://servlib.com/lg/tv/50ln5400-ta-50ln542y-tc-chassis-lb36b.html ]

Unit does not turn on backlights; a flashlight against the screen shows video. The connector "+led" and "-led" for the backlights is dual; like two sets of drivers. "+ led" referred to chassis shows 80V at each branch; "+led" referred to "-led" shows no voltage. [P801 at top edge, near left on image]

If I had a known healthy string of LED backlights I could rule out the supply by plugging and confirming; but not.
If I knew the healthy voltages at the backlight connector pins, I could rule out things too.

If the two mosfets are not doing the grounding path at the schematic below; well that would match symptom.

Guidance, please ?

View attachment 216774

View attachment 216777View attachment 216774View attachment 216777
Check the LED strips. IRRC, these units will shutdown the backlight supply if a problem is detected. I have seen cases where there is an open LED that would cause this shutdown. Normally, you can see the one that is bad as it may show a black/char spot. I would focus on the strips first rather than the supply.

Dan
 

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Thanks, gentlemen.
Check the LED strips
Of course that is first step. And very risky. A 50 inch piece of 1/16 inch thin glass has to be removed with zero fingerprints hitting nothing and no flexing and no lint around just to reach/probe the individual LEDs under a bunch of layers of diffusers.
I need some magic to probe if there is a bad emitter on a strip without the risk of dismantling... and finding emitters are good and the driver circuitry is not...:oops:
 

Thread Starter

Externet

Joined Nov 29, 2005
1,546
Found in many videos that the typical way people gets to the LED emitters for diagnosis and replacement is wrong.
No need to remove, nor even touch ! the LCD panel nor the difussers to separate this LG 50LN5400 television in two halves, front shell with the panel and rear shell with the emitters.
It is designed with latches to separate and reassemble very quickly with no risk of damaging anything, done front down on a padded surface. With no fear of damage, is now work in progress; bad emitters found and replacements ordered. Will update. :p

Updating... Success in the repair. Image of the two half shelves pried apart with a small screwdriver to insert plastic shims at 20 catch locations along
the perimeter :

LG TV.jpg
 
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