My LG 55LM6200-UE TV has backlights but no picture or sound. Would that be a bad main board, TCON board, both, or neither?

Just change everything, or take this advice.

https://www.fixya.com/support/r29790007-repairing_own_television

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Hi Pat, I'd test F1 on the tcon board to see if it's blown, if it tests ok try a main board, only way to know for sure if the main is outputting video to the tcon would be with a scope

Shopjimmy tests their boards but a main can be damaged if it's not handled properly, it's very sensitive to static damage. Remove the flat strips going from the tcon to the panel boards, clean them and look closely to be sure there isn't any bent or broken contacts, do the same with the LVDS cable going from main to tcon. Next thing to do is check the fuse on the tcon board, if it's blown don't order a new board but throw the set out as the panel is defective. If the fuse is ok I'd send the main back and get another one from shopjimmy as it sounds like this board is bad.

If you did the flashlight test and no picture that means its not a backlight issue so probably main board failure!

If you could see a faint picture then it is a backlight issue and it would have been the led backlights blown or the T-con board!

Mostly is the back lights problem.

most probably a faulty controller board

Check the backlight if you have backlight only two things will solve the issue . The Tcon board board need to be changed. The other would be to get another main board those tow parts I mentioned will solve your issue would try the tcon board first

The Tcon Board or the LCD panel is faulty
If you wish to get some details; check the site linked here. Pull up older posts.
http://electro-medical.blogspot.com/

This could be 1 of a few things here. Without opening up your set and trying to look at voltages or indiciator lights I couldn't tell you exactly without some more info.

Without looking at it myself here are my thoughts and where I would first look at.

It may be a main board because:
1) Main board processes all your audio/video signals. Bad main board means bad video processing. Could do that. Especially if the main board is not sending the right signal or a bad signal to the rest of the boards so they don't do their jobs. 

It may be a power supply because:
1) Power supply is well, the power supply. If it's sending inadaquete voltage to the main board you may not be recieving enough juice to keep that main board doing your video processing or further up the line keeping enough juice flowing to your TCon board to keep it telling things where to go and what to do. The power supply sends voltages to your Ballast board(controls your lights) and to your main board through which power is supplied to your TCon board via a LVDS line and a +12/+16V signals. If the power supply isn't pumping out enough juice to keep the TCon or Main Board running, it could be a problem. By the same token it could be that your P/S isn't sending power out to your ballast board. 

It could be a TCon because:
1) If the TCon is bad, you won't get your pictures. This could be the TCon board itself or the LVDS cable from your TCon to your Main. However, with few exceptions, if it's a bad TCon you're looking at a total panel replacement. 

It could be a ballast board because:
1) The Ballast Board controls the lighting behind the LCD screen. If this isn't working right you won't be seeing much of anything without that back lighting. If this is bad, it's a panel replacement. These are not field replaceable items by LG. 

Based on the symptoms, without more data or a eyes-on view of it, I would say you're most likely looking at a Main Brd or PSU problem.