My TV has lost brightness to about 80% of the screen?

that looks like a potential tcon issue. its sometimes hard to determine which is the cause of image issues like this ( tcon and lcd display) as some of the symptoms overlap such as this dark image. what you can try is to unplug the tv, remove one ribbon cable at a time (there is two) leading from the tcon board to the panel driver boards. after disconnecting one, apply power back to the set and see if that makes a change. repeat on the other cable. you will not have any picture on half the screen depending on which side is disconnected (should alternate sides of display) , but if there is a difference when disconnected, its most likely the tcon board. if not, its your display. post back here with your findings for any further assistance.

i left this out because its a sharp line in contrast, but being me, i would also check to see what your voltage is reading on your led strips. typically, but not always, if one goes they all go. again, just to cover all bases....id make sure that its not just one strip lighting the whole deal. ( if this was the case, you would see a gradual decrease in light and not a hard line like this, but you never know)

@jostewcrew I've seen laptop displays fail in the middle, but not entirely. Maybe it's different on TVs, but generally I've seen LED arrays partially fail, rather then a full failure.

@nick , my original diagnosis of tcon was correct...i was just covering the bases as it were. the give away here is the sharp, crisp line. my experience with a failed array is a gradial type dimness. in the example of a middle array failing it would look bright on bottom, dark in middle, and bright on top. that is if it isnt wired like a christmas tree ( which most are) where if one goes out, they all do. thanks for the info on the pc displays though. good to know.

@jostewcrew It was weird - fine on the charger, but on battery I lost most of it. I ended up finding a complete assembly for $4 more then the bare panel at the factory resolution (768p). At that point, I replaced the LCD assembly and took the baby out with the bath water, even though it was probably fine to just buy the LCD and the cable is fine. I mentioned it because I figured it would be helpful for someone like you since some small (usually disposable) TVs use laptop panels.

I think the extra power helped it limp along, but putting it on battery temporarily killed the damaged part of the array as you seen there. On laptop displays, I've never found a non-destructive way to get them out and you usually can't find them so a bad backlight is the end for a laptop type display.

I didn't know TVs usually use Christmas tree wiring where one takes it all out - I figured it would be like the small scale displays where it's split so you lose some but not all.