Model P52920bl Have sound but
Test the TV with different inputs: over-the-air antenna, DVD player, game box, cable or satellite box, etc. Picture problem with only one input? You've partially solved it: bad source, jack, or cable.
Picture problem on every input? First, unplug the TV and let it set for 15 minutes or so, then plug it back in and try it.
You most likely have a backlight issue with this TV set. Could be the inverter circuits for the backlights. The inverter is the power supply source for the backlights. there could also be a screen driver problem. This would entail changing the entire board. You should call in an authorized LG repair center to check this set out for you. Good Luck and please let us know what you found out about this TV set.
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If you have no volume bar or menu showing on the screen, then it cannot be anything you have wired wrong.
to see the picture on an lcd tv, you need to have back lights. the light shines through the lcd panel and you can see the picture. I suspect that you have video on the screen, but you cannot see it because the back lights are not lit. To prove this simply turn the tv on in a very dark room and tune in a channel so you can hear the audio. then with a powerful flashlight, point it into the screen and look around the edge of the flashlight for evidence of video. Be careful not scratch the panel with the flashlight as you place it on it..
Back to your problem, the back lights are tiny florescent bulbs and a ballast to run them, similar to the ones in a shop light. If the lights go bad or the ballast fails them you will get your symptom. Many times if the lights fail then not all of them will fail all at once, so the others will start up and then after a second they will cause the ballast to quit trying. In this case when you first start up the tv, you will see a slight glow on the screen (if the room is really dark). Many times you can see which light isn't working, there will be a darker horizontal stripe. This usually means the panel must be replaced, and unless it's cover with a warranty, you will probably not fix it due to the costs.
If it never glows slightly, then replacing the ballast may fix it. On some set this board is not available separately, but many have now been released. And on newer sets the ballast is part of the power board, and they have issues with a diode shorting so the part is cheap. With out a model number, I'm spit balling.
The last thing that can happen, is there is a slight back glow and no picture is being displayed. This is almost alway the fault of the lcd control board. Like the ballasts, many of these boards are now available without buying the whole panel.
At any rate, you know what to expect, there is nothing you can do, because a technician will need to fix it.
Problem is in either power supply or the Y-Sustain board.
The Y-Sustain is a relatively large board out of which go connections to the side of the display; it's therefore closer to either the left or the right side of the display, has at least one rather large heat sink on it and fat power cables connecting it to the main power supply.
Often there are fuses on that board - one or more. Check those.
if they are blown you can try to replace them, but expect them to blow again - usually the whole board needs to be replaced.
Fixing it will not take long..
Hoping this will help in solving the problem....