LED screen comes on and goes black - LG 47LS4500

Sounds like a backlight problem to me. If you input a signal to the HDMI or Composite input and then it goes black have you tried taking a flashlight to the screen to see if the picture is still there but just dark? That would indicate a backlight issue. That model appears to be an LED edge lit backlight, so if the picture is still there after going black your going to have to trace out the backlight circuit and see whats going on. Usually with LED backlighting if it comes on at all then the LED's are good, its just the driver supply thats failing to keep them on. Some TV's have a small separate board that provides the backlight control, but i suspect with this model its on the power supply board and not the motherboard. Have you checked that board? It could be bad caps on the power supply board, check the voltages to the backlighting when it lights up and then when it goes dark. It could just be some failed capacitors on the power supply causing that voltage rail to drop off.

If you're able to see the logo on the screen, it's likely a bad main board.

Samsung HL-R5087W TV screen goes blank -tv/sound on - Forums ...

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CNETJun 11, 2015 - About a month ago, the at various times the tv screen would go blankeven though the tv ... Try turning off the cable/sat box when the happens. ... In reply to: Samsung SP50L7HX DLP TV linked to Sky HD box - same problem .... on it comesback to normal. however, once it went off again (Black Screen) after only 2-3 minutes. ln-t4069f picture problem lcd warmup - Forums - CNET
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Hello, do you keep the sound? if you do, take a flashlight when it goes out and shine it across the screen, can you see a dim image? if you do the L.E.D. driver/power supply board may be bad. let me know....tks

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TV with Black Blank Screen Repair How to Troubleshoot Symptoms for Faster...

Check filter caps in power supply.

Hello,

The picture board on your monitor is bad, since it goes off after few seconds and the power led or button is on, it means the picture board of your monitor has developed fault.

The best thing for you to do now is to take the monitor to a local repairer or service center within your vicinity to help with the replacement of the picture board because the fault is not something you can fix yourself, in order not to complicate issues.

Take care.