Does anyone know what the most common problem with a tv? that will turn on stand by light comes on but no picture or sound. It's a proscan 50in Led

Bad caps on power board

In most of the case, the problem is defective LED back lights strips.

You may try a new power supply first.

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Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

Sounds like either the CCFL tube (like a mini florescent tube) or the inverter that powers it has blown.

Look for the setup menu on the TV. It is common that you need to select either TV speakers or external speakers as only one will work at a time. If you want the sound to come out the audio output, you may need to select this option. Worth checking.

is there a cadence to the blinking timer light?

The power supply (AC adapter) is bad. It has enough power to light the red LED, but not enough to power the TV.http://shop.ebay.com/?_from=R40&_trksid=m38&_nkw=12+volt+5+amp+power+supply&_sacat=See-All-Categories