Sanyo dp50740 turns on, with still picture and no sound. then powers off then back on again repeating it's self

it is only a little pc board that can be change

There is no reset button on televisions. I see that is a plasma set, if you are just losing the picture that is one thing, if the whole set is shutting down, that is another. Please advise

Main board fault. Contact any service technician. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Pull up older posts. Surf the site with patience. Do a search by type in the brand name of your device at its search box and press "Enter".
http://electronicshelponline.blogspot.com/

It sounds that the TV switches ON, and switches OFF, after three relay clicks. It is likely to the fault to its main board. Contact any service technician. This is not a user repairable fault.
google.com/+GopakumarGopalan

Try to change the Inverter board (Buffer board).

What you have is most likely a failed main board. These sets are known for this problem. There are several different variations of the main boards. Make sure you order one that matches yours. The main board part # will start with J4@#. The main board part # should be on a sticker either on the main board itself or on the coax input terminal casing. I hope you got the TV for a good price !!!

Change power outlet and see. If problem persists, it might need attention by a qualified service technician.