I have a VIZIO P502ui-B1E with no picture or sound

The power light in the left corner would come on and then fade. the VIZIO logo would appear in the middle of the screen. then fade off and nothing. no picture, no sound. Then every 45 seconds the TV would automatically go through the same loop without me doing anything. was told it was the power board, replaced it and didn't even get the logo. was told it was the mother board. replaced it. power light but no logo. tried a different mother board and power light comes on and fades, no logo. press the power again nothing, like turning off the TV. press the power button again and the power light comes on quickly and off quicklyno fade) then flashes 15 times. I have tried using a flashlight and there is nothing there. Jerrad