Samsung TV not turning on

Hello,

I’m looking to fixing a friend’s TV. I’ve already taken it apart and read some topics regarding mainboard/Tcon-board/powerboard failures.

The red LED indicator on the front blinks 5 times when I connect it and then turns off. There’s no apparent damage to the powerboard.

The mainboard’s optical connector does light up, the tcon-board shows 3 blue LEDs and the IR-receiver board lights up when I press the accompanying button to turn the TV on.

I’ve seen somewhere that I should unhook the mainboard to check if the backlight turns on, is this right? I tried, but there was no backlight.