While this may totally not be the case here:

my plus would reboot over and over again when i turned it on, turned out to be a broken solder joint on the analog board connector to the logic board (J4 IIRC).

try gently slapping the plus casing on the side of the analog board, to see if you get it to do anything.

This might likely not be your problem since you get no power at all, but it certainly fixed my power and video issues.