Then to figure out which board is bad, pull one board out at random from the device (with it disconnected from all other devices) and then power it On each time, until some major function (that is still detectable despite pulling out boards) is detectable as active when you push the power-On switch.
Each time, unplug it from the power again before removing anything else.
Inspect every board you remove, of course, for burnt-out fuses, or burnt-looking components, or even brown spots on reverse sides of circuit boards.
(Note: if board has 'chips' on it, be sure not to touch any part of the board at all, except the edges, or you may blow-out the chips. This is the most basic of anti-static precautions, and if you don't already know this, you are by no means at the high level of electronics repair skills.... I say this to help in case it is needed, and just in case it is needed.)