Usually there are 2 or 3 board in a cheap DVD players. the power supply, MPEG board or maybe video output board. You can check voltage coming out of the power supply board. Usually 5v, 12v. If the fault is in the MPEG board, don't even try to fix it. It's more than just video in and out. It has digital processing, MPEG conversions...etc. All the components are surface mount, and the board could be 4 layers. DVD player, especially cheap ones are not made for servicing and repair.