Steps to try:
Clean and check all connection including all socketed EPROMS and connections between A B and C board.
Fixed great
If not fixed try the B and C board on a different A board. If the problem persists, as a long shot check all the EPROMS match a known reference set but its probably a PAL or C board
If the problem is fixed on a different A board try pressing down on the PPU a little as they can get dry and lose connection on some pins. If this fixes it, clean and reflow or resolder the PPU. If the problem persists visually inspect the entire A board, probing for continuity and looking for obvious damage but things get difficult after this point. It could be a poofed PPU but it could also be a range of other things.
