First thing to do is check the edge connector foil traces to make sure power is even getting onto the board. If you have game boards where two or more boards makeup a complete game board check for bad ribbon cables or interconnector pins that may be tarnished preventing communication. If still dead try changing the CPU chip(s). Usually Z-80-CPU, 6502, 6800, etc. are typical numbers of the chips. If the CPU chip is bad ain't nothin' gonna happen on the board and it will appear dead.