Have 2 MK133’s hooked up and having problem with player 8. Whenever the switch on 8 is pressed, 6 and 8 light up. When you press any of the others including 6, they light up normally. Is something blown/shorted? Everything looks identical on the other board for players 1-4 and works fine.
Does the board work fine in 4 player mode (i.e. when not connected to the second board)?
Negative, the 4th switch still lights 2 and 4 on the single board.
So there is definately a problem with the second board.
As you have one good board, it is easy to compare voltages at different point to find the cause of troubles.
Possibly, any good starting points as to what is causing this “bridging”.