I recall a diode that needs to be switched in or out (or installed/removed) depending on the mode these aftermarket boards that simulate bally mpu35 or stern mpu100/mpu200 modes.....

Something about some slight differences between the models that will make the batteries 'run' the board unless the jumper/switch is in the right position etc....i think even the homepin Mally MP35 board mentions it too...

MM