Quoted from VALIS666:Mine won't boot anymore.
First off, I don't have this game, never been inside of one, and don't know the layout of the internals. That said, I've worked on a couple of custom games that use the P3-ROC, so I know just barely enough to be dangerous.
Sounds like you hit the nail on the head, here. The computer that drives the machine is off. That's why the P3-ROC seems to be working fine. It is, but the USB is not connected, meaning, the computer that drives it is not on. Figure out how to override the power button/find the power button/reset button for the computer. Voltages look normal since all the appropriate LEDs are on. Might be as simple as a power connector to the computer that came unplugged. The P3-ROC is not the computer. Follow the USB cable from the P3-ROC and where it terminates will be the source of your problem.
That's my guess. Good luck!