As noted, check the fish paper on the start button and any other switches on the coin door. They put 110V right to it and used metal buttons to boot. A recipe for shocks.

If the door has a microswitch on it, check the physical switch itself. Melody Lane shocked the crap out of me once because a switch on the door that attached to the coin counter had its housing broken open. The bare metal contacts inside the switch were directly touching the coin door. I yanked that counter out with great satisfaction.