That could be the payout relay or override solenoid chattering or the payout relay switches arcing. the plug connections are flaky or a wire is breaking off a plug pin in which case the override solenoid would stay powered but the pinwheel would stall for some reason mechanical issue with the hopper motor or hopper. If the hopper is stopping, it could only be one of: certainly long enough to see what the payout relay is doing as the first step of narrowing it down. I was assuming it's the early circuit, in which case timeout shouldn't happen for a pretty long time. It's possible the machine is a very late model 873 with the newer circuit, or the newer circuit/motor was retrofitted in, but it sounds like he's got plenty of time to see if the payout relay is unpowering. The later circuit with the diode/capacitor/ratchet microswitch kept resetting the safety motor, and if it didn't the shutdown happened extremely fast compared to the early design.

The 873's appear to have the early safety circuit that just runs the safety motor when the hopper motor is running, so the timeout/shutdown has to be long enough for the highest payout + some margin.

Bally slot machine 873 worn reset pawl