Hardened factory grease is the main reason old EMs fail
The grease the factories put on stepper units sets hard over the decades and stops steppers advancing or resetting. This one problem is behind most EM games that do not work, so clean every stepper before chasing other faults.
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Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)