A copper pad on a stepper's bakelite board is burnt, with heavy sparking as the wiper leaves it.

Burnt pad on a stepper unit's contact board

Cause: A low-ohm coil in that pad's circuit; its kickback arcs as the stepper fingers break contact moving off the pad.

Checks

• Identify the circuit on that pad and measure the coil it drives.

Fix

• Replace the low-ohm coil, then clean or repair the pad.

Systems

• Electro-mechanical

Related tips

• What stepper units do in an EM
• Kinds of stepper unit
• Faults that point to a stepper unit
• Cleaning and aligning an EM stepper unit
• Set stepper wiper finger tension and alignment
• Stepper cleaning, part 1: mark, check and remove the wipers
• Stepper cleaning, part 3: rivets, wipers and lube
• Repair a broken or worn stepper wiper arm

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)