After a game starts, the player 1 display repeats the credit and ball-in-play numbers; everything else plays normally.

Player 1 display shows credits and ball in play

Cause: A switch strobe (column) line shorted to ground, for example a wire pinched between backbox and cabinet, or a trough switch touching its metal bracket.

Checks

• Look for switch wiring touching grounded metal.

Fix

• Free and insulate the shorted wire or switch.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Self test will not start from the coin door button
• Switch reads closed even with switch connectors unplugged: shorted resistor on the MPU
• One drop target only registers if it is the first one down: reseat and clean the PIAs
• Odd switch problems from corrosion at the DIP switches
• Coin door switch shorted to ground causes lamp and other odd faults
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Some games add a sixth switch strobe ST5 (switches 41-48)
• MPU switch connector pins J2 and J3

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)