Many lamps fail as if a data line were missing, or the game acts strangely.

Coin door switch shorted to ground causes lamp and other odd faults

Cause: A coin door switch touching the grounded door, or worn insulating paper behind the start button.

Checks

• Unplug MPU connector J3 (lower right). If the fault clears, a coin door switch is shorted.

Fix

• Insulate or realign the coin door switches and replace the fish paper behind the start button.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Odd switch problems from corrosion at the DIP switches
• Self test will not start from the coin door button
• Switch matrix confused by a bad MPU DIP switch diode
• Rollover switch only registers with two diodes fitted: failing PIA at U10
• MPU switch connector pins J2 and J3
• Some games add a sixth switch strobe ST5 (switches 41-48)
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Prove the MPU switch inputs with the harness unplugged

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