Strange switch matrix behaviour on a board that had battery leakage.

Odd switch problems from corrosion at the DIP switches

Cause: Corrosion around or under the DIP switches is shorting switch lines together.

Checks

• Inspect around and under the DIP switch banks for residue.

Fix

• Remove the switches if needed, clean and neutralize the area, and refit.

Systems

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

Related tips

• Switch matrix confused by a bad MPU DIP switch diode
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Coin door switch shorted to ground causes lamp and other odd faults
• MPU switch connector pins J2 and J3
• Some games add a sixth switch strobe ST5 (switches 41-48)
• Prove the MPU switch inputs with the harness unplugged
• Switch reads closed even with switch connectors unplugged: shorted resistor on the MPU
• Rollover switch only registers with two diodes fitted: failing PIA at U10

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