The switch matrix acts erratically and settings may read wrongly.

Switch matrix confused by a bad MPU DIP switch diode

Cause: The 32 DIP switches are read through the matrix with their own diodes; a failed diode upsets the matrix.

Checks

• Turn all DIP switches OFF to take them out of the matrix and see if the fault clears.
• Game off: test each DIP switch diode on diode range, 0.4 to 0.6 V one way and open the other.

Fix

• Replace the failed diode.

Systems

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

Related tips

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

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