One rollover lane switch works only intermittently. The switch and its diode test fine, the MPU connectors have been repinned, and the switch only works reliably when a second diode is added in parallel.

Rollover switch only registers with two diodes fitted: failing PIA at U10

Cause: The switch matrix PIA at U10 on the MPU was failing; other switches in the matrix were slightly erratic too, though not enough to notice.

Checks

• Swap in a known good diode on its own to rule it out.
• Check for missing or added capacitors on the switch and for stray wire strands or solder splashes on the blades.
• If the switch side checks out, suspect the switch matrix chips on the MPU.

Fix

• Replace the U10 PIA on the MPU board.

Systems

• Bally MPU AS-2518-35

Games

• Lost World

Related tips

• MPU switch connector pins J2 and J3
• Switch matrix confused by a bad MPU DIP switch diode
• Prove the MPU switch inputs with the harness unplugged
• Coin door switch shorted to ground causes lamp and other odd faults
• Some games add a sixth switch strobe ST5 (switches 41-48)
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Odd switch problems from corrosion at the DIP switches
• Lost World ROM set numbers

Source: Pinball Repair help group!! (Facebook group)