During play the match display changes along with the player 1 score, and the player 3 display shows odd numbers that change in time with player 1. A known good PIA made no difference.

Player 3 and match digits show wrong numbers: wire loose in the IDC plug at 1J5

Cause: The wire for display data line BCD A2 at pin 9 of connector 1J5 had worked loose in its IDC crimp.

Checks

• Swap the display PIA with a known good one to rule it out.
• Check each display data line at 1J5 for a wire that has pulled out of its IDC slot.

Fix

• Push the wire back into the IDC connector, then recrimp or replace the connector for a lasting fix.

Systems

• Williams System 7

Games

• Laser Cue

Related tips

• MPU boots only sometimes, weak clock signal: failed 6800 CPU chip
• Displays show only every other digit ("0_0_0")
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)
• Six- and seven-digit score displays
• A segment missing on a score display
• Digits burning or faint on players 1 and 3 (or 2 and 4)

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