Odd matrix faults, such as player 1 display copying the credit/ball display, or a game that will not play properly; common after a coin door break-in.

Strange game behaviour from a grounded start button

Cause: The grey fish paper that insulates the start button from the grounded coin door is worn or torn, grounding that switch line (page 43).

Checks

• Switch off, unplug the coin door connector, power on and see if the fault clears.

Fix

• Replace the insulating paper behind the start button.

Systems

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

Related tips

• Player 1 display shows credits and ball in play
• Right outlane adds players or starts a new game: start button shares a matrix row
• Odd switch problems from corrosion at the DIP switches
• Self test will not start from the coin door button
• Coin door switch shorted to ground causes lamp and other odd faults
• Credit button does nothing in switch test: daisy-chained coin door wire come loose

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