Score displays flicker or flippers work only some of the time.

Flickering displays from the foil backbox ground board

Cause: On later games the backbox ground plane is foil over cardboard; it warps, can touch the board backs, and grounds the boards poorly.

Checks

• Look for warped cardboard touching the boards.
• Check that solder mask is not insulating board grounds from their brackets.

Fix

• Daisy-chain a wire to one metal bracket on each backbox board and on to a solid metal ground in the cabinet.

Systems

• Bally MPU AS-2518-35

Related tips

• Flickering displays and flipper drop-outs from a poor backbox ground
• Three flashes and 8s or 9s in the hundreds digit
• Rectifier board J3 (to backbox boards) pins

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