The game sometimes locks up at power-on or in play (for example one start in four, with both CPU LEDs lit), and coils can lock on.

Intermittent lockups with both CPU LEDs on

Cause: The 40-pin interboard connector between CPU and driver boards; it carries the whole data and address bus, so a microsecond dropout crashes the game.

Checks

• If reseating the boards makes it work, the connector is the problem.
• Look for gray, green or cracked joints, especially under the batteries.

Fix

• Replace the female half of the 40-pin interboard connector on the driver board.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• "Adjust Failure" at power-on
• Random lockups from the 12 V logic pin
• World Cup will not boot after a CPU board swap
• CPU never starts: check the reset line at IC1 pin 40
• Game powers up in audit mode instead of attract mode
• Game only boots after several power cycles: corrosion under the 6808 CPU socket
• What the adapter LEDs show
• Reset or lock-up: which points where

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part One, Clay Harrell (pinrepair.com)