Match the head-to-cabinet plugs before power-up

Steps

• Find the plugs at the backbox-to-cabinet split; one white pair and one black pair are physically identical.
• Join white to white and black to black.
• Check the wire colors on both halves match, pin for pin; wire color matters more than plug color.
• On games where both plugs are the same color, match them by wire color only.

Swapping these plugs puts 28 V solenoid power on the +5 V logic and destroys chips on the CPU, driver and sound boards.

Systems

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

Related tips

• Burnt 3J1 square power input connector
• Replace the 3J6 logic power connector
• Replace CPU board power connector 1J2

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