After power-up with crossed plugs: GI and displays out, flippers and drop target reset coil stuck on, sound board fails its self test and fuses blow.

Damage after the black and white plugs were swapped

Cause: 28 V reached the 5 V logic; exactly what fails varies by game and how long it ran.

Checks

• Check the sound board amplifier chip.
• Check CPU chips IC7 (7404) and IC5 (7402) for +5 V to ground shorts.
• If the game always starts in test mode, suspect CPU IC12 (7408).
• If many switches do not read, check driver IC17 (7406) and the IC11 6821 PIA.

Fix

• Replace the shorted chips found, socketing them.
• Fix the plug matching before powering up again.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• A coil or flasher comes on and stays on at power-up
• How a coil or flasher is switched: PIA to TTL to pre-driver to driver
• Lamp column still dead after a new TIP42
• Coil fires when tested but not in game play: TTL driver chip
• Which logic chips drive which solenoid transistor pairs
• No sound effects in play though speech works: check the sound board PIA
• One switch closing lights up its whole row in switch test: bad column driver chip
• Lamp rows stuck on: the 7406 at IC19

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