The CPU runs alone (even with Leon's code) but locks when the driver board is attached.

CPU locks up as soon as the driver board is connected

Cause: A failed driver board PIA, most often the switch matrix PIA IC11.

Checks

• Test the driver PIAs with Leon's code and a tester LED.

Fix

• Replace IC11 (switch matrix PIA) with a socketed 6821 first.
• If still locked, replace IC5 (solenoid PIA).
• If still locked, replace IC10 (lamp matrix PIA).

Systems

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

Related tips

• Test the CPU board PIA IC18 with Leon's code
• Leon code running but LEDs dark: bad CPU PIA IC18
• Test the switch PIA IC11 with the Leon test EPROM
• Leon's test EPROM fitted but the LEDs do not start flashing
• Damage after the black and white plugs were swapped
• Displays show only every other digit ("0_0_0")
• Leon's code with the driver board attached
• Transistors, diodes and chips to keep for CPU and driver repairs

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