Test the CPU board PIA IC18 with Leon's code

Steps

• Connect the tester LED's plain lead to +5 V (TP9 on System 6/7, or interboard pin 1 at the far right).
• If testing in the game, remove fuses F1, F2 and F3 first.
• Touch IC18 pins 2 to 17 in turn: each should blink about once a second.
• Pins 19 and 39 (CA2 and CB2) should also blink, though they may not with the driver board attached.
• Data pins 26 to 33 should pulse on a logic probe; pin 34 (reset) should be high.
• Any of pins 2-17 not blinking: replace IC18 with a new 6821 in a socket, after checking the chips that pin also feeds.

Remove F1 (display high voltage) before probing a board in the game.

Systems

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

Related tips

• Leon code running but LEDs dark: bad CPU PIA IC18
• Leon's test EPROM fitted but the LEDs do not start flashing
• Test the switch PIA IC11 with the Leon test EPROM
• CPU locks up as soon as the driver board is connected
• Damage after the black and white plugs were swapped
• Test the driver board PIAs IC5, IC10 and IC11 with Leon's code
• CPU board PIA positions and jobs
• CPU chips Leon's test code does not need

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