A CPU PIA pin does not blink the tester LED with Leon's code running.

A PIA output fails Leon's test but the PIA is good

Cause: Another chip on the same line is holding it high or low.

Checks

• IC18 pin 2 also goes to IC6 pin 23; pin 3 to IC6 pin 22; pin 4 to IC6 pin 21 and IC7 pin 11; pin 5 to IC6 pin 20; pin 39 to IC7 pin 5.
• Snip the suspect chip's pin where it meets the board, lift it clear, then retest. If the PIA pin now blinks, that chip is bad.

Fix

• Replace the chip that held the line (for example a 7404 at IC7), and resolder or replace any lifted leg.

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
• Test the CPU board PIA IC18 with Leon's code
• CPU chips Leon's test code does not need
• Leon's older memory test read with a tester LED
• Running Leon's test EPROM with the boards still in the game
• Add two test LEDs to a System 7 CPU board
• Why the CPU LEDs stay on when the program does not run
• System 7 CPU LEDs are swapped versus System 6

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