The CPU LED flashes nine times and stops.

Boot stops after the ninth LED flash: IRQ fault

Cause: The IRQ signal is missing or wrong. Suspect the IRQ counter U29 (4020), then U35 (74LS10) and U36 (7404); the ROMs are unlikely by this stage.

Checks

• Logic-probe U15 pin 4 (IRQ) for pulses.

Fix

• Replace U29, U35 or U36 as found.

Systems

• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Last resort on a locked CPU: replace IC11 and IC12
• Display chips on the CPU board fail after swapping displays
• Scores show only a single 0 and the credit display is blank: failed 7404 at IC31
• Missing digit on one or more score displays
• Missing segment on one or more score displays
• Displays never show the digit 1
• Damage after the black and white plugs were swapped
• Blanking circuit parts: C32/C84, IC7, Q5/Q1 and IC23

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)