The board never settles: U15 pin 40 keeps going low and high, or never goes high.

CPU keeps resetting: reset line at U15 pin 40

Cause: A fault in the reset section, or a bad 6821 PIA (U10 or any of the others) holding the reset line.

Checks

• With a logic probe on U15 pin 40, it should go low briefly at power-on, then high and stay high.
• On the bench, ground pin 40 momentarily; if the board then boots, the power-on reset section is at fault.

Fix

• Repair the POR parts (C30, R55, R56, Q34/Q36/Q38, ZR1, ZR2), or replace the PIA holding reset.

Systems

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

Related tips

• Won't boot after restoration, new EPROMs fitted: try the original ROMs
• Game will not boot consistently: power-on reset (POR) circuit
• Four beeps at power-up, GI only and no display: game has no batteries
• Won't boot even with a new CPU board and new chips: the new ROMs were bad
• Resets continue after diode repair: power and connector checks
• Displays flash bright and speakers noisy, game does nothing
• Spinner works in switch test but scores nothing in play: factory reset
• MPU won't boot even though a test ROM passes every PIA and RAM: bad game ROMs

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