The CPU boots only sometimes or not at all: a 7, 8 or blank on the System 11 LED digit, or 7 or 8 flashes on 11A-11C boards.

Game will not boot consistently: power-on reset (POR) circuit

Cause: A fault in the power-on reset circuit, often caused by out-of-range +5 V from a bad bridge or filter capacitor, or by a failed POR part on the CPU board.

Checks

• Take the CPU board to the bench and run it from an external supply; if it still boots erratically, the fault is on the CPU board.
• After a failed boot, momentarily ground /RESET (pin 40 of U15). If the board then boots correctly, the POR section is at fault.

Fix

• Check C30 (22 mfd 10 V): clip a known good 22 mfd capacitor in parallel, observing polarity; if boots become reliable, replace C30.
• Then check R55 (4.7k 1/4 W), R56 (10 ohm 1/4 W) and transistors Q34, Q36, Q38 (2N4401).
• The POR waits for +5 V and +12 V as judged by zeners ZR1 (1N5996A, 6.8 V) and ZR2 (1N5990, 3.9 V); check them too.

Systems

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

Related tips

• CPU and driver board transistors by position
• Quick check of all TIP122/102 drivers before first power-on
• No flippers at all: flipper relay K1 not pulling in
• A coil or flasher comes on and stays on at power-up
• CPU flipper enable relay RY1
• Solenoid driver transistor positions on the CPU and PPB boards
• One or a few coils lock on at power-up
• Flash lamp stuck on and melting plastics on L/R reversed games

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