Board boots with game ROMs and passes tests, but blanking stays low.

Blanking stays low: timer, C31 and Q5

Cause: A failed timer chip at IC23 (556 on System 3-6, 555 on System 7), capacitor C31 (1 uF 25 V), or transistor Q5 (2N4403).

Checks

• Test Q5 with a meter on diode test.
• On System 6/7, check the IC25 (4020) outputs with a logic probe; without the 1 ms interrupt the IC23 timer cannot be retriggered.
• On System 3/4 the 556 also makes the IRQ, so check C31 and Q5 first.

Fix

• Replace C31, Q5 and the timer chip as needed.

Systems

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

Related tips

• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Blanking circuit parts: C32/C84, IC7, Q5/Q1 and IC23
• Blanking LED wrong, or all solenoids pulse and blow a fuse at power-on
• Displays flash bright and speakers noisy, game does nothing
• MPU boots only some of the time
• Five flashes only: U12 timer or open R113
• Install the Andre Boot test ROM
• Andre Boot test ROM flash codes

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