The blanking LED never comes on, or blanking is high from the first instant of power-on, briefly firing every solenoid and the flipper relay and often blowing the solenoid fuse.

Blanking LED wrong, or all solenoids pulse and blow a fuse at power-on

Cause: A fault in the blanking circuit, built around a 555 timer and Q50 (2N4403), or a shorted logic chip on the blanking line.

Checks

• Blanking should be low for a moment at power-on, then go high when the CPU has booted (LED on).
• Power the board with +5 V and ground only (no +12 V): reset stays low, so blanking should stay low. If it is high, the blanking circuit is faulty.
• Probe the blanking signal at U43 pin 3.

Fix

• Replace the 555 timer and Q50 2N4403 as needed.

Systems

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

Related tips

• Displays flash bright and speakers noisy, game does nothing
• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Blanking stays low: timer, C31 and Q5
• Fliptronics flipper coil wired wrongly
• CPU flipper enable relay RY1
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• STNG: under-playfield diverter coils lock on during play

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