Blanking low with game ROMs fitted while Leon's tests pass.

Blanking circuit parts: C32/C84, IC7, Q5/Q1 and IC23

Cause: Failed part in the blanking one-shot. A shorted Q5 (Q1 on System 7) can also damage capacitor C32 (C84).

Checks

• With a scope or logic probe, check the signal entering and leaving C32 (System 3-6) or C84 (System 7), 0.047 uF 50 V.
• No input signal: suspect IC7 (7404).
• Test Q5 (Q1 on System 7), a 2N4403, on diode test.
• If those are good, suspect timer IC23 (556 on System 3-6, 555 on System 7).

Fix

• Replace the failed part; replace C32/C84 if the transistor was shorted.

Systems

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

Related tips

• Blanking stays low: timer, C31 and Q5
• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Damage after the black and white plugs were swapped
• Which transistor drives which WPC solenoid and lamp circuits
• Missing digit on one or more score displays
• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• Scores show only a single 0 and the credit display is blank: failed 7404 at IC31

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