+5 V measures below about 4.9 V DC; the game may reset at random or show PIA or IRQ errors.

Low +5 V (below 4.9 V) from the power supply

Cause: Capacitor C8 (47 mfd 50 V) on the power supply has gone open, or the main +5/+12 V filter C10 (18,000 mfd 20 V) is worn out.

Checks

• With the game on, set the meter to AC volts and measure across C10's leads; more than 0.3 V AC means replace it.

Fix

• Replace C8 (a 100 mfd 50 V part also works).
• Replace C10.

The power supply board also makes about +100 and -100 V DC for the score displays; keep hands and probes clear of that section while the game is on.

Systems

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

Related tips

• What the System 11 power supply board feeds
• Display supply low (under 90 V) after the resistors are changed
• Rebuilding the +100 V section of the power supply
• Rebuilding the -100 V section of the power supply
• Measure a rebuilt supply before reconnecting the displays
• GI connectors left the power supply after Cyclone
• System 11 power supply versions D-8345 and D-12246
• Routine high-voltage changes on every System 11 power supply

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