The game still resets or locks up after the coil diodes are good.

Resets continue after diode repair: power and connector checks

Cause: A switch matrix fault, low or noisy +5 V from the power supply, low wall voltage, or cracked header joints on backbox connectors.

Checks

• In switch test, press each playfield switch; one switch must not show as two or more.
• Measure +5 V on the CPU board in attract mode: 4.9 to 5.2 V DC (a reading in range does not clear the supply).
• Measure the wall outlet: 115 to 125 V AC; below 115 V resets can happen.
• Inspect power supply fuse holders F5 and F6 for tarnish or cracks.

Fix

• Replace the +5 V filter capacitor C10 (18,000 mfd 20 V); every game is now past its life.
• Replace C8 (47 mfd 50 V) on the power supply.
• If still low or resetting: replace bridge BR1 (35 A 200 V, lug type), then Darlington Q5 (2N6057; fit a 2N6059), then regulator IC1 (MC1723C).
• Resolder the header pins on backbox boards, starting with power supply 3J6 (power out) and CPU 1J17 (power in).

Line voltage is present in the cabinet; unplug the game before working near the transformer, line filter or wall wiring.

Systems

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

Related tips

• +5 V fuse blows the instant the game is switched on
• No +5 V on a System 7 power supply
• No or low +5 V from the power supply
• Sound aux supply keeps blowing its 2N6059: shorted capacitor on the 30 V rail
• Choosing replacement bridge rectifiers
• Lockups and odd behaviour from a heat-dried C1
• Game resets when the flippers are used: recap supply and renew MPU power connector
• Random resets, days or minutes apart: burnt power supply AC input connector

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