The game will not start, or resets and locks up at random.

+5 V logic low or noisy

Cause: +5 V outside 4.9 to 5.2 VDC or not smooth, from bad connectors, a failed regulator circuit or a CPU board fault pulling it down.

Checks

• Measure +5 V at 3J6 pins 7-10 (ground on pins 11-15); it should be 4.9 to 5.2 VDC.
• If it is good at the power supply but under 4.9 V at the CPU board, suspect the connectors or a CPU board load.

Fix

• First replace connectors 3J6 (power supply) and 1J2 (CPU board), then work on the regulator circuit.

Systems

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

Related tips

• Replace the +5/+12 V logic filter capacitor
• Upgrade logic rectifier diodes D7 and D8
• Judging the logic filter cap by AC ripple
• No +5 V on a System 7 power supply
• Display supply low (under 90 V) after the resistors are changed
• Cross the legs when MJE15030/15031 replace SDS201/SDS202
• High-voltage supply rebuild parts, System 3 to 11C
• Fitting the high-voltage kit

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