+5 V is low or missing.

No +5 V on a System 3-6 power supply

Cause: Usually a failed D7 or D8 diode (open drops the voltage, shorted blows a fuse); the heat-sinked X3 LM323 regulator rarely fails.

Checks

• Test D7 and D8 on diode test.

Fix

• Replace D7/D8 with 6A4 diodes; replace X3 (LM323) only if the diodes are good.

Systems

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

Related tips

• Power circuits in a System 3-7 game
• What the power supply board takes in and puts out
• Test the power supply with its outputs unplugged
• Swapping power supply boards between generations
• High-voltage supply rebuild parts, System 3 to 11C
• Bally solenoid driver 5 V reads only about 4 V: swap the drop-in regulator
• Parts of the 28 V solenoid supply
• Display supply low (under 90 V) after the resistors are changed

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