+12 V at 3J6 pin 6 reads below 10 or above 14 VDC (pins 11-15 are ground).

Unregulated +12 V out of range

Cause: Most often the logic filter capacitor (C15 on System 3-6, C10 on System 7); then diodes D7/D8 (System 3-6) or bridge BR1 (System 7); rarely the transformer.

Checks

• Measure 3J6 pin 6 to ground with outputs unplugged.
• Test D7 and D8 on diode test: 0.4 to 0.6 V one way, nothing the other way.

Fix

• Replace the filter capacitor.
• Replace failed D7/D8 with 6A4 diodes, or replace BR1.

Systems

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

Related tips

• The unregulated 12 volts can read 10 to 14 V
• 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
• Spare diodes, capacitors and resistors for System 3-7
• Early Black Knights use the System 6 power supply
• Swapping power supply boards between generations

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