+12 V measures below about 10.5 V DC.

Low +12 V (below 10.5 V)

Cause: The 2N6057 power transistor on the power supply board, or its socket, which can corrode or lose contact.

Checks

• Inspect and reseat the 2N6057 in its socket.

Fix

• Clean or replace the socket, or replace the transistor.

Systems

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

Related tips

• Power supply gives no +5 V
• Display supply low (under 90 V) after the resistors are changed
• Measure a rebuilt supply before reconnecting the displays
• Game boots only sometimes, 5 V drops out: cracked diode D6 on the power supply
• System 11 backbox boards and supplies
• What the System 11 power supply board feeds
• System 11 power supply versions D-8345 and D-12246
• +5 V fuse blows the instant the game is switched on

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