Checking ground and +5 V on the CPU board

Steps

• Game off, meter on ohms: TP1 (left of the batteries) to the backbox ground strap must read zero ohms.
• Game on, meter on DC volts, black probe on the ground strap: TP2 (right of U21) should read +5 V.
• Check +5 V at the chips: U15 (6808) pin 8; U26 (2764) and U27 (27256) pin 28; PIAs U10, U38, U41, U42, U51, U54 pin 20. Pin 1 is at the notch or the printed dot.
• On the power supply board: TP1 +5 V, TP2 ground, TP3 +12 V, TP4 -12 V.

The power supply board also makes about +100 and -100 V DC for the score displays; keep hands and probes clear of that section while the game is on.

Systems

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

Related tips

• Which System 11 CPU boards fit which games
• Checking the CPU clock signal
• CPU RAM at U25
• CPU board test switches SW1 and SW2
• What each 6821 PIA on the CPU board controls
• Which System 11 CPU boards swap between generations
• How to tell a System 11C CPU from an 11B
• U15 CPU and U25 RAM substitutes

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