Dead CPU board with good +5 V.

CPU never starts: check the reset line at IC1 pin 40

Cause: The reset circuit is holding the CPU in reset.

Checks

• Check +5 V at IC1 pin 8 (and pin 35 on System 6/7), slightly less at pin 2, and ground at pins 1 and 21.
• With a meter on IC1 pin 40 (/RESET), power on: it should rise from 0 to at least 4 V after about 50 ms.
• If it does not, briefly connect +5 V to IC1 pin 40. If the CPU then runs, the reset circuit is the fault.

Fix

• System 6/7: test transistors Q1-Q4 and Q6-Q9 and diodes D19, ZR1 and ZR2.
• System 3/4: test transistors Q1-Q4 and Q6 and diodes D17, D18 and ZR1.
• Work through the schematic testing each part until the bad one is found.

Systems

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

Related tips

• System 3 CPU board reset circuit modification
• World Cup will not boot after a CPU board swap
• Intermittent lockups with both CPU LEDs on
• Reset or lock-up: which points where
• Usual causes of lock-ups and resets
• "Adjust Failure" at power-on
• Random lockups from the 12 V logic pin
• Game keeps resetting: replace the 5101 RAM (an NVRAM works)

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