CPU seems to run but solenoids, CPU lamps and displays stay off.

Check the blanking and IRQ signals on a booting CPU

Cause: Blanking (interboard pin 37) never goes high, often because the IRQ timing it depends on is missing.

Checks

• At power-on, interboard pin 37 should rise from low to about 4-5 V. With Leon's EPROM it toggles once a second instead.
• Check the IRQ with a logic probe at IC1 pin 4 (or TP5 on System 6/7). It comes from IC25 (4020) on System 6/7, or IC23 (556) on System 3/4.
• Check the IC1 socket, especially if it is still a Scanbe socket.

Fix

• Replace the failed IRQ or blanking part, or the socket.

Systems

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

Related tips

• Board boots and passes SW1, but no displays or CPU lamps
• System 7 diagnostic display stays lit on a working game
• A System 7 CPU board can run any System 3 to 7 game
• Run a System 6/7 CPU board on +5 V only
• Which CPU boards can run which system firmware
• Why the CPU LEDs stay on when the program does not run
• Run the System 3-6 built-in CPU test (SW1)
• Is it the CPU board or the driver board? Boot the CPU alone

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