With game ROMs the LEDs (or System 7 "0") flash and go out normally and the SW1 memory test passes, yet displays and CPU lamps stay off with good fuses.

Board boots and passes SW1, but no displays or CPU lamps

Cause: The blanking circuit itself has failed.

Checks

• As a quick test, briefly jumper +5 V to interboard pin 37. If the game comes alive, the blanking circuit is bad.

Fix

• Repair the blanking circuit (see the component entries).

Systems

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

Related tips

• System 7 diagnostic display stays lit on a working game
• Is it the CPU board or the driver board? Boot the CPU alone
• CPU LEDs flash once, then come back on and stay on
• System 3 CPU board ROM layout
• System 7 CPU ROM sockets
• CPU LEDs come on and stay on with no flash
• Add two test LEDs to a System 7 CPU board
• Check the blanking and IRQ signals on a booting CPU

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