Dead game: CPU LEDs (or the System 7 "0") stay on.

Is it the CPU board or the driver board? Boot the CPU alone

Cause: Either board can lock the system, since the CPU board will not run its game code without the driver board.

Checks

• Remove the driver board completely and power the CPU board with the game ROMs fitted.
• System 3 to 6: if both LEDs flash on, go off, then come on and stay on, the CPU is running and is waiting for the driver board.
• System 7: the same pattern shows as a "0" that blinks, goes off and then stays on.
• If the LEDs (or "0") light solidly from the start with no blink at all, the CPU board itself has a fault.
• If the CPU passes this but locks up again with the driver board connected, the fault is on the driver board.

Fix

• Repair the board indicated, using Leon's test EPROM for detailed checks.

Systems

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

Related tips

• Add two test LEDs to a System 7 CPU board
• System 7 CPU LEDs are swapped versus System 6
• Late System 7 CPU boards lack LEDs and DIP switches
• System 3/4 CPU: no clock at the 6800 or MC6875
• System 6/7 CPU: no clock at the 6802/6808
• Identifying a System 3 to 7 CPU board
• What changed on the System 6A CPU board
• System 6 CPU board test points

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