The LEDs flash once and go out as normal, but no score displays light.

CPU LEDs flash and go out but the score displays stay dark

Cause: The program is running, but there is a ROM fault, no display high voltage, a blanking problem, or a fault on the master display driver board or the CPU display circuit.

Checks

• Power off and refit fuse F3 (lamp matrix).
• Power on, then off and on quickly. If the feature lamps start the attract mode but displays stay off, the fault is in the display driver board or the CPU display circuit.
• On System 3 (no attract mode beyond the flashing high score lamp), add credits and try to start a game instead.
• If the lamps do not come on, or a System 3 game will not start, go on to Leon's test EPROM.

Fix

• Repair the display circuit, display high voltage or blanking fault found.

The score display supply (fuse F1) is high voltage; keep hands clear when testing with power on.

Systems

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

Related tips

• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)
• Whole digits missing
• Seven-digit scoring on six-digit displays for green-ROM games
• Displays have high voltage but stay blank
• Digits burning or faint on players 1 and 3 (or 2 and 4)
• Ball and credit displays show only even numbers after header work
• CPU LEDs (or System 7 digit) never light at power-up
• CPU LEDs come on and stay on with no flash

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