The seven-segment diagnostic display on a System 7 CPU board shows nothing, so the board looks dead.

System 7 CPU digit display dead: check IC34 7447

Cause: The 7447 segment driver at IC34 fails often, or the display itself has failed.

Checks

• With a logic probe, check for pulsing at CPU IC1 pin 15. Activity there means the CPU is running.

Fix

• Replace IC34 (7447) or the seven-segment display.
• Fitting two test LEDs makes later diagnosis independent of the display.

Systems

• Williams System 7

Related tips

• CPU LEDs come on and stay on with no flash
• Is it the CPU board or the driver board? Boot the CPU alone
• System 6/7 CPU: no clock at the 6802/6808
• Identifying a System 3 to 7 CPU board
• System 7 CPU board test points
• 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

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