At power-on the System 3-6 CPU LEDs light steadily, or the System 7 digit shows a steady "0", with no flash.

CPU LEDs come on and stay on with no flash

Cause: The CPU is not booting, is stuck in reset, or locks up at once. Usually bad Scanbe sockets or a worn interboard connector.

Checks

• Check reset and clock (see those entries).
• Run Leon's test EPROM with the driver board removed.

Fix

• Replace the sockets or interconnector found faulty, then any failed chip.

Systems

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

Related tips

• CPU LEDs (or System 7 digit) never light at power-up
• Add two test LEDs to a System 7 CPU board
• How a good System 3 to 7 CPU behaves at power-on
• Power a System 3 to 7 CPU board on the bench with a PC power supply
• System 7 CPU LEDs are swapped versus System 6
• Late System 7 CPU boards lack LEDs and DIP switches
• Is it the CPU board or the driver board? Boot the CPU alone
• System 7 CPU digit display dead: check IC34 7447

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