With the test EPROM fitted, the PIA outputs and indicator LED stay still.

Leon test EPROM does not run: check the 6808 and its pins

Cause: A bad 6808 CPU or socket, or a missing basic signal at U15.

Checks

• Swap in a 6808 known to work, then check the socket.
• With a meter: U15 pins 2, 3, 8, 35 and 40 should be positive, around 4 V.
• Pins 39 (clock), 4 (IRQ), 5 (VMA) and 37 (E) should read about 2 to 3 V.

Fix

• Repair the socket or the circuit feeding the missing signal.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Checking PIA outputs with the test EPROM running
• Testing U25 RAM with Leon's EPROM
• System 11 CPU board won't boot after cleaning: worn ROM socket contacts
• What each 6821 PIA on the CPU board controls
• Testing the 6821 PIAs in circuit with a meter
• Using the Williams System 11 test fixture EPROM
• Find out how the board died: failures cascade to the PIAs
• Other quick PIA checks: swap or touch test

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)