CPU board locked with good reset, clock and bus lines.

CPU locked: check the EPROM chip select

Cause: Failed chip-select logic, so IC17 is never enabled.

Checks

• With a logic probe check for pulses at IC17 pin 20 (chip select).
• System 3/4: the select logic is IC15 (7442), IC11 (7410) and IC25 (7402).
• System 6/7: it is IC15 (74LS139) and IC11 (74S10).

Fix

• Replace the failed select chip.

Systems

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

Related tips

• Good reset and clock but LEDs locked: check IC17 and its bus lines
• System 3/4 CPU: no clock at the 6800 or MC6875
• System 6/7 CPU: no clock at the 6802/6808
• Leon's test EPROM fitted but the LEDs do not start flashing
• Obsolete chips on System 3 to 7 CPU boards
• Check the CPU address and data bus for shorts
• CPU chips Leon's test code does not need
• Last resort on a locked CPU: replace IC11 and IC12

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