Check the CPU address and data bus for shorts

Steps

• With a logic probe, look for pulsing on IC1 A0-A7 (pins 9-16) and D0-D7 (pins 33-26). One line stuck high or low stops the program.
• Power off. With the meter on continuity, check A0 against every other address line (IC1 pins 9-20 and 22-25) and data line (pins 33-26); there must be no shorts. Repeat for each line.
• Look for solder bridges, especially where sockets were replaced.
• Check each address line runs from IC14 to IC17 to IC20 (and IC26 on System 7) and back to the buffer chips and IC1.

Systems

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

Related tips

• Data buffer check: 8T28s at IC9/IC10 (System 3 to 6)
• Good reset and clock but LEDs locked: check IC17 and its bus lines
• Bypass the 8T28 data buffers IC9/IC10 on a pre-System 6A CPU
• CPU locked: check the EPROM chip select
• Replacing the 6802 CPU with a 6808
• Identifying a System 3 to 7 CPU board
• What changed on the System 6A CPU board
• System 6 CPU board test points

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