CPU board locked with good reset and clock.
CPU locked: check the R/W and VMA lines
Cause: A bad gate in buffer IC8 or support logic blocking R/W or VMA.
Checks
- With Leon's code running, R/W should be high; it passes through IC8 pins 2 to 3.
- VMA leaves IC1 at pin 5 and passes through IC8 pins 14 to 13; IC1 pin 5 and IC8 pin 14 should be high. System 3/4 boards also route VMA through IC7, IC11 and IC12.
Fix
- Replace IC8 or the support chip that blocks the signal.
Systems
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
- Address buffer check: 8T97s at IC3, IC4 and IC8
- Replacing the 6802 CPU with a 6808
- Identifying a System 3 to 7 CPU board
- What changed on the System 6A CPU board
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)