Checking the CPU clock signal
Steps
- With good RAM and EPROMs and +5 V present, power the board.
- Probe U15 pin 39 with a logic probe or oscilloscope: there should be a square wave.
- A steady high or low means a fault in the crystal clock section.
Systems
Related tips
- Checking ground and +5 V on the CPU board
- Which System 11 CPU boards fit which games
- Running the CPU board on the bench
- CPU RAM at U25
- CPU board test switches SW1 and SW2
- What each 6821 PIA on the CPU board controls
- Which System 11 CPU boards swap between generations
- How to tell a System 11C CPU from an 11B
Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)