Very early System 7 CPU board locks up.
Early System 7 CPU: unmarked 74125 above IC33 locks the board
Cause: Some early boards carry an unmarked 74125 chip above IC33 that Williams later dropped from the design; when it fails it can lock the CPU.
Checks
- Look for an extra 74125 above IC33.
Fix
- Cut the chip off the board; it is not needed.
Systems
Related tips
- Identifying a System 3 to 7 CPU board
- System 7 CPU board test points
- A System 7 CPU board can run any System 3 to 7 game
- Run a System 6/7 CPU board on +5 V only
- Which CPU boards can run which system firmware
- System 7 CPU ROM sockets
- System 7 CPU jumpers that are the same on every game
- Obsolete chips on System 3 to 7 CPU boards
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)