A bad SR14 can short switch rows on System 11A and later
Each row enters a two-input 4011 NAND gate. The second input was meant for the opto header 1J9; once 1J9 was dropped, that input is held high by the 3.3k network SR14. If SR14 fails the input falls low and a row reads as shorted.
Systems
Related tips
- Testing the switch rows with a clip lead
- System 11 switch matrix: connectors and chips
- Connector 1J10 switch row pins
- Row or column error: CPU board or playfield?
- A whole switch row or column is dead
- How the Williams switch matrix is scanned
- Testing the switch columns with a clip lead
- Testing the switch matrix with a logic probe
Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell