Testing CPU input buffers Z29 and Z27
Steps
- Power the board on the bench. Clip a lead to ground.
- Z29 (7405): ground pin 1 and read pin 2, pin 5/6, pin 11/10, pin 9/8.
- Z27 (74H21): ground pin 1 and read 2, then 3/4, 5/6, 9/8, 11/10, 13/12.
- Each output should go to +5 V when its input is grounded; one that does not means a bad chip.
Systems
Related tips
- Aftermarket System 1 CPU boards
- Bench test of coil buffers Z6 and Z7
- Check the TC3 jumper plug on the CPU board
- Powering a System 1 CPU board on the bench
- Ni-Wumpf board coil buffer chips
- The six Rockwell "spider" chips on the CPU board
- Using an LED to see a bench CPU boot
- CPU board clock check
Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)