Check the TC3 jumper plug on the CPU board

The TC3 test-connector plug passes -12 V to the spider chips and the coil drive circuit, so a missing or poor TC3 plug stops the board. TC1 and TC2 are factory test points and can be left alone.

Systems

• Gottlieb System 1

Related tips

• Aftermarket System 1 CPU boards
• Powering a System 1 CPU board on the bench
• The six Rockwell "spider" chips on the CPU board
• Using an LED to see a bench CPU boot
• CPU board clock check
• Testing CPU input buffers Z29 and Z27
• CPU address and data line check
• Swapped U4 or U5 spider still does not work

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)