Firing coils from CPU buffers Z6 and Z7

Steps

• Game on; clip a lead to +5 V at the top of CPU C16.
• Touch Z6 pins 1-6 and 8-13 (pin 7 is ground, 14 is +5) and Z7 pins 1-4. Each input/output pair should fire its coil.
• Only one pin of a pair works: replace that 7417.
• Neither pin works: check the CPU to driver plug and the driver transistor.
• All fire but the coil still fails in play: U4 is bad and the CPU board must be replaced.

This check is done with the game on; keep the clip lead away from other parts and touch only the named point.

Systems

• Gottlieb System 1

Related tips

• A CPU controlled coil does not fire
• Many coils and lamps on, or no coils working
• Why System 1 coils lock on: the ground shift
• Lamp test can hold coils on for five seconds
• A shorted coil means replacing its whole driver path
• Coils or driver transistors run very hot
• Bench test of coil buffers Z6 and Z7
• Testing CPU input buffers Z29 and Z27

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