How the CPU keeps lamp phases in step

Zero-crossing detectors on the CPU board watch the phase A and phase B lamp lines. The phase B detector raises an interrupt that tells the 6803 to switch to the other set of SCRs; because all the AC windings cross zero together, phases A and C are switched together and then B and D. A missing lamp phase therefore upsets the CPU, not just the lamps.

Systems

• Bally 6803

Related tips

• Lamp driver SCRs on the CPU board
• Some feature lamps out after the backbox was lowered
• Lamp driver SCR blew apart with a crack
• Expected LED-strip results on lamp headers J10-J13
• An LED stays dark on a lamp header during the test
• What is on the 6803 CPU board
• CPU board test points
• Interrupt circuit failure from the 2x12 VAC signal

Source: Repairing Bally 6803 Pinball Games from 1985 to 1989, Clay Harrell (pinrepair.com)