The board fails the lamp phase or interrupt tests even though the CPU runs.

Interrupt circuit failure from the 2x12 VAC signal

Cause: The circuit that turns the transformer's 2x12 VAC into interrupts has failed, usually U12.

Checks

• Measure about 2 V at U12 pins 2 and 6.
• Check the related diodes and resistors on schematic sheet 2.

Fix

• Replace U12 if the voltage is missing.

Systems

• Bally 6803

Related tips

• Check the 6803 clock at U1 pins 2 and 3
• Find a bad 6803 or a shorted bus line
• What is on the 6803 CPU board
• CPU board test points
• Factory CPU jumpers by game
• Bench-test a CPU board with the 6803 test EPROM
• Power a 6803 CPU board on the bench with a PC supply
• CPU board landmarks: battery, LED, relay and ROMs

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