With the test EPROM blinking fast, a pin on U7 or U8 (6821) is not switching between 0 and 5 V.

PIA outputs not toggling under the test EPROM

Cause: A bad PIA, a short on that output line, or (if every output is dead) a missing select signal.

Checks

• Output pins 2-17 and 19 should all pulse. Bend a silent pin out of the socket: still silent means a bad PIA, pulsing means a short on the board.
• If all outputs are dead, check select pins 21-25, 35 and 36 for 2 to 4 V and the gates at U10, U11 and U14 that drive them; also check the bus pins.

Fix

• Replace the PIA, or remove parts on the shorted line one at a time to find the short.

Systems

• Bally 6803

Related tips

• Test EPROM fitted but the LED does not blink
• Unplug all but J1 when using the test EPROM in the game
• Bench-test a CPU board with the 6803 test EPROM
• CPU jumpers JW1-JW6 for game EPROM size
• Check the 6803 clock at U1 pins 2 and 3
• Find a bad 6803 or a shorted bus line
• Electronic spares for 6803 board repair
• Cybernaut can have a -35, 6802 or 6803 boardset

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