Score display data is missing or wrong while the CPU runs.

Display drive signals missing at J2

Cause: PIA U7 or the U13 buffer between U7 pins 9-13 and connector J2.

Checks

• With the test EPROM running, U7 pins 2-18 should read 1 to 2 V and pin 1 a steady 5 V.
• If a U7 pin 9-13 signal is fine but missing past U13, U13 is bad.

Fix

• Replace U13 (or U7 if its outputs are missing).

Systems

• Bally 6803

Related tips

• Interrupt circuit failure from the 2x12 VAC signal
• Check the 6803 clock at U1 pins 2 and 3
• CPU LED stops just short of the full count
• CPU never starts: check the reset line at U1 pin 6
• 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

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