In display test the digits count in a scrambled order rather than 0 to 9.
Display test counts out of order (0,1,0,1,4,5...)
Cause: A BCD data line not read properly: MPU resistors R49-R53 (20K), cracked display header joints, or a bad MC14543 on one display (one bad display can affect the others).
Checks
- Swap displays; if the fault moves, the MPU is not at fault.
- Measure R49-R53, which should be within 5% of 20K.
- Plug displays in one at a time (game off between) and run the test to find a bad one.
Fix
- Replace off-value resistors, reflow headers, or replace the decoder.
Systems
Related tips
- One digit missing on a score display
- All digits on all displays show 2s or 7s
- Seven-digit and six-digit displays
- Score display digit parts by position
- One segment always lit on a display
- Making Bally and Stern 7-digit displays swap between makes
- Same digit missing or stuck on every display
- One segment never lights on a display
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)