The same segment is dark on the digits of one display, so numbers read wrong.
One segment never lights on a display
Cause: A failed MPS-A42 segment driver (Q13 for segment a through Q19 for g) or the 4543 decoder U1.
Checks
- Display test (two presses). Put a logic probe on the base of that segment's driver; if it pulses, replace the transistor.
- If it does not pulse, cut the base lead and probe the board side; pulsing there means the transistor was loading it (replace the transistor), no pulse means replace U1.
Fix
- Replace the MPS-A42 or the 4543, fitting the chip in a socket.
The solenoid driver / regulator board makes about 190 V DC for the score displays (around 230 V on some Stern boards); a shock from it hurts. Work with the game unplugged and let the filter capacitors discharge, and use one hand and insulated probes for live checks.
Systems
Related tips
- All displays show strange scores (one bad decoder)
- Display glass dim, digits missing or slow to light
- One digit missing on a score display
- Seven-digit and six-digit displays
- Score display digit parts by position
- Score displays dim
- One or two score displays completely blank
- Display test counts out of order (0,1,0,1,4,5...)
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)