On a Bally/Stern 6-digit score display one or more whole digits are dark, rather than single segments missing.

Bally 6-digit display, whole digit dark: reflow header, check the 100k resistors

Cause: A whole missing digit is usually a cracked solder joint on the display board's edge header or an open (often burnt) 100k digit resistor on the display board, not the decoder chip, which causes missing segments instead.

Checks

• Swap displays between positions to confirm the fault follows the display.
• Reflow the solder on the display board's .156 header pins.
• Check the 100k digit resistors on the display board (members name R3, R5, R7, R11, R13 and R15) for burning or an open reading.

Fix

• Replace any burnt or open 100k resistor (half-watt parts are recommended) and reflow the header.

Score displays run on high voltage (around 190 V): unplug the game and let it discharge before touching the display boards.

Systems

• Bally MPU AS-2518-35

Related tips

• Bally score display looks gassed with good voltages: replace its 4543 decoder chip
• Bally and Stern score display boards
• One digit missing on a score display
• Bally 7-digit display misbehaves wherever it is fitted: replace the decoder chip
• Making Bally and Stern 7-digit displays swap between makes
• Bally -35: same digit positions dark on every display: open digit drive resistors
• One segment never lights on a display
• No score displays light at all

Source: Pinball Repair help group!! (Facebook group)