The small 4-digit credit display never lights. Credit display is enabled in the settings, the game is not on free play, and a display that works in another game stays dark too.

Gottlieb System 80: credit display stays dark: broken filament supply wire

Cause: No AC filament ('offset') voltage reaches the display: a wire from the transformer secondary to the display was broken.

Checks

• Put the game in display test and see whether the credit display lights there.
• At the display connector, carefully measure the high-voltage DC supply, the roughly 3 V AC filament supply and the 5 V logic supply against the schematic.
• If the filament voltage is missing, follow its two wires back from the display towards the transformer, reseating any connector on the way and checking each wire for a break.

Fix

• Repair or replace the broken wire between the transformer and the display (a new wire run straight from the transformer connection to the display connector cured it here).

The display connector carries high voltage and the transformer primary is at line voltage: measure carefully, and switch off and unplug the game before touching wiring.

Systems

• Gottlieb System 80
• Gottlieb System 80A

Related tips

• Gottlieb System 1 and System 80 power harnesses look alike: check before fitting
• Status display is off but the score displays work
• A display segment or digit is always on or always off
• Haunted House and Black Hole fuse panel
• How the System 80 fluorescent displays light
• Gottlieb System 80: bottom segment missing on one display that follows it: replace Z1
• System 80 schematics' 'offset' voltages are DC bias for the display filaments
• Displays never show the digit 1

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