One bulb not working on Gottlieb System80 pinball machines can be anything from this list:
- bad bulb socket
- wire or ground braid to that particular bulb socket broken-off somewhere
- bad pin in connector header on the driver board for that lamp
- bad drive transistor for that bulb on the driver board (easier to find-out which transistor with the game manual)
Some level of electrical troubleshooting and knowledge would help to fix this.
Plus for safety...
Note: Never unplug or reconnect connectors on boards with the machine powered ON,
or poke under the playfield with a metal tool... or serious damage can occur on boards.
Good luck!
-Sylvain.