Display voltage is provided by the power supply.
The 42 and 60V test points on that supply are labeled.
Measure DC voltage relative to game ground.
If you connected or disconnected a display with power on, it is possible that the MPU display drivers (7448) were damaged.
Gottlieb is (AFAIK) the only system that can be damaged in this manner.
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Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info