To accurately troubleshoot problems with the machine, you'll need to reassemble the machine so it is ready to start a game. When you power on a Gottlieb machine of this era, it will go into tilt mode and the lights would be on in the backbox and the coin door. Playfield lights would be off. With credits on the machine and pressing the credit/start button, the playfield lights would turn on and the score reels would start to reset. What you're describing is a "no start" issue when pressing the credit/start button. Do you have a schematic for the machine?

It could have been a engineering change during the production of the machine. I had a Jet Spin many year ago, and when the machine was powered on, it woulld be in tilt mode with lights off on the playfield. Also, there could have been wiring modification by the operator. Hopefully, the OP will give us more details about the issue...