It could be a variety of things, but I would check for the typical voltages at the power supply rectifier and if those appear to be correct, I strongly suspect the 5v regulator on the solenoid driver board. You should be able to find a set of schematics online (if not for Night Rider perhaps another machine of that era. Most of the Bally machines are pretty similar). These kinds of problems are generally pretty easy to find and correct.

Let us know how it goes or if you have any additional questions.

Good luck!

Jeff

One other thing I forgot to mention, and this is pretty basic and perhaps you have already done this, make sure all of the fuses are good

Jeff