You can find helpful repair information at the Pinwiki: http://www.pinwiki.com/wiki/index.php?title=Bally/Stern
Your manual (ipdb.org has one if you don't have a copy) has the switch matrix near the back in the schematics area. Looks like switch 56? is for the N. (not sure I'm reading it right). Try going into switch test mode and activating the N.
If the N switch works, maybe you have a dirty or bad bulb socket. Game should be off when cleaning the socket. You can get a cleaning tool at http://www.pbresource.com/tools.html (leaf adjuster tool is helpful too).