How can I fix my very old 3M 732 Photocopier?

It is a 3M 732 Photocopier pretty old and basic, it takes only a4 paper, toner is pored into a loose trough which can be removed, acces to its mechanical innards is through lifting the top half up and backwards on a hinge, paper is fed via a removable plastic tray on the side with a trigger/lever type mechanism which feeds sheets one at a time & automatically starts the copy process, rather than the actual light moving back and forth the actual top cover and glass plate move sideways mechanically. It was given to me by a freind who found it whilst clearing out a old building and I have no other information other than what I have discovered through messing with it.
Everything About the copier seems to be working fine, the problem being that paper either comes out completely blank or with the occasional black splodge. The paper feeds fine, the light works. I dont know what to do and cant afford an engineer, any help would be greatly appreciated.