I don't think I have the service ("maintenance") manual for the KNS80.
Given the age of this unit, a PDF would be a scanned-in one and probably rare, but a lot of "old" avionics people might have a paper copy.
They often come up on U.S. Ebay (ebay.com not ebay.co.uk). I recently bought a service manual for a pretty expensive instrument, 1980 dated, for $200. I don't need it now but one day it will be worth it - in the right hands.
I'd imagine a KNS80 would be a pig to repair, with all the analog/RF circuitry. All one can do is check for obvious mechanical damage, loose/dried out electrolytic caps, bad solder joints. In the olde dayz when people used to repair TVs, blindly replacing all the electrolytic caps would fix the fault a lot of the time.