I'm 1980 too....anyway, Ed's advice is always sound! For the knotmeter, I assume you have tried the basics - removing it, spinning by hand, seeing if anything registers?
I had a similar problem. I had problems with my electrics, which I fixed. uh...you found the second on/off switch? My boat had two, one on the main panel, one an the companionway. Caught me out at first, especially as there was a wiring problem between the two (loose connection came apart when tugged). The instruments then displayed, but nothing past zero.
My knotmeter issue was caused by a little sea-critter setting up home there. Gently relocated. My transducer, however,was dead. I was very tempted to replace the lot with a new set (Raymarine ST-60), but in addition to the cost (~$1500) that would have required a lot of work, as they are slightly smaller than the Datamarine so the current holes would have been too big. In the end I found DMI Marine (
http://www.dmimarine.com/), set up by a guy who used to work for Datamarine, and bought up lots of parts/spares etc. Specialises in repairing old Datamarine kit. He can repair/recondition our old instruments, and I believe also has his own range which are exactly the same dimensions. He ain't cheap - maybe $150 to recondition my depth display, plus the cost of a new transducer - but it was cheaper than a new set, and works fine (for now).
Incidentally, I set up my transducer in an oil bath, to work through the hull. No haulout required - my old trandsucer, now dead, remains glued in its through-hull beside it. Works fine, and should be a snap to replace. Also, I have been reading about solid-state knotmeters...they are expensive, and I know nothing about how they work; just seemed kinda cool as I am eternally cleaning "stuff" out of my paddle wheels!
(Disclaimer: Not associated with DMI Marine. Just a happy customer)