M.B.
Wow! We also have a Sharp Carousel that still works...... we don't use it, though, since we now have a built-in over the stove that works great, but we have that as a back-up (or in case one of the kids needs a microwave).
We got ours in 1982 as a wedding present from my in-laws....... All we've had to do is replace the mica "screen" over the microwave generator... that has gotten pretty nasty a few times.
But... unfortunately, it would cost more to have a professional clean/repair it than a new one would cost...... hubby says you should be able to pull the knob straight off and clean behind it..... most likely it is gunked up, but hopefully it isn't anything more than that.