Thanks for that. It's definitely the shaft that's loose, but as I recall the nut is on the inside, not the outside, isn't it? I'll have a look. I think I'll need to take off the top paneling.
I've got a fairly old MicroKorg which has developed wobbly volume and cut-off knobs. Has anyone been in the same situation and managed a successful home repair? If so, could anyone give me a few handy hints so I don't make a hash of it.