I've used Elmer's wood glue and water with tissue paper to fix speaker cones. For the plastic knobs I can only think of 3d printing a replacement. For the knurls that got polished off from use the only thing I can think of is using a brass shim cut and rolled into a tube then glued into place in the knob. You may have to drill the hole in the knob a little bigger to accommodate the increase in shaft size with the custom made brass shim tube.
I've used Elmer's wood glue and water with tissue paper to fix speaker cones.
For the plastic knobs I can only think of 3d printing a replacement. For the knurls that got polished off from use the only thing I can think of is using a brass shim cut and rolled into a tube then glued into place in the knob. You may have to drill the hole in the knob a little bigger to accommodate the increase in shaft size with the custom made brass shim tube.
Thanks for the ideas. Well 3D printing is out since I don't have a 3D printer but I do like your idea about an insert.