If you know what to look for and have a good DVM then yeah its repairable as a DIY project. I was posting to highlight what can cause the hum.
I'm sure I could repair mine for the hum however I would probably replace before repair since I would still have an older switch with old contacts. Even if you could replace the contacts you would never find replacements.
It could also be the contacts are pitted and need touching up as posted previously that could be an easy fix. If you don't have a burnishing tool use 800 grit sand paper. This fix could last a year or two.
Another easy check make sure the unit is fastened down tight and the whole unit is not vibrating.
If you take it to a shop for repair it will cost about the same as a new one $75 to $90 new. I would say if the two easy fixes don't work replace and be done with it.