Hard to turn speed dial

Hi!  I purchased a used, newer model Ankarsrum from a woman who went on hospice, and the speed knob is very difficult to turn.  I pulled it off to see if dough had dried behind there and got it stuck, but that isn’t it.  It also smells when it’s running, like a new belt is hot or something.  I disregarded the belt issue because I also bought a refurbished Ank the week before and it smelled the same while r

is there anything I can do to get the knob to turn smoothly, or is it something else I don’t understand? I bought two so folks could try out the different mixers in my bread class, and decide which one they might want to use, but I don’t want them using one that isn’t working right - that’s no good for anyone. The other is now my personal mixer

Thanks for any help or advice.
Melanie Carr