My sander has been running fine for years but
it slowly started smelling like it was running
too hot--the smell of hot insulation. I think
its the smell of the armeture getting too hot.
Could short brushes be the cause of this, meaning it gets intermittent contact from one or both of them. They are about 3/8" of an inch long now. The brush retainer screw cover can't be screwed in anymore.
Idea #2: Its not the brushes at all, but excessive resistance in the bearings and thus the need to be replaced. Without the motor belt installed, the motor runs fine and does not emit an odor. There are still quite few sparks coming from the brushes though.
Put the motor belt on and do some sanding, (put a load on it) the odor returns. It smells like it will burn itself up.