My Craftsman belt sander is about 30 years old, but is not used very often. I had a problem with it today. When I squeezed the trigger, it would just hum. I got it going again by manually rotating the drum (while off), but the problem repeated itself several times. I also noticed a lot of sparks and a kind of clicking noise inside. I don't know if there were more sparks than usual, since I never paid much attention to it before. The brushes look like they still have a lot of "meat" on them. Is there anything I can do to fix it myself? If not, is it worth having repaired?