another one who needs help with motor repair

I have an older 1/2 hp motor that I picked up somewhere. I think an Amish guy gave it to me. It came off something he purchased. The motor can be wired 110/220 and it was wired for 110v. The motor,when turned by hand,was a little tight but would spin freely after a little work. Under power, there was some sparking at the brushes. I tried to help the shaft turn but it would not turn under power. I could not budge it but it was fine once power was removed. I used fine sandpaper and sanded the oxidation off the armature and cleaned the brushes a bit. This did not help. The motor is no great loss, but it is an oldie and appears to be very well built. I would like to get it running as much to learn as anything. Could someone give me some steps to follow to diagnose the problem and learn at the same time. There is an earlier post about a motor repair and someone said to pitch it because motors burnout. Is there more useful advice like the sparks mean there is a poor ground or??? Thanks, Bob