If the motor buzzes and armature moves but won't spin if you manually assist it, it may be a bad armature with one of the three windings shorted or open. This could have happened if someone tried to run it and kept applying power when it was bound up from lack of lube or whatever. Or if the motor overheated from running too long and too hard. Armatures are easily tested with a VOM meter. Check resistance and continuity between each of the three commutator faces. Also check the field winding for a short.
To test-run the motor without the reverse unit, make sure you have a complete circuit through the motor with the field and armature in series.
