Motor
In my experience, (featherweights only), I have seen my share of bad armatures. The stator of the motor usually is not the problem, unless it shorted on the metal motor housing and then the motor obviously won't run at all. What I have heard is that the armatures are most prone to overheating if the machine has been overloaded or tight from lack of maintenance. Heat is what kills the armatures and some of these vintage machines have had a hard, overloaded life. Below is a procedure that can help rule out the electrical components inside the motor.
On brush-type motors, inspect the brushes to make sure they are still serviceable. Generally, brush length should be at least twice their width or replace them. Also make sure the brushes are free in their tubes and the springs are not broken. Check for shorts or open armature windings by touching one test lead to one commutator (the grooved copper section that contacts the brushes) segment and the other test lead to another segment. Compare each segment with every other segment and look for either open (infinity ohms) or shorted (zero-ohms) armature windings. Either will be cause to discard or rebuild the motor.