My Sears circular saw threw some sparks at the brushes and stopped abruptly.
It had been working normally, no symptoms of trouble. The brushes and
commutator look fine, no obvious burn marks that I can see. There is
current to the brushes. When you press the trigger nothing happens (no
groaning or whatever), but if you hold down the trigger and give the motor a
spin by hand, it gives a repeat of the sparking, and kind of tries to keep
running.

Any ideas as to what might be wrong? If it is a shorted armature, I might
be willing to pay for a new one, but getting it serviced would cost at least
$50 plus parts, and this makes no sense. The saw is 20 years old and has
had some fairly rough use.