Small Engines (Lawn Mowers, etc.): weed whacker repair?, permanent magnet motor, rf emissions

Question
Dear Michael,

My 1/2 horsepower weed whacker started smoking.  I took it apart and discovered 4 diodes  (equivlanet to 1N4007's) and two wimpy capacitors (ceramic disk, 1000 pF) configured as a bridge recitifier feeding into a "permanent magnet motor" (says so right on the label).   The diodes seem to check out when tested on the board--at least they give different numbers (~500 in one direction, ~1200 in the other) when tested on the diode setting of a DMM. There are two brushes which have LOTS of life in them, and they move freely.  But when I start the motor, BIG, bright white sparks appear where the brushes contact the armature.  I could see sparks occurring due to poor contact between brushes and armature, but I think the contact is solid.  Any ideas?
Thanks,
Whackless in Fresno,
Bruce Ratcliffe

Answer
Dear Bruce,

Some questions persist about the exact type of motor on your wacker.  I'll assume it is plugged into 120 VAC.  Diodes are in question, remove them from circuit and test again. Test meter should indicate about 0.5 volts in the 'forward' direction and over scale in the 'reverse' direction. If all energy goes thru the the diodes they are tightly sized. The caps are most likely there to reduce RF emissions and extend brush life and are not vital to motor function.  So if the wiring goes like this: 120VAC, switch, good diodes, brushes, armature and you still get sparks it's probably a shorted wrap in the armature.  If so RIP.

Good luck.
Michael