So I traded a guy a motor that would trip the breaker for one that wouldn¡¯t. Since I am an electronics tech and I have overhauled a half dozen U90 motors I was confident in the swap. Noise suppression caps were good, brushes good, but what I found was one of the field wires had worn through right where it comes through the plates, that was the short. A piece of shrink tubing, cleaned and dressed the commutator, lubed it, and put it back together. Runs like a new motor.
So look where the wires pass through the field plates and make sure there is nothing rubbing there or on the commutator itself, I have seen both modes of failure.
You can check the caps (3) by using a multimeter each end to center and end to end, they should show open or a cap charge curve (slow rise in resistance to open).
The caps are only there to shunt the arc noise generated by the brushes to ground, (emi).
Pat
