Accidentally, I swear! I have a U4189 that is a daily driver downstairs. I usually vacuum every other day because of the dog. I was using the machine today and it was noisy, a little more than normal, but I was suspecting brushroll bearings or a belt. Then it happened, the motor got slower and slower along with the electrical smell. I shut it down and its currently lying in state at my workbench awaiting an autopsy. I suspect carbons or armature. I have many spare one speed motors, so I don't plan on repairing it. Just swapping motors. The machine was a thrift store find. No big loss, but I never pass up convertibles, metal or plastic. Parts is parts.
This is the first time I had one die on my watch. My first Dial A Matic to bite the dust was last spring. Oh well, its bound to happen with all the machines I have. Good thing it wasn't a true vintage machine that parts are harder to find.
A green one, model U4381. I swapped the power surge switch for a regular two speed switch, then used the cleaner for work... it lasted about 2 months and went in a blaze of sparks... the motor actually threw bars off the commutator!