well, since it's pretty clear that the better half does not want to see that old vacuum ever again, I did an autopsy on it. Unfortunately, the motor housing was built in such a way that it was not openable without cutting and prying. IOW, not serviceable. And sure enough one of the brushes had the lead broken. Must have been intermittent contact. (Although not sure how that could have either led to an increase in rpm or noise or big sparks when running.) Not that surprisingly, the bushes pads themselves had plenty of meat left even after 20 years, cuz admittedly, we're not obsessed with vacuuming in my house... .
Had fun with the cutting wheel, though
and got some wheels, screws, and a long extension cord out of the exercise...
ah, clever. That would be true of synchronous motors IIRC?
I could see 2 places where there were sparks when the motor was running so seems like both brushes were still going most of the time yet it seemed to run much faster than normal.
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