Mixer was on stir. Suddenly sped up, settled back down for a second, then sped up again. After that it was essentially only 'off' or 'fast'.

I order replacement speed control plate and governor, and replaced both.

With the new parts, it tends to pulse on stir, and won't go below - say - 80-100 rpm. If I continue to tighten the speed control screws, it eventually just stops, but never gets down to 60 rpm.

I tried letting it run on a higher speed, and smelled something.

Took the new controller plate off, and I can see that the governor has been boring into it, removing material from both the soft white and the hard black plastic. Stumped

Phase control board was it. Back in business!