Actually, I think the centering ring built up a static charge. I focused on the centering ring because it was the only conductive surface I was aware of contacting, and it made the multimeter flash strange readings like FFFF for overflow.
I think this the culprit:
The exposed rivet is at the lower side of the yoke, hidden from my view. It connects the "DO NOT MEASURE" voltage to coils.
I insulated all the potential conductors which I might accidentally contact and I was still wary of the centering rings, so I made handles with epoxy putty.
Now I'm proceeding with adjustments without getting electrocuted. Thanks for your thoughts on the matter.
- Henry