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Paul,

Provide DC, voltage boost and polarity all go together.

It seems like I've seen small cheap 120 / 28 volt transformers, can't remember exactly where right now.

For testing purposes you could just meter the output of the small variac driven from 120 Volt, probably wouldn't be a good long term method. Sooner or later, if it were me, I'd screw up and hit the motor with more than I wanted - - and let the magic smoke out.

Although - I've seen some industrial uses of variacs for voltage adjustment that have a mechanical stop which won't allow the variac to be adjusted to a voltage above a pre-determined maximum.

Mike
B)