William Noble wrote:
the speed control is a triac based phase control, the same as a light
dimmer - typically it has two wires, just short the two wires together and
the dremel will run full speed all the time.

by the way, typical failure is just noisy pot, try cleaning carbon track

Mine had a smd triac BT134W which was faulty.

I replaced it (easy!!) with a new one, and
the dremel has worked years after that. The
triac costs about 1 usd.. The parts are on a
white ceramic circuitboard.

Kristian Ukkonen.