I would bet that the extra pair of wires bring variable power to the
stator, providing a variable magnetic field and thus varying the speed.
With no power on these leads (no power on the stator), there is no
field, so the motor won't turn, of course.

Since you just got a new speed control unit from Dremnel, you have to
assume that the speed control unit is OK. This leads you to suspect
that the problem is in the wiring inside the motor, or the cord from the
speed control unit to the motor (assuming you didn't get a new one with
the new speed control). So start at the speed control - does it put
power on the yellow and white wires, as well as on the red and blue
wires? If so, does the power make it through the cord to the pcb
mentioned in another post? If so, does it make it to the motor?

Find out where you are losing power in the yellow / white wire circuit.

Bill Jeffrey
===========================

There's 4 wires to the motor? I had assumed the brushes would be in series
with the field coil, you need voltage to both pairs of wires for the motor
to run.

The 2 wires to the brushes have the 100VDC, I do not know what the others
are for.

Here is a picture, the red and blue wires go to the brushes:

http://www3.telus.net/martik/motor.jpg

It appears that the white and yellow wires are possibly connected to
some other item (thermal sensor ??) on the other side of the
fibreglass pcb which is clamped under the brush housings. Can you
remove the pcb to have a look?