as suggested elsewhere, I did this mod, I bought two PWM controllers (1 for spare). Tested controller using a light bulb for a load, then removed fan face (shroud), tested PWM directly on fan, then swapped wiring over on the face (shroud) of the fan and installed PWM to shroud body , tested, reinstalled. Note that PWM output seems to not like a common negative rail. Its a PITA working overhead. I did shorten the knob and the potentiometer shaft. For the shaft - use a small hacksaw - do not use a Dremel - a Dremel generates far too much heat. Insulated spade connectors were used between the shroud and the fan body (and supply wiring). The rheostat speed controller is totally disconnected.