The short answer is yes, the controller is indeed a simple voltage regulator. I built one for my own model M without too much trouble. My own ETX-90M moves at around sidereal rate with 1.71V to the motor. I'd also claim that a stepper motor would be overkill. Of course, overkill is sometimes the point.
The long answer is all about if it's worth doing.
The RA drives on the original ETX-90 were very cheaply made, and out of the box they would typically not track well until you made certain modifications to them. Weasner's site mentioned above has all the details, pictures, circuit diagrams, etc, on those mods. You may or may not want to go to all that trouble. I did, and I got the drive to work pretty well. The two big problems are:
- When you clutch the drive in, there's a LONG delay before the RA axis starts to move. As in, much more than one field-of-view's worth.
- Once you get movement, there can be an intermittent slop, like a pause-and-jerk movement on about a 10 second cycle. Not everyone gets this problem, but I did.
Both of these issues come from the same cause - there's a big clearance between the main drive gear and its bearing surface. Again, if you're interested, Weasner's site has the details, but in short, there are two common fixes (I had to do both):
- Build a voltage regulator that puts out either 1.7V for tracking, or a higher voltage that speeds up the motor to take up the initial slack. My own regulator gives either 1.7V or 5.0V. That solves problem 1.
- Shim up and grease that bearing surface. That solves problem 2, and also helps with problem 1.
After all that, my ETX tracks quite well. Doing the mods was worth it to me, at least "in the hobbyist sense", in that I got to tinker around, and learn about voltage regulators.
But deforking it would have been FAR easier.