When their RTX tool first came out it was an infinitely variable speed tool with markings from 1 to 6. The new ones are marked as only 3 speed. I had one of the original models. I actually got an awesome kit on clearance at walmart for only $25 which came with tons of bits, the snake extension, and a hard case with bit storage in the top which works great. This is actually one of the only tool cases I have kept and used to keep the tool in because it stores the tool and its accessories soo well. This was purchased shortly after the RTX came out over 10 years ago.
I used my RTX harder than most, constantly exposing it to wet, dusty, dirty conditions. I used it at least 2 times a week much of the time for heavy duty tasks like cutting and grinding metal. The metal filings building up on the magnets in the motor and probably some gravel that got into it while dragging it around under a vehicle is what finally killed mine. I seriously put it through more hell than most will ever put a rotary tool through. I of course took it apart and indeed something had lodged in between one of the brushes and the contacts and ripped one of the brushes out and it disintegrated into the motor. Motor was fried, even if brushes were replaceable it would not have been salvageable especially with all of the metal fillings built up in there from years of metal working.
I of course needed a replacement and I have used Dremel Tools complete line as well as a Craftsman rotary. The Craftsman was complete garbage and failed within a year. It had a plastic/rubber motor linkage which stripped out and failed. The Dremels are great, well built tools and you can't go wrong with them, especially the 4000. The replaceable brushes are a nice feature, but after over 10 years of hard use my one good brush was still about 1/4 so I think that is not a big deal. I actually like the weight, ergonomics, collet lock, size and weight of the RTX more than of the Dremel 4000. So I bought another RTX, which are advertised as 3 speed now.
I ordered just the base tool from Amazon as I plan to continue using my original case. Got the tool and was pleased to see that it is exactly the same tool in every way except for the speed dial. I took it apart and all of the parts are exactly the same including the motor control board, and all circuitry. This includes the potentiometer that controls the motor speed. It is infact infinitely variable, they just used a little spring that catches on notches on the dial to lock it into the "3" speed locations. You can easily use your RTX with the dial in any position for variable speed out of the box. Or, you can do this easy mod and make it infinitely variable without those annoying locking positions. It will not hurt the tool, or its circuitry, and will give you fine motor control. I'm guessing Black and Decker gimped it out to keep from having legal trouble with Dremel because it is basically just as good or better as the variable speed dremels.
All you need to do is take the tool apart and remove the little spring that locks the dial in the 3 positions.
To complete this you will need a security torq bit set (the ones with the holes in the middle) and a medium sized philips screwdriver. I got a ratcheting screwdriver set at Home Depot that came with these bits as well as every other bit a person might want. Pics are self explanatory.
Note: I actually replaced the dial with only a "H" "M" & "L" markings with my original dial with the numbers 1-6 on it.