I think I came up with a quick solution. I added a small drop of Superglue to the motor shaft, and the tubing wheels seem to be hanging on -- so far! It still won't zero white, but I'm less concerned about that.
My next project is one of the Controller III boxes I have has the stupid "no fan" error. It doesn't matter which head I plug it into. And the other controller I have works fine on both heads. Sigh. I sure hope deOXit will fix it.
Thanks again for your help, IC!
-MD