Here's what I do periodically to renew the Cab-1 red knob.
1) Remove the 6 screws from the back of the Cab-1. Note that the two screws at the bottom (by the red knob) are shorter than the other 4.
2) Separate the halves of the Cab-1 by gently pulling them apart. There are some wires near the top that will keep the halves from completely separating, but that is okay.
3) The metal knob shaft fits into a hole in the rear half of the Cab-1 housing. Stick a small (1/4 x 1/4 inch or a little larger) piece of masking tape over the hole.
4) Put the Cab-1 halves back together. Be sure that the optical sensor board next to the red knob shaft fits into the plastic slots that hold it. You'll need to squeeze the Cab-1 slightly so the knob shaft can pierce the tape. A flap of tape will get pushed into the side of the hole effectively making it smaller and creating some friction for the shaft.
5) Replace the 6 screws, making sure that the short ones go in the bottom of the case.
6) If the knob still rotates too freely, open up the unit and add a second piece of tape on top of the first.
This fix has worked for me many times.