If I remember right, the knob pushes on a part of the circuit board that is vertical, and that with time, angles backward. If you take radio out, take top cover off, you will have no problem seeing what is going on as you push the knob.
Little piece of wood nudged between the back side of the little vertical circuit board and a capacitor or something like that behind, stopped the motion of my knob.
It has been a long time so I hope I remember correctly.