Sewing machine, upper tension problem, Montgomery Ward model 267?

Our Montgomery Ward sewing machine is about 40 years old but still looks and runs good except now the upper thread tension knob does not work. The tension disks are mounted on a hollow splined pen. The groves of the spline catch the end of a coil spring that hooks the thread. The hollow splined shaft turns freely but it seems to me it should not turn in order to tighten the tension disks. What is supposed to keep the hollow pen from turning? The spline engages the coil spring but the spring is not strong enough to keep the hollow pen from turning.