The tuning knob on the FT-857 free wheels too easily. If you prefer a tighter knob with more drag, here is what to do per a Vertex Standard Technicion.
Remove the rubber ring around the big tuning knob. Using an allen wrench, loosen the allen screw and remove the knob off it's shaft.
On the shaft you will now see a steel spring. This spring can either add tension or loosen the tension on the knob's shaft. To add more drag just tighten the spring's tension on the shaft. Do the opposite for making less tension on the knob. After you adjust to your likes, put the knob back on, tighten the allen screw, and put back on the rubber ring on knob.