The Speed Control Dial (indicator ring) has teeth (see upper arrow in the picture below) under the Speed Control Handle which is turned by the gear mounted on the front of the Control Bracket (see lower arrow in the picture below). That gear is turned by the teeth on the inside center of the Speed Control Handle where it attaches to the worm gear shaft. If the teeth on the inside of the Speed Control Handle are broken off it cannot turn the gear to turn the Speed Control Dial or if the Speed Control Handle is not on the worm gear shaft all the way it may not be engaging the gear to turn the Speed Control Dial. It will still change the speed by turning the worm gear shaft it is attached to. There is a cut out in the Speed Control Handle for a 3/32" Allen Wrench (if I remember the size correctly) to loosen the set screw that holds the Speed Control Handle on the worm gear shaft. Remove the Speed Control Handle to see which problem it may have.
Mark V completely upgraded to Mark 7
Mark V 520
All SPT's & 2 Power Stations
Model 10ER S/N R64000 first one I restored on bench w/ metal ends & retractable casters. Has Speed Changer, Model 4E Jointer, Jig Saw with lamp, a complete set of original accessories & much more.
Model 10E S/N 1077 oldest one I have restored. On bench w/ metal ends & retractable casters.