I assume you have the dimmer control with the "resistor" circuit card (Moss P.N. 146-300). The unit in my '78 B wasn't jammed, but it had only a very small range of dimming before the lites would just shut off. The device has a metal plate riveted to a zamac body casting. Grinding or carefully chipping off the rivetting will enable removal of the plate and reveal a phenolic circuit card with a pattern of resistance foil on which the wiper blade moves in rotation to connect various diminishing resisitances into the circuit. Its possible some of this foil has been burned through and the wiper has picked it up and jumbled it into a blockage that won't allow the shaft to be turned...could also be the zamac casting corroding or deteriorating and binding the shaft...try a bit of penetrating oil from the outside. If the card does have damaged foil, it might be repaired with some judicious lite duty soldering...bridging burned out spans with a small glob of solder. Use a very active liquid soldering flux sparingly just at the desired spot to facilitate good bonding...and a magnifying glass or some sort of magnification lite or hood is helpful to see the small working area. My repaired dimmer has been working fine...under test conditions...I prefer brighter dials so don't ask the dimmer to do much dimming. From the purist's perspective, a working dimmer brings satisfaction and it can always be shown to an appreciating colleague then turned up bright for running. After repairs, prepare the two rivet bases in the body for drilling and tapping...I recall using 2-56 round head screws...or maybe the next size up. I do a lot of model rail stuff and have the small tools to do this work but they are not expensive.
Bill