I just purchased a used 1983 240D in pretty good shape
The AC and Heat work fine, however the dash fan knob is hard to turn, and also will not turn the fan all the way off. It will turn the fan from low to high with some effort - but not off. Is this easy to repair/replace?
Also - in an attempt to see if the switch knob was defective I accidentally broke the little tiny light bulb that is embedded into the switch control stem - is that something I can purchase and replace myself - or would it be easier to take it to the shop?
The bulb is replaceable - if you can get the broken bits out of the shaft.
The fan switch on these manual W123 heat-AC setups also has a vent cable control that runs from the fan switch to an air flap in the duct behind the glovebox. The cable often gums-up after so many years, making the fan-switch very hard to turn. If you keep forcing it, eventually it can break the pot-metal shaft the fan-switch knob sets on. Then you'll need another fan-switch, as well as freeing up or replacing that cable.
To free up the cable, you need get behind the fan-switch to disengage the cable-coupling and also disconnect the cable at the flap behind the glovebox. Then liberally squirt automatic transmission fluid at the ends where the cable slides into the sheath. You'll also need to pull the cable ends back & forth with a needle-nose pliers numerous times until it frees-up.
After you remove the knobs, three cups, nuts and faceplate for the rotary controls, getting at the rotary fan-switch/cable coupling will still be tough as it's secured behind a plastic plate bonded to the heat-AC box. Unless a PO cut this plate, you'll probably wind up pulling the center console for enough room to get at it.
You may try disconnecting and lubing the cable behind the glovebox first to see if it will loosen enough to avoid pulling the console.
Happy Motoring, Mark