Funny yesterday I just cleaned out all the carbon build up in my 78 rheostat.

On my model - one phillips head screw was in a recessed area on the top that could not be seen untill I used a dental mirror, after removing the screw the knob stayed in place. I unplugged the unit.....took it up to my hobby desk....bent out the two metal tabs and ( this is where you need to be careful ) IN MY VERSION inside the casing there were two small springs that held the contacts to the rhio winding and center contact area.

I used a fine emery board to clean the contact surfaces on the contact piece and a dremel bit to polish the center contact part in and on the bottom of one of the part halfs....I then put it all back together and WHAMO! my inter instrument lamps came on just fine!

Hope that helps!