Dave, I just took a quick look at mine and reminded myself how it works. The mode selector switch is inside the bandwidth selector switch (dual function). Most of the knobs are similar, though the shaft for the mode switch is much smaller in diameter. Drake used a fiber reducer bushing to bridge the gap, and the set screw squeezes the bushing into the needed "D" shape for the shaft. There's another small fiber bushing on the shaft to isolate it from the bandwidth selector.
If your mode shaft turns easily with pliers, your problem is with the reducer bushing. Unlike the other knobs, tightening that knob it a bit touchier. It never gets fully tight like it would on a fill-sized shaft. Perhaps a previous owner stripped it out. I suspect you could substitute it with another knob by retrieving the reducer bushing.?
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73 de Steve K9NUD ..
Nothing happens until someone calls CQ!
Steve K9NUD