The tuning knob on the 8600 is held on like most other Icom's.
That being, the knob has a "rubber tire" outer band around the knob. You just slip a tiny flat blade screwdriver like a pocket screwdriver or smaller between the rubber tire and the silver trim of the knob. Using the screwdriver, it's very easy to work the rubber tire off the main knob.
Under the rubber tire on the main knob is single hex head set screw. The set screw is located 180 degrees from the tuning knobs fingertip hole. You need a 2.0 mm hex wrench to insert in the small set screw hole and loosen the set screw.
A 1/16th inch hex wrench may also work but don't do it if your hex wrench is a sloppy fit. If you round off the hex set screw, getting the knob removed could be hard.
My set screw was not very tight.
When reassembling, there is a flat on the 1/4 inch shaft. The hex head set screw tightens to the flat of the shaft.
While the knob is removed, see if the shaft turns freely. If so, look for burrs on the back side of the knob and remove anything found. Also look for burrs sticking out from the chassis behind the knob from around the mechanical brake parts that you will see with the knob removed.
If you still feel a snag at the 1 o'clock position while the knob is removed, it may need a trip back to Icom if you don't think rotating it back and forth over the snag area is making it feel like it is freeing up.
Working it back and forth past the 1 o'clock position may very well clean it up over time.
Mine both spin nice and freely and did so since day one.
It's an optical encoder and something you don't want to mess with if you don't find a burr on the knob or the chassis behind the knob and it also does not seem to be getting better over time. Optical encoders are tricky to work on and easy to break if you try!
I do wish the R8600 tuning knob was heavier allowing it to coast easier like the larger tuning knobs do on the larger receivers like the R9000 or similar size transceivers. I don't think the slight tightness of the tuning knob is due to shaft friction but it could be. It's hard to tell.
Behind the knob and around the 1/4 inch shaft is a 1/2 inch or larger nut that holds the assembly tight.
You can try loosening that nut slightly and see if it helps. If you loosen it too much, the knob get sloppy and wobbly so don't loosen it much. Maybe loosen it until the shaft has excessive side to side play and then tighten the nut just to the point any excessive free play is gone.
I found on one of mine that the nut was a bit too tight compared to the other radio. Once I loosened up the nut a little, both radios tuning knobs spin equally to each other.
I'd also test the brake positions and function while the knob is removed just to make sure that is all intact and working like it should before you put the knob back on.
The brake lever is three positions. All the way to the right gives the knob detents that you can feel as you turn it. The center position of the brake lever removes the detents but leaves a fair amount of friction on the knob. The left brake lever position is supposed to be the free wheel position.
