Assuming it’s a Chrome bumper unit, you’ll need a miniature screwdriver - flat head IIRC. loosen off the locking grub screw then just pull the knob off the shaft. Reverse the process to re-install. Make sure you don’t over-tighten as the plastic can get brittle with age and split or break.
Of course, it may be that the switch is worn, in which case it should be replaced when the amp is next serviced.
If the rotary switch has been worn it doesn’t matter as long as it functions properly.
But is there a possibility that the knob locking bolt does not completely press the switch shaft?
I’m just curious, but I will do everything carefully
I tried to open the knob of the NAP-140 with the following screwdriver, but I am not sure if this screwdriver is correct. The flat width of this screwdriver is about 2mm, is this enough or not wide enough?
You have to make sure the screwdriver is properly fitting the slot of the screw and not ended up past the screw and wedged between the screw and the plastic on the knob.
Make sure the screwdriver is entering the hole central and heading for the centre of the knob, hard to explain but I think you will feel it when it is right.
And of course once the knob is off you can look and see how to fit the screwdriver for next time!!
I see that the diameter of your screwdriver looks too big compared to the diameter of the hole where the fastening bolt is, is it not advisable to use a smaller screwdriver diameter of the screwdriver neck that goes into the hole in this knob?
The picture is a close-up, and even though it looks big, the blade passes through the hole ok.
In the picture of the back of the knob you can see the problem of age and over-tightening, and I filled the part of the knob shown, with some plastic just the right size, and placed as shown to strengthen the part that encloses the shaft. Mine may well have had a similar crack that I was keen to strengthen, I cannot remember exactly, but this seems familiar.
the picture is not a knob, I am just a description of the position of the tightening bolts where, I can’t open mine myself because I don’t know what the shape of the fastening bolt looks like, it looks like using a flat screwdriver with a width of 2-3mm