The only way I found on my psr-600 (same knob setup) was to pull both knobs off and then wrap a thin strip of rubber tape from the inside of the bottom knob to the outside of the bottom knob. This makes a small "drag point" for the bottom knob when it is re-installed. This bottom knob for me is the squelch, which I rarely use. But now when I turn the upper knob (volume) the bottom knob stays put. Some have had luck spraying electronic cleaner around the knob, but that didn't work for me.
