Dirty contacts. I sold Kenwood in the 70’s into the early 80’s and many came into our service department with noisy controls.
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It’s at that age where one possibility, the ugliest of all.. is the one I made with the last three integrated amps I worked on.
Complete switch disassembly (of all switches), and scrubbing of the contacts with 2000 grit sandpaper, reassemble switches, re solder, and then reassemble the gear. They might never suffer a fault in the entire life of the gear, ever again.
That’s the only ’real’ solution that works. Contact cleaner works for a bit, but then......
Contact cleaner like DeOxit works well and works fast. Got my 80's receiver running like new. But, that was only 8 months ago so I cannot comment on the duration of that solution. As teo-audio mentions it might not be a long term solution but could serve as a cheap(ish) (a tiny can of DeOxit is $20...and doesn't spray well either !) stop gap solution.
I don't have the soldering skills to do the permanent job he mentioned or the money (my receiver isn't worth it) to get someone else to do it.