Squelch was kicking in initially from ground interference which initially started when flying near high output antennas or the hospital district. Checking grounds and antennas helped somewhat... the low flight radio interference would trigger the squelch as if you pulled it. Initial ideas from here and red board were that there might be an ELT problem (excluded immediately).
Gee, there's an idea ...
Comm2 was the worst ... again, symptoms were squelch as if pulled (not pulled) worse on comm2 due to interference. Your ideas are the ones you try the first day things occur. After reducing RF problems, the original KX155 you could force it to do it by bumping the side of the volume/squelch knob. Until the other RF problems were reduced, you couldn't even figure that part out.
What's so hard with eliminating factors one at a time? All of your ideas were exhausted in the first several minutes of trouble shooting. Compounding problems is that it took 3 years to become continuous ... it was so intermittent and short lived at first that it was hardly noticeable and was multi-factoral in origin. The knob was never loose, switch itself wears allowing you to engage squelch by slightly bumping it left to right rather than pulling it. The avionics shop dropped my radio and had to replace mine with a replacement AFTER getting repaired ... guess what? The replacement did the exact same thing the first time it was checked.
That's actually my story, through two very good avionics shops. It'd be nice if everything in the world was as straight forward as you'd like it to be ... but it often isn't. I also don't have a basic panel ... the previous owner spent a ton upgrading the panel and to say its maxed out would be an understatement. These certified AC don't qualify for wiring harnesses like the experimentals that make things as easy as you'd like them to be if you have a complex panel.
All of that still doesn't explain why you would hear it at all if Comm 2 was NOT selected on the audio panel.
It sounds like you're saying the potentiometer went bad, (bumping the knob made it intermittent) which has nothing at all to do with the (removable) knob.
As far as certified vs non-certified wiring harnesses, I have no bloody idea how that matters in this case. If the harness was suspect, swapping the two KX155s in their trays would have adequately shown that the problem followed the radio, not the harness.
Look, I'm just saying if your local shop is filling your head with this garbage, find a better avionics person. A busted pot that you can wiggle hard and cause to open and close a squelch is like "two-way radio 101" class.
The rest of the story makes so little electronics or standard troubleshooting methodology sense, that I'm concerned they're feeding you some really weird ideas about how it's done. If they charged a lot of labor hours looking for this, caveat emptor.
Been there, done that... watched an avionics shop simply MOVE a problem instead of addressing it, and damage a second radio in the process. Well, they didn't follow a process, actually... they just made something up and tried it without thinking.
Or as the funny commercial says, "That's not how this works. That's not how any of this works!"
