Loud squeal on Push to Talk

[BalsaBat]

I have a dual screen HDX system with SV-intercom-2S, but my nav/comm is from a Garmin GTN-650-xi. This is an existing installation (from about 2020), but has recently begun giving an overpowering squeal when I hit the passenger side push-to-talk button,. The squeal starts immediately and is clean like a tone generator was switched in, maybe at 600 Hz or so. It is also transmitted. Today it started doing it on the pilot side as well, but is more "scratchy". If I turn off the intercom power, the radio works fine with the fail safe (except I don't get the Dynon lady warning me about all the stuff she always likes to complain about). I saw a couple old strings here-in with a similar issue, but I don't think they mentioned that their system worked normally with intercom power off, or that it seemed primarily (or only) on one side of PTT. Getting to the antenna connection on the back of the GTN-650 will be a chore if its likely that. FWIW, I have a composite airplane with eight radial 3 foot wires for a ground plane.

Any thoughts?

Thanks
Ted

 

[GKC Aviation]

Because the squeal is being transmitted, and you can hear it in your sidetone, it is obviously on your mic audio line.
This won't be anything anything to do with antenna connections.
Personally, I would check the shield grounds of the shielded wires, especially the mic hi and lo wires.
I would also check the intercom unit ground itself. It might be as simple as a terminal has come loose.
There have been a few threads over the years here, and other forums where intercom or com squeal has been rectified with checking and repairing grounds.
The main ground wire comes out of pin 1 of the intercom connector. It is usually grounded at the same place as the VHF is grounded.
If this wire from pin 1 is not a solid ground, the intercom can attempt to get a ground through anywhere it can, including mic and phone shields (as they are all connected to this wire).
All the shield grounds are connected to this wire usually inside the backshell of the connector.
I have seen people do this shield jumper connection badly, and the shield wires start pulling out of the splices.
Might be a place to start anyway.
Come back to us with what you find.

 

[BalsaBat]

Thanks for that. I'll inspect/redo those grounds first. I did see that in general the shield connections were in the right overall configuration per the manual (though not "as close as possible" to the connector), but I've found some bad crimps elsewhere. Hope it's there.
.

 

[BalsaBat]

Well, the third party that installed my system really didn't do very good job, even using sub-standard wire (for aircraft anyway), and it is not a tidy job. The shield jackets were quite a long distance (about 5") from the D-sub connector, but that had not been changed. I remade a few connections, but I really couldn't find a specific smoking gun. After that, the squeal was slightly less "solid", almost intermittent. It actually, went away for awhile until I started the engine, but then returned even with the engine off. When squealing, it was with the intercom energized, and the radio was still fine with intercom switched off. I will be rewiring all of the intercom/NAV/COM wiring per dynon instructions, since I think the problem is most likely in there somewhere.

Today, just for fun, I took the cover off the SV-Intercom-2S (no longer in warranty anyway). It has a single printed circuit board with hundreds of solder joints in close proximity, maybe 1/16", to the metal box. I was kind of surprised that there was no insulator between the bottom of the circuit board card and the bare metal box interior. It looked like perhaps the board/card mounting was intended to keep the solder joints from touching the box, but again, not a LOT of clearance. One solder connection wire protruded enough from the bottom of the board that it MIGHT have been lightly touching the metal box. Hmm.... Not sure about that, but I added a craft paper "insulator" across the bottom. (mylar would be better) After that, the squeal SEEMS to have disappeared, with or without engine running. Time will tell if the gremlims moved on or are just resting, but the full rewiring is still going to happen.

However, I'm curious if there is normally an insulator between the circuit board and the bare metal box inside the SV-intercom-2S?