Comm Gremlins

[Boatguy]

First flight after annual...

Main battery was R&R'd for desulfation. HIRF box not opened.

Massive static on the radio. I'll skip the story, here's what my diagnosis in the hangar has determined:

1) Limited to COM2. NAV2 seems OK (or at least successfully tuned the ILS at my airport).
2) Acts like the squelch is open, massive static which varies with volume setting and the Rx is lit.
3) Begins at power up, but eventually ends after 2-10min.
4) Seems to be sort of an intermittent period where I don't get reception, but intermittent Rx and static.
5) Seems to be shorter or not at all after the avionics are warmed up. So after sitting in the hangar for an hour with half a dozen power cycles (avionics off, master off, wait, master on, avionics on...) the problem didn't appear at least once.
6) Resetting CBs did not seem to do anything useful (though I did not that pulling GPS2/NAV2 also disabled COM2 which was a surprise)

My first thought was that some connection was disturbed during the annual, but I don't know why it would then cure itself after 2-10min.

Is the lower antenna the COM2 antenna? That presumably has a different route to the HIRF box than the top antenna.

So what's the collected DAN wisdom think about this?

** Further thought

My radio setup has ATC on COM1 and Tower/Ground/ATIS on COM2. I switched to COM2 about 10nm from my home and the problem recurred while descending through some turbulence to pattern altitude. This was probably 50min after engine start. If the lower antenna is COM2, then it could have been disturbed when they removed the interior and leads me back to the "disturbed connection" theory.

[Chris]

So it doesn't matter what frequency you are tuned to at all? Also, does "don't get reception" mean that you can't hear anybody else transmitting on frequency (when tuned to a known-busy frequency) while the Rx it lit up? Once the static clears up, the radio behaves normally after that? Can you transmit anything successfully on COM2 when the problem is occuring?

I think you're correct that COM1 uses the upper antenna and COM2 uses the lower one. The antenna wiring seems like a good place to start.

[Rich]

FWIW I find all sorts of stuff (including COMMs) really suck in the hangar. Whenever I want to test something regarding any external RF I pull the plane out in front of the hangar. (I have the luxury of being at the end of the row and don’t block anyone needing to taxi. I could sit there all day.) Even then the steel hangar structures can compromise reception, but it is definitely to a lesser degree.

[Boatguy]

FWIW I find all sorts of stuff (including COMMs) really suck in the hangar. Whenever I want to test something regarding any external RF I pull the plane out in front of the hangar. (I have the luxury of being at the end of the row and don’t block anyone needing to taxi. I could sit there all day.) Even then the steel hangar structures can compromise reception, but it is definitely to a lesser degree.

That was an issue at O69, but at KSTS I can hear Oakland Center, KSTS Tower/Ground/ATIS/ILS, and GPS lock on both GPS' with the doors open.

[Boatguy]

So it doesn't matter what frequency you are tuned to at all? Also, does "don't get reception" mean that you can't hear anybody else transmitting on frequency (when tuned to a known-busy frequency) while the Rx it lit up? Once the static clears up, the radio behaves normally after that? Can you transmit anything successfully on COM2 when the problem is occuring?

I think you're correct that COM1 uses the upper antenna and COM2 uses the lower one. The antenna wiring seems like a good place to start.

Correct, when the problem is present, I can't hear anyone else on COM2 while the Rx is lit, just static. Once it stops, then it behaves normally.

I've briefly pulled the PTT on COM2 and see the Tx light up, but I haven't attempted to actually talk since I can't hear what's happening on frequency and don't want to step on them more than the brief pull.

And yes, I confirmed in the AMM that the lower antenna is COM2, which I suspect was "disturbed" when removing the interior.

[Colin]

After some maintenance which involved pulling the seat, I had a similar issue. And I noticed the "Manual Squelch" button was lit on the Comm panel. I used Com2 exclusively for a 2hr flight. I figured I would try to get a really sharp diagnosis before bringing it back to a shop. After about three hours of flying the problem vanished.

I hate things like that.

[Steve]

And yes, I confirmed in the AMM that the lower antenna is COM2, which I suspect was "disturbed" when removing the interior.

Russ:

On my aircraft, removing the interior does not expose the COM2 antenna connector or ground plane, which is located behind the aft bulkhead. The antenna coax is completely (except for the last 8 inches before the antenna connector) enclosed within a conduit within the passenger compartment, and would be almost impossible to "disturb". I suppose the NG airframe or those aircraft with extended baggage compartments may be different. On my aircraft, the ELT is in close proximity to the COM2, and since this has to be removed to perform the procedures required by the ICA, I'm always careful to make sure I don't disturb the antenna coax or ground connection. My aircraft is not a G1000 airframe, so the location of various components may be different in yours.

[Boatguy]

Steve, that's also what I saw today, the antenna is completely protected and routed through a shield.

Two hour roundtrip today (1hr each way) the problem was only on the ground in selected locations so I'm crossing my fingers that it was a localized problem that won't be chronic.

[Lou]

I just found this post after experiencing what sounds like a very similar problem. It occurs only at my home airport and only in some consistent spots on the ground and possibly at low altitudes. I typically have tower on COMM 1 and ground on COMM 2. I can see the RX light up but at most I hear weak garbled comms. Usually I don't hear other aircraft at all. I have found this workaround: I turn off one radio and use only the other and everything is fine. If I turn on the second radio, the sound is suppressed. Weird.

If anyone has some insight I would be grateful. But I think the most likely solution is continued sacrifices to the god of avionics and hoping for the best. These things are impossible and expensive to troubleshoot.