Engine noise on COM1 but not COM2?

[Jim_R]

I've got an IFD540 as COM1 and an SL30 as COM2. Last weekend, mid-flight, COM1 suddenly began exhibiting a very loud, staccato interference during reception (i.e., the interference doesn't break squelch by itself--it's only present when squelch is broken by a received transmission). The interference is not present on COM2. The radio is still useable, but the interference is very noticeable and annoying. (Two passengers have asked me, "Is that normal?")

The staccato frequency varies with engine RPM.

Plane is about to go in for annual. Not sure that this is something my A&P can help with, but afterward I'll immediately take it to a different shop for the pitot/static cert, and they do avionics work too, so I plan to ask them to check it out.

In the meantime, is there anything "easy" I could/should be checking?

 

[Bell206]

In the meantime, is there anything "easy" I could/should be checking?

One quick check if the antennas are compatible is to swap the coax between #1 and #2. If the noise follows maybe a shielding problem with that specific coax.

 

[xander75]

One thing to clarify Jim is whether the noise you are referring to is truly engine noise only, or ambient noise in general or a soft whistling sound.
It could be one headset of lower quality that is picking up more ambient sounds especially when the boom is in an airstream of your airco ( have that with a rugged I got for my daughter in the rear seat and it is consistent in the summer when she moves the boom off her mouth), it can also be the squelch setting of a heatset or audo panel input picking up that noise, but if it is solely engine noise hissing, it could be a grounding situation on the com2 only.
Especially if they are different brands/installed at different times.
See if your alternator has the capacitor for noise cancellation, some newer ones are said they don't need it, but still do, it's expensive, $70 for the part ( a cap!!) and the install is painful because the right side cowling needs to come off so it is laborious.
I'd say can you live with it or is it worht hte expense(s)?
xander!

 

[Racerx]

It could be one headset

Well, considering he and two other passengers heard it...

How long has it been installed? I'd shoot avidyne an email just for fun

 

[Half Fast]

...COM1 suddenly began exhibiting...

“Suddenly” makes me suspect a ground came loose, possibly a mounting fastener. When my radio did something similar, I could push on the radio chassis and the noise would clear up. Re-tightening all the connections and fasteners took care of it.

 

[Jim_R]

One thing to clarify Jim is whether the noise you are referring to is truly engine noise only, or ambient noise in general or a soft whistling sound.
It could be one headset of lower quality that is picking up more ambient sounds especially when the boom is in an airstream of your airco ( have that with a rugged I got for my daughter in the rear seat and it is consistent in the summer when she moves the boom off her mouth), it can also be the squelch setting of a heatset or audo panel input picking up that noise, but if it is solely engine noise hissing, it could be a grounding situation on the com2 only.
Especially if they are different brands/installed at different times.
See if your alternator has the capacitor for noise cancellation, some newer ones are said they don't need it, but still do, it's expensive, $70 for the part ( a cap!!) and the install is painful because the right side cowling needs to come off so it is laborious.
I'd say can you live with it or is it worht hte expense(s)?
xander!

Reasons I don't think it's ambient noise:
- It's not present in intercom conversations
- It's not present in my transmissions (I pulled some LiveATC audio to check)
- It's still present when only my headset is plugged in (again, if it were coming from my mic I'd hear it on LiveATC)
- It also just sounds like signal interference--a very sharp staccato, probably ~5 beats per second or so. It's not a whistle or whine or rumble or hiss.

How long has it been installed? I'd shoot avidyne an email just for fun

I'm an early adopter. It's long out of warranty. I guess if there's noise-suppression circuitry inside the radio, it could be that, but is that likely?

“Suddenly” makes me suspect a ground came loose, possibly a mounting fastener. When my radio did something similar, I could push on the radio chassis and the noise would clear up. Re-tightening all the connections and fasteners took care of it.

Yeah, I agree the instant onset makes it seem likely that something, somewhere, "came from together". But the IFD540 is secure in its chassis, and none of the functionality has altogether stopped, so it seems unlikely to me that it's in the IFD itself, or its installation. I wasn't sure if there might be something else obvious, like a noise suppression capacitor near the alternator or something like that, that I should be looking at. But anything more general like that I'd expect to affect both COM1 and COM2, so that doesn't seem real likely, either. Xander did suggest such a thing, so I'll take a look there.

I haven't had a chance yet to try the earlier suggestion to swap antenna cables and see what happens.

 

[JAWS]

Reasons I don't think it's ambient noise:
- It's not present in intercom conversations
- It's not present in my transmissions (I pulled some LiveATC audio to check)
- It's still present when only my headset is plugged in (again, if it were coming from my mic I'd hear it on LiveATC)
- It also just sounds like signal interference--a very sharp staccato, probably ~5 beats per second or so. It's not a whistle or whine or rumble or hiss.


I'm an early adopter. It's long out of warranty. I guess if there's noise-suppression circuitry inside the radio, it could be that, but is that likely?


Yeah, I agree the instant onset makes it seem likely that something, somewhere, "came from together". But the IFD540 is secure in its chassis, and none of the functionality has altogether stopped, so it seems unlikely to me that it's in the IFD itself, or its installation. I wasn't sure if there might be something else obvious, like a noise suppression capacitor near the alternator or something like that, that I should be looking at. But anything more general like that I'd expect to affect both COM1 and COM2, so that doesn't seem real likely, either. Xander did suggest such a thing, so I'll take a look there.

I haven't had a chance yet to try the earlier suggestion to swap antenna cables and see what happens.

Does it do it with just the master on and the engine not running? Different head set? Different jacks? What other items are running that can be turned off to see if they affect the radio.

One of my first go to's is to "re-rack" the box. Just the action of moving the box often clears up issues. Cheap and easy.