Radio Feedback Troubleshooting

moto657

Filing Flight Plan
Joined
Dec 5, 2019
Messages
9
Display Name

Display name:
moto657
I am getting really bad high pitched feedback in the headset. It’s always been there, but I’ve been flying a bunch lately. It’s really annoying and has my tinnitus ringing really bad now.

My Equipment:
- TKM MX 11 Com
- Softcomm panel mount 4 place stereo intercom.

Last night I did some testing to understand interference sources.
1. Turning the intercom off or on made no difference.
2. Radio on with engine off and all loads off is quiet.
3. Alt field switch on with engine running. RPM tracking high pitched whine. Biggest contributor.
4. Strobes on. Pulsing high pitched. Second biggest contributor.
5. MX11 display when on bright causes high pitched whine. Really weird. It has a photocell to auto dim. When it’s dimmed no noise. Lucky I was testing in the dark and noticed this when the headlamp hit the photocell.
6. Nav lights and Skybeacon on. Very quiet high pitched.
7. Map/interior lights, no effect.
8. Taxi/ Landing lights. They are blown out now, or not working. Add that to the list!

Ground loop issues, right?

Based on some research I started troubleshooting by checking all of the jacks for headsets to be sure they are isolated from the panel/airframe.
1. None of the jacks have fiber insulation washers.
2. To remove the jack to panel ground loop I pulled the retaining nuts off all 8 jacks and moved them away from the panel.
3. Plugged in headset and turned radio on to test. Radio out put was minimal. At full volume sound is very faint.
4. Headset speaker jack body needs to be touching panel (grounded) to hear normal radio volume output. Same result at all 4 headset positions.

So now I am thinking that the jacks are not wired properly. Seems they need to have a ground lead ran back to the common radio ground. Then isolate the jack bodies with fiber washers.

I am not great with electrical, and no audio experience, so please let me know what you guys think.
 
You will have to hunt around to find a copy of the installation manual, as Softcomm is defunct. Maybe an avionics shop might have one.
 
I am not great with electrical, and no audio experience, so please let me know what you guys think.
I would say you definitely have a ground issue. But you may want to get your mechanic involved before you start ripping and tearing anymore wires loose given your lack of experience in this. Things can get damaged by intermittently grounding powered circuits. From your explanation it appears you may have several different issues that you may not find on your current route of troubleshooting.
 
I agree with @Bell206. Audio is a tricky bit** sometimes, especially when multiple individuals and/or shops have been into bundles and harnesses over the years.

Magnetos can be noisy in audio systems that aren't properly shielded or terminated. If the audio wires going to your actual jacks aren't shielded, that's an issue. Moreover, if they are but the shield is grounded/terminated at both ends you'll get a ground feedback loop. I like to leave the jack end floating and terminate the shield at the audio panel ground lug or connector back shell (however the install manual specifies).

To properly troubleshoot the problem, you really need to have a look at installation manuals for the equipment installed in addition to looking at the wiring to your jacks (some should be grounded, some not), intercom, and audio panel. Most audio surgeries lead to more problems if not chased properly.
 
Thanks for the input so far everyone. I’m going to take a close look at how the jacks are installed and compare to the recommended practice from the install guide.

I’m having a tough time finding an install guide for the Softcomm ATC-4PS intercom. The Sigtronics SPA-400 is similar and I have one for it.
 
Also, make sure your spark plug leads are tightened properly. If you are getting feedback in proportion to engine RPM... I second the magneto theory. Yes, it is a grounding issue but if you have high-tension voltage flying around your airframe because of poorly tightened leads then you'll get feedback.

Make sure you tie all audio related shields and grounds to the same point. Ground blocks and grounding straps help a lot with this. So, yeah your right, ground loops ;). Now, solving them can be a physical and mental challenge.
 
Last edited:
Back
Top