Radio transmission with a lot of noise - do I need a new intercom?

[German guy]

Our Cessna 172 is equipped with 2x KX-155 radios and a Sigtronics SPA-400 intercom. When I turn the intercom on, the communication between pilots and co-pilot work flawlessly.

When I however try to make a radio call, the transmission contains a lot of noise. The noise also changes with rpm. This problem however only occurs on the pilot's side, the co-pilot's side works just fine. When I turn the intercom off, the problems disappears and the pilot can also transmit without problems. I put together a few audio samples, with the intercom turned off, what puts it into fail safe mode, in which the pilot's headset is directly routed through to the radio, as well as when the intercom is turned on, to show you the issuet: https://youtu.be/pvzOJ5lzZ_U

Using another headset has no effect, the jack is also properly grounded. I also thought about the ignition, if this would be it, we would however also have some static when we are just talking, the co-pilot would the most likely also have the some issues.

I wonder know, whether I need a new intercom or if I should look into something else? I strongly suspect that it is the intercom, but would also like to hear your opinion.

 

[flyingron]

The SPA isn't the greatest intercom, but I suspect that the issue is indeed some grounding issue rather than a fault in the intercom itself. The wiring between the intercom and your audio panel (or radios if you have no audio panel) is likely a culprit.

 

[German guy]

Hello Ron,

Thank you for you response.
I figured that if the problem would be in the audio panel or the connection between the intercom and the audio panel, the same effect would also occur when the co-pilot transmits, what is however not the case.

 

[flyingron]

I wouldn't be too sure of that. THere's no passthrough feature on the copilot's side on that intercom if I remember right. There's possible you're getting some crossfeed from bad ground.

 

[weirdjim]

Our Cessna 172 is equipped with 2x KX-155 radios and a Sigtronics SPA-400 intercom. When I turn the intercom on, the communication between pilots and co-pilot work flawlessly.

When I however try to make a radio call, the transmission contains a lot of noise.

What kind of "noise" are you talking about. If the intercom function isn't squelched during transmit, the combination (usually phase inverted or delayed) will cause garbley lousy audio. If it is a "static" noise, Ron gave you some ideas.

Try turning the sidetone as far down as it will go on one radio and see if the noise goes away.

Jim


.

 

[German guy]

It appears as whether Ron was right, we seem to have a ground issue.

Our A&P had an identical intercom in his shop, which we tried - same issue. Lots of noise when transmitting from the pilot's side. We figured that if static from the ignition or the generator would be the cause, we would have the same problem on both sides and also when talking over the intercom. Finally, our A&P came up with the idea that it might me caused by vibration. So, I pressed the PTT on the yoke, with the engine off, while he was wiggling on all the components and there is was - when we moved the yoke and the spiral cable which goes to the PTT, we were able to create our own 'static'. With the engine running, I was also able to get rid of the 'noise' by holding the cables in a certain position. Next, I need to figure out, whether it is the cable itself or its connection to the rest of the wiring, I am however glad that we were able to narrow it so far down.

@Jim: I uploaded an audio sample to Youtube and posted a link in my initial post.