Intermittent transmit issue - both radios

[SunnyBeach]

Hi all. Looking for suggestions here on an intermittent transmit problem with my radios.

First off - I'm very new to all this. I have a 63 Cherokee 180 I purchased in March. Everything has worked really well with it. I had its annual done in November - no real issues - tires/brakes, that kind of thing. It has a KM24 audio panel, a KX 155 with GS, and a KX 170B w/208.

About a month later, I experienced a transmit problem with my radio stack. I can receive just fine, but nothing is heard when I transmit, regardless of which radio I use. I tested both pilot and co-pilot jacks. I tested with different headsets. Nothing.

Had some trouble arranging getting it looked at. Finally arranged to take it over to another FBO. Of course, when I did that, it was transmitting fine. And continued to for the next flight or so. Next time I went out, same transmit problem.

I'm headed back to the shop, but I'm wondering if you guys have any suggestions where to start looking. This feels like one of those elusive issues that could haunt me for a long time.

Thanks!

 

[Rgbeard]

I had that issue when I bought our current plane.

Fortunately, I knew I had a total avionics replacement happening shortly after purchase, including the audio panel. My slash-and-burn approach "fixed" it. Haha.

Since it's happening to both, I suspect it's a loose connection between the ptt or mic and the audio panel, imho. You have to look at the part of the circuit that's common to both radios, not unique to each, and that means pre-audio panel.

 

[Dan Thomas]

Since it's happening to both, I suspect it's a loose connection between the ptt or mic and the audio panel, imho. You have to look at the part of the circuit that's common to both radios, not unique to each, and that means pre-audio panel.

That right there. The push-to-talk circuit is the most likely suspect. The switch itself might be bad.

What sort of switch is it, I wonder? One of those velcroed-on things, which often had two microswitches in them, either of which could fail? The cables from those are also famous for breaking internally where they flex at the switch and jacks. Or a built-in switch that could be 40 years old?

 

[SunnyBeach]

So the issue occurs whether you are using the pilot PTT switch or the co-pilot. So I don't think the switch itself is bad. I assume they come together somewhere though, so that is the first point of commonality to look at.

The switches are mounted to the yolks - not velcroed. Beyond that IDK.

 

[Chris Ellington]

I had exact same problem in a Cherokee I just bought with an old King KN24 audio panel and two KX-170b radios I brought home just before Christmas. First two times I flew it to buy it and later for insurance checkout the radios both transmitted out, but I found the side tone weak in my headset when I transmitted.

On flight home by myself , neither radio would transmit out. I had a spare KN24 audio panel on my shelf at home from an old plane and tried the simplest approach by switching it out. Both radios started transmitting with strong side tone. I’ve heard the KN24 panels are known to go bad if the radio selection knob is never used to switch to second radio.?? It should be an easy check for a shop as they are usually easy to find laying around.

hope you can figure out quickly. I suspected my PTT or audio connection between intercom and audio panels too, but luckily that wasn’t the root cause.

 

[SunnyBeach]

Thanks. I'm going to look into all this, and I'll let you know. Thank you!