Comm Radio Transmits in Air, Not on Ground

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
Have an SL-40 in my T6. On a recent trip home I noticed that when on the ground, I could not transmit on the radio. FBO at a fuel stop said they heard me keying the mic, but that was it. When attempting to transmit, there was no side tone, couldn't hear anything. While in the air, however, radio transmits just fine.

Any thoughts on what would cause that?
 
Sounds more like a problem with your intercom or audio panel (or the headphone jacks) than the radio itself. Have you tried headphones on the other side of the plane? Can people hear you on the intercom when you are in a situation the radio appears not to work? Have you tried other radios or the "PA" feature of the audio panel?
 
flyingron said:
Sounds more like a problem with your intercom or audio panel (or the headphone jacks) than the radio itself. Have you tried headphones on the other side of the plane? Can people hear you on the intercom when you are in a situation the radio appears not to work? Have you tried other radios or the "PA" feature of the audio panel?
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It's a T-6. Only one set of jacks in the front pit. There are jacks in the back seat, but nothing plugged in.

No audio panel. Just a single SL-40 comm radio.

Intercom (separate push to talk) works fine in air and on ground.
 
Weird. Is the RPM too low on the ground to generate adequate voltage to transmit?

Got a friend with a sl-40 you could slide in and see if it follows the radio or not?
 
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Is the wind in flight helping the antenna to obtain a better ground?
 
did you wire it to a squat switch? :eek:
 
eman1200 said:
did you wire it to a squat switch? :eek:
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I had a goofy problem with my XM Radio (GDL69A) that only worked intermittently. One day I realized it only worked when the gear was down. One day I sat in the hangar, jacks ready to try to figure out what was going on. I had the service manuals for the audio panel, the GDL69A, the MX20 (which controlled it), and test equipment lined up. I open up the GDL69A manual and the first thing in the installation is that it says if you have a gear horn, you need to wire it to one of the six mute terminals they give you (three active high, three active low). Ah shoot. The Navion gear horn works by going from the battery bus through the throttle switch to the horn to the gear switch to ground. There's no single point you can connect to that lets you know the gear horn is sounding. Sure enough, they just connected it to the gear switch side of the horn. I cut the dang wire and everything was happy after that.
 
I don't think it has anything to do with the way it is wired. Nothing has changed since I've owned the airplane - the problem just developed during a recent trip down to Jacksonville.

After landing at CRG, the first transmission to Tower after clearing the runway was fine. Then when contacting Ground, it sounded wonky - no side tone when talking, BUT the Ground controller was able to hear me. Two days later, same thing on departure. Sounded wonky, but it was good enough for controller to hear me and I departed normally. Then when landing at my fuel stop was when it stopped working completely on the ground and again when on the ground at home.

So whatever happened, started after landing at Jacksonville and then got worse.
 
FORANE said:
Weird. Is the RPM too low on the ground to generate adequate voltage to transmit?

Got a friend with a sl-40 you could slide in and see if it follows the radio or not?
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I would think it should be able to transmit off of the battery alone.
 
I’m guessing antenna problems. When on the ground the impedance changes due to proximity, just enough she ain’t working.

I’d pull the antenna loose and look for corrosion. Wouldn’t take much. And the kind of thing that could have just been getting progressively worse to where she finally just won’t work.

An SWR meter will tell you for sure, but those aren’t common it seems. Then start tracing your way from the antenna to the radio looking for problem connections or maybe a chaffed or frayed antenna transmission line.
 
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Maybe find a local ham with an antenna analyzer. The hobby versions are pretty affordable these days.

No idea if this one is reasonable or not, appears to be a "NanoVNA" clone. They can work way better than you'd think, but still can be tricky to troubleshoot intermittent ground/wiring problems, as you already know.

 
I had a bad mic jack on the RV10 that caused me a month’s worth of grief before I just cut it out and soldered in a new one. The jack LOOKED fine but went from “transmits fine” to intermittent to “no workee” in a matter of days.
 
Fearless Tower said:
...the first transmission to Tower after clearing the runway was fine. Then when contacting Ground, it sounded wonky - no side tone when talking, BUT the Ground controller was able to hear me. Two days later, same thing on departure. Sounded wonky, but it was good enough for controller to hear me and I departed normally. Then when landing at my fuel stop was when it stopped working completely on the ground and again when on the ground at home.

So whatever happened, started after landing at Jacksonville and then got worse.
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Tools said:
I’m guessing antenna problems.
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Will antenna problems cause issues with sidetone? I wouldn't expect that.

I'm guessing it's a problem with the radio itself or the wiring inside the cockpit. There's a clue in how the performance degraded over a short period rather than failing instantaneously. Not familiar with the SL-40 innards, but maybe there's something going on on the transmit side...capacitor leak? Internal connection point gradually working its way loose? Something like that took some time to "spread" across a circuit board or multiple wires?

Sliding it into another plane to see if the problem follows the box would be the "easiest" option (assuming you can find another plane to swap it with). Next stop would be an avionics shop for a bench test.

But the "works in the air but not on the ground" points against a problem with the radio itself (unless maybe there's something loose that's affected by acceleration at takeoff vs. deceleration at landing) so I wonder if it could be wiring inside the cockpit. Are all the cable connectors and pins tight and solidly mated?

Not sure how accessible the wiring is, but can you visually scan for potential issues, especially for things that change between ground and air ops? Example: Do you land with the yoke pulled way back, and often taxi that way too? Does yoke travel cause any of the radio wiring to move (is there a PTT switch? is it "pulling" on a cable bundle during a portion of yoke travel?). Could the wires have chafed and worn through such that a short is occurring during portions of yoke travel? Etc.
 
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still at CRG? there's an avionics shop there...Bragg Avionics
 
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