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Tx on ground, not in air

gnuse

Well Known Member
I am certainly no expert on these matters, so I hope the collective knowledge of this group can help guide me to a solution.

On a recent flight I realized that my RV-8's comm radios, a Garmin 430W and a Bendix/King KX135 with a PMA4000 audio panel, were not transmitting while airborne. They work flawlessly on the ground. Occasionally I will get some static filled partial transmission while airborne. Receiving is a good and clear as ever, on the ground or airborne.

Switching from Comm 1 to Comm 2 sounds about the same. The Tx indicator on the 430 and the KX135 is missing or blinking on and off depending on if the radio is actually transmitting.

I started to find the problem today within my own limitations. I replaced the PTT switch in my Infinity grip without any effect. I replaced my Bose Lemo plug headset with a D/Clark in the standard plugs with no effect.

I tried various RPM settings in the air from 1200rpm to 2400rpm with no effect noted.

I could really use some help, as the plane is useless without the ability to communicate.
 
Both radios do not work in the air? Does each radio have a separate antenna?

Have you done any mods of late?

Carl

No mods excepting a smoke system. Both radios do not transmit in the air, they do receive in the air.

I am working on the answer to your second question. There are two bent antenna on bottom of fuselage, so I think they each have their own antenna.
 
If reception is OK then that would seem to indicate that the antenna is OK.

It is strange that transmission is OK on the ground, but not in the air! An obvious thing to check is the squelch adjustment on the audio panel.

Otherwise it could be the connections to the PMA4000, or the unit itself, since both radios are behaving the same. I would try re-seating the plugs to the PMA4000 in case one of the connections has degraded, but strange if this is the case only in flight.

I've been chasing my own PTT intermittent problems for a couple of weeks, now partially resolved by process of elimination and testing connections. It is annoying that a small thing can "ground" a plane!
 
Today I did take the PMA4000 out of the panel and spray both connectors with DeOxit. I got the airplane all back together, however was unable to test fly it.

I did reread the manual this evening and read that there is a mode where the intercom can be bypassed by turning it to OFF and pulling its circuit breaker. For sure that will be the next test.

I will check the squelch as you suggest.

It is strange that this just cropped up. I'm just hoping someone helps me get back to flying it.........after all, it isn't a NORAD J3 Cub. ;)
 
Glad you tried different headsets. If any of them are are stereo headsets make sure to slide their selector switch to mono as the PMA4000 is a mono unit. While not a sophisticated test to try different make/model of headsets it's super simple and a fast way to rule out headsets.
 
Glad you tried different headsets. If any of them are are stereo headsets make sure to slide their selector switch to mono as the PMA4000 is a mono unit. While not a sophisticated test to try different make/model of headsets it's super simple and a fast way to rule out headsets.

As I stated initially, I tried my Bose and a David Clark today with the same result...no TX in the air but excellent on the ground.
 
transmit testing

How are you testing to see if it transmits on the ground but not in the air?

Can you test on the ground during a runup with the canopy open to see if it's not something to do with the squelch?

Do you hear the "sidetone" both on the ground and in the air when you transmit?
 
Check that all of your shield wire grounds are good at the intercom plug. On my PM3000 I daisy chained my shield grounds via solder sleeves, and one of the wires pulled out of a not-fully-melted solder sleeve. This interrupted my shielded wire ground chain at the intercom plug.
 
If reception is OK then that would seem to indicate that the antenna is OK. SNIP...

Not at all typical. As we only use ears to determine reception, a good or bad antenna can sound similar on reception.

After you eliminate the obvious things, then I assume something is happening to the antenna or feedline when in the air. I would next do some YES/NO testing. For example:
- The best thing to do is use an antenna analyzer to find truth. All EAA chapters should have one in the shared tool bin, if not perhaps you can convince them to do so. I check every antenna and every feed line - and have found issues long before the first panel power up. Here is a good, inexpensive analyzer: https://www.dxengineering.com/searc...toview=SKU&sortby=Default&sortorder=Ascending
- If you cannot get an analyzer, connect a temporary antenna to the radio and see if transmit works. This can be a simple dipole.

Carl
 
Not at all typical. As we only use ears to determine reception, a good or bad antenna can sound similar on reception.

After you eliminate the obvious things, then I assume something is happening to the antenna or feedline when in the air. I would next do some YES/NO testing. For example:
- The best thing to do is use an antenna analyzer to find truth. All EAA chapters should have one in the shared tool bin, if not perhaps you can convince them to do so. I check every antenna and every feed line - and have found issues long before the first panel power up. Here is a good, inexpensive analyzer: https://www.dxengineering.com/searc...toview=SKU&sortby=Default&sortorder=Ascending
- If you cannot get an analyzer, connect a temporary antenna to the radio and see if transmit works. This can be a simple dipole.



Carl
Thanks, Carl.
The problem is that the no TX is only happening in the air. Both radios receive and transmit perfectly on the ground - with engine off, taxiing out, and during run-up. Both radios do not TX, but do RX in the air.

The infrequent instances of TX that I got in the air was scratchy/static filled to both me and the friend on the ground with a handheld. It was apparent to me the quality wasn't good, not only in headset but also because of the flickering of the TX light on the radio.
 
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Glad you tried different headsets. If any of them are are stereo headsets make sure to slide their selector switch to mono as the PMA4000 is a mono unit. While not a sophisticated test to try different make/model of headsets it's super simple and a fast way to rule out headsets.

If this could be a cause, I am sure that there is a possibility that the Bose might have gotten switched inadvertently. I will check how to do that and confirm it is set to MONO.

Would this cause the two Comm radios to fail to transmit?
 
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How are you testing to see if it transmits on the ground but not in the air?

Can you test on the ground during a runup with the canopy open to see if it's not something to do with the squelch?

Do you hear the "sidetone" both on the ground and in the air when you transmit?

I am testing on the ground and then taking off. I'm not sure I can answer your question other than that.

The sidetone is silent when not transmitting, as is the TX light missing on the Comm radios. Airborne, when activating the PTT switch on the stick, there is no sound in the headset -- when airborne. On the ground, I can hear it click on and off with PTT switch. Of course the TX light shows also.
 
Really? I don't understand your point.

You did not answer the question. I know how frustrating these things like this can be. But why would any of us spend the tine to help with meaningless questions? My answer to your question is, "Really". Let's try all the troubleshooting we can here. Point being an automotive wax can cause static build up when you are moving through the air that can interefere with radio antennas. Only happens in flight. So IF you had a good wax job lately that could cause issues that only manifest themselves when in the air. So IF you have had any kind of exterior paint protection done lately that might be a problem. As a rule make sure a quality aircraft wax with anti-static specifcation is used. But we still don't know if a wax finish is the case. Is this a high probability? Who knows.
 
Thanks, Carl.
The problem is that the no TX is only happening in the air. Both radios receive and transmit perfectly on the ground - with engine off, taxiing out, and during run-up. Both radios do not TX, but do RX in the air.

The infrequent instances of TX that I got in the air was scratchy/static filled to both me and the friend on the ground with a handheld. It was apparent to me the quality wasn't good, not only in headset but also because of the flickering of the TX light on the radio.

New information! The flickering light tells me you have intermittent power going to the radio. First look for a loose connection on the associated breaker. This is a fairly common cause for your “flickering TX light”.

Carl
 
X two radios ?

Can’t say how things are wired, but issue on OP was both radios. Common item(s) Audio Panel and altitude, airspeed, since does not happen on the ground. Multiple headsets, PPT , jacks, radios, and assume antennas X2 have been used in troubleshooting. Happens only once in flight. I say the audio panel in some way is the issue. Now I want to know why!
 
Thanks, Carl. I miss worded the TX light thing, it is just an TX or RX indication on the screen of the radio, not an actual light.

It appears that sometimes the simplest procedure can solve a problem. I know for a fact that the solution to this problem has been "the" solution for other people with electrical issues. A hangar neighbor stated that we really need to move that procedure up to the top of the "to do" list.

Another test flight, after the reseating of the two connectors on the back of the the PMA4000, and the problem is fixed. I think the working on the ground vs not working in flight confused the issue.
 
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