Can receive on comm 1 but can’t transmit on comm 1. Can transmit and receive on comm 2. GTN750

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Jamie Kirk

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JamieK
So on the ground I can transmit and receive on comm 1 and ground and tower can hear me just fine.

However once up in the air comm 1 receives perfectly but I can’t transmit. On the GTN750 I see TX when I push the PTT. Tower doesn’t receive muffled or static transmitting, they get nothing. Like no one is even transmitting yet I am and the GTN750 says I am.

I program tower frequency to comm 2 and swap mic to comm 2 on the GTN750 and I transmit perfectly clear.

I changed headsets. I

plugged headset into copilot and used that PTT to transmit and the GTN750 again says TX for transmitting but nothing goes out.

I went to 3 different airports, same result.
 
Are both comms using the same antenna? Unlikely, but possible.
 
Cap'n Jack said:
Are both comms using the same antenna? Unlikely, but possible.
Click to expand...

I don’t know to be honest, how can I check? I am not missing any antenna and none have damage.

If the antenna wasn’t working then why would I be able to receive perfectly fine but not transmit?
 
Jamie Kirk said:
I don’t know to be honest, how can I check? I am not missing any antenna and none have damage.

If the antenna wasn’t working then why would I be able to receive perfectly fine but not transmit?
Click to expand...
Most planes I've seen have an antenna for each transmitter, so you should have two antennas. If this is the case, COMM 2 is transmitting on a separate antenna.

My thought now is that maybe the connection to the antenna is flexing during flight, causing a poor connection for transmission.
This is only my opinion, people like @weirdjim know a lot about this and hopefully he'll chime in.
 
Cap'n Jack said:
Most planes I've seen have an antenna for each transmitter, so you should have two antennas. If this is the case, COMM 2 is transmitting on a separate antenna.

My thought now is that maybe the connection to the antenna is flexing during flight, causing a poor connection for transmission.
This is only my opinion, people like @weirdjim know a lot about this and hopefully he'll chime in.
Click to expand...

I have 1 large antenna on top and a second small on top of plane. Third antenna is the ELT on the bottom of the plane.
 
Probably a little different... My 530 would not transmit but the TX would light up when PTT was pushed. I slid my buddies 530 in and it worked perfect. I sent my 530 back to Garmin for rebuild and now it works $1320 later...
 
pigpenracing said:
Probably a little different... My 530 would not transmit but the TX would light up when PTT was pushed. I slid my buddies 530 in and it worked perfect. I sent my 530 back to Garmin for rebuild and now it works $1320 later...
Click to expand...

That’s the consensus between those of us at the airport trying to brainstorm.

Issue is I have my checkride Friday that I’ve been waiting 2 months for
 
Jamie Kirk said:
If the antenna wasn’t working then why would I be able to receive perfectly fine but not transmit?
Click to expand...
This may happen when you have a failing antenna or cable to the antenna. It is much easier to receive than to get power out when transmitting.

I had this occur with one of my comms. In my case, upon closer evaluation, I could transmit but only for very short distance. It never lost reception. We tried a handheld comm to see if my transmit could be received. Turned out my transmit was only received for a couple hundred yards. I got a SWR meter and placed it between the comm and its antenna. The SWR was very high. The RG400 between the comm and the antenna had failed due to repetitive stress secondary to bouncing in turbulence right at the connector to the antenna. The RG400 appeared to be in perfect condition on visual inspection but had internal cable damage.

Get a SWR meter and check your reading. In the meantime, stop transmitting on that comm; if you have power being reflected back to your transmitter due to failing antenna/cable it may damage your transmitter.
 
FORANE said:
This may happen when you have a failing antenna or cable to the antenna. It is much easier to receive than to get power out when transmitting.
Click to expand...

This. Another possibility, less likely, is that the transmitting section inside the radio is bad. That would account for the other poster's $1320 repair. (Man, I'm in the wrong line of work!) If the transmitter is only making fractional power, you would be able to talk on the ground, but not at any distance from the airport. Receive would still work fine.
 
FORANE said:
This may happen when you have a failing antenna or cable to the antenna. It is much easier to receive than to get power out when transmitting.

I had this occur with one of my comms. In my case, upon closer evaluation, I could transmit but only for very short distance. It never lost reception. We tried a handheld comm to see if my transmit could be received. Turned out my transmit was only received for a couple hundred yards. I got a SWR meter and placed it between the comm and its antenna. The SWR was very high. The RG400 between the comm and the antenna had failed due to repetitive stress secondary to bouncing in turbulence right at the connector to the antenna. The RG400 appeared to be in perfect condition on visual inspection but had internal cable damage.

Get a SWR meter and check your reading. In the meantime, stop transmitting on that comm; if you have power being reflected back to your transmitter due to failing antenna/cable it may damage your transmitter.
Click to expand...

Thank you. Plane is coming apart next weekend for annual so I’ll add this to the todo list.
 
Stephen Poole said:
This. Another possibility, less likely, is that the transmitting section inside the radio is bad. That would account for the other poster's $1320 repair. (Man, I'm in the wrong line of work!) If the transmitter is only making fractional power, you would be able to talk on the ground, but not at any distance from the airport. Receive would still work fine.
Click to expand...

GTN750 was installed last June so should be a warranty issue. But I’m assuming they won’t cover the labor testing it. I’ll call Garmin Tuesday.
 
Resolved!

Little over 4 hours checking ground on antennas, chasing cables and plugging in meters to test signal strength.

Avionics shop who installed all new avionics less than a year ago at a cost of more than $70k failed to attach coax from radio to antenna completely on the back of the radio. What a difference a 1/4 of a turn locking the connections in does.
 
This is what I expected. Not airplanes, but I spent 10 years doing IT and pulling cables. Only twice was the problem inside the computer.​
 
Jamie Kirk said:
Resolved!

Little over 4 hours checking ground on antennas, chasing cables and plugging in meters to test signal strength.

Avionics shop who installed all new avionics less than a year ago at a cost of more than $70k failed to attach coax from radio to antenna completely on the back of the radio. What a difference a 1/4 of a turn locking the connections in does.
Click to expand...
A bit of a surprise to me, since I would have looked at the antenna end first. The lesson for me is to check the connections first, as your shop did. IOW, start with the basics.
If I dived in with an SWR meter, I might not have noticed the connection, found no problems, and have been puzzled upon putting it back together to find it worked fine.

Thanks for letting us know the outcome!
 
Cap'n Jack said:
A bit of a surprise to me, since I would have looked at the antenna end first. The lesson for me is to check the connections first, as your shop did. IOW, start with the basics.
If I dived in with an SWR meter, I might not have noticed the connection, found no problems, and have been puzzled upon putting it back together to find it worked fine.

Thanks for letting us know the outcome!
Click to expand...

First we checked the antenna and found they didn’t remove the paint for the mounting ground plate. Instructions stated it must be secured directly to a non painted surface. Fixed that.

They had the entire plane apart but didn’t run a new coax cable from the radio.

Then we started testing signal strength with HAM radio equipment my friend has this is where we found the antenna was fine but when we plugged into the coax and keyed the mic it barely moved the signal meter.

Then I get to play contortionist and got under the panel to plug the coax from the tester into the radio and bingo problem found.
 
Since there were two antennas, one on top and the other on the bottom, we had to know which one was comm 1.

We used the standard electrical plug tester and when you put it near the antenna and key the mic it lights up.

Luckily it was the bottom antenna since it was much easier to access.
 
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