Garmin GTN 750 Issue - Thoughts?

[Spectrepilot]

I have a Piper Saratoga TC II with a G500 and two GTN 750's. Recently whenever I key Com the radio with Com 1 and only Com 1 selected on "Mic", I get a RX alert on Com 2. I usually fly with 121.5 in Com 2 and I confirmed I am transmitting on both Com 1 and Com 2. I've gone into the audio panel settings and confirmed only Com 1 is set for mic, I do "monitor Com 2", and I don't see anything remarkable.

I took it to my local avionics tech and he thought it was the Garmin GMA 35 which is the communications interface unit. The new GMA arrive and we swapped it today and ended up with the same result. Somewhat flummoxed...avionics tech is leaning towards one of the GTN 750s being bad...but I am not convinced.

Any thoughts or suggestions?

Thank you!

 

[bnt83]

This happens on a lot of aircraft, I wouldn't throw money at it.

A transmitter extremely close to a separate receiver is not easy to keep from behaving like this.

 

[Jesse Saint]

With the com radios so close, it's not a surprise that there's some cross-talk when you key the mic on one. If you have Com 2 set to listen, and Com 1 set to listen and transmit, do you get any feedback when you key the mic?

Some of these radios have a transmit interlock, so when you key one radio the other one kills the receive function so there is no cross-talk. I don't know if the 750 is supposed to have that or not. If you get feedback when you do as I described above, then ask your installed about the transmit interlock.

 

[Spectrepilot]

I've had the airplane for several years and have never had this problem. The issue is I am now always broadcasting on both radios at the same time. I don't want to fly around trashing guard freq, for now I've set 123.45 but I don't think ignoring it is the way forward.

 

[Jesse Saint]

You said you are getting the RX annunciation on the second radio. That is Receive. If you were getting the TX, then you would be Transmitting on both radios, which is not what appears to be happening. The cross-talk will give you an RX on Com 2 when you talk on Com 1, and vice versa.

 

[Spectrepilot]

You said you are getting the RX annunciation on the second radio. That is Receive. If you were getting the TX, then you would be Transmitting on both radios, which is not what appears to be happening. The cross-talk will give you an RX on Com 2 when you talk on Com 1, and vice versa.

Yes, the other 750 shows "RX" next to he frequency but I have confirmed with other pilots at the airport that I am indeed transmitting on both radios at the same time.

 

[sourdough44]

A strange issues, both on the same freq for now? Or 2nd one off until needed?

Yes, negates reason to have 2.

 

[Ryan Klems]

Yes, the other 750 shows "RX" next to he frequency but I have confirmed with other pilots at the airport that I am indeed transmitting on both radios at the same time.

If you are actually being received on multiple frequencies, my suspicion would be that your GTN 750 that is COM 1 has an issue with it's COM transmitter and is essentially transmitting a much wider signal than it should that is bleeding in to other frequencies. If you pull the breakers to your 2nd GTN 750 and transmit on the #1 GTN 750, are you still received as transmitting on multiple frequencies? Does your shop have a spectrum analyzer? See this thread on BT for an example of what this looks like https://www.beechtalk.com/forums/viewtopic.php?f=21&t=179153

 

[Spectrepilot]

If you are actually being received on multiple frequencies, my suspicion would be that your GTN 750 that is COM 1 has an issue with it's COM transmitter and is essentially transmitting a much wider signal than it should that is bleeding in to other frequencies. If you pull the breakers to your 2nd GTN 750 and transmit on the #1 GTN 750, are you still received as transmitting on multiple frequencies? Does your shop have a spectrum analyzer? See this thread on BT for an example of what this looks like https://www.beechtalk.com/forums/viewtopic.php?f=21&t=179153

Ryan,

Thanks...you mirror my thoughts on the GTN but wanted to make sure it wasn't a setting I was missing. My Avionics tech has another 750 coming for a separate project and intends to swap that out one by one with both of mine to see if that fixes the issue. Greta input everyone, grateful.

 

[Jim Carpenter]

This shouldn't have any effect at all, but, any chance you have one unit set for 8.33khz frequency spacing, and the other for 25khz? (Just a total WAG, not based on scientific principles whatsoever).

 

[bnt83]

Backup and start from scratch. Is this happening in flight? When did you notice it happening? How long have you been familiar with the airplane?

If the non-selected com was actually keyed up and transmitting it would change the RX to TX. If the audio panel was causing this it would be pulling the mic key pin on the unselected GTN to ground.

Did avionics ring coaxes and swap GTNs around?

What make & model ELT is installed in the airplane?

What is the locations of the com 1 and com 2 antennas?

Transmitting on both simultaneously where one is indicating RX sounds impossible to me.

Transmitting on one com and the other com showing RX due to the transmission on one is pretty common. Not only that but if you have the audio panel set up to hear both coms the one in RX can spew trash into your headset when your transmitting on the other com.

 

[Spectrepilot]

This shouldn't have any effect at all, but, any chance you have one unit set for 8.33khz frequency spacing, and the other for 25khz? (Just a total WAG, not based on scientific principles whatsoever).

I have not gone into the startup setup menu...my avionics guy didn't think that was the issue because I have been flying this setup for two years with zero issues.

Backup and start from scratch. Is this happening in flight?

If the non-selected com was actually keyed up and transmitting it would change the RX to TX. If the audio panel was causing this it would be pulling the mic key pin on the unselected GTN to ground.

Did avionics ring coaxes and swap GTNs around?

What make & model ELT is installed in the airplane?

What is the locations of the com 1 and com 2 antennas?

Happens in flight and on the ground.

I understand what you are saying about RX and TX, regardless, broadcasts on COM 1 are heard on BOTH the frequency set in COM 1 and the freq set in COM 2...I have confirmed this three times and on multiple freqs.

Avionics has not done anything with the Coaxes or antenna's. He mentioned having a similar problem on another aircraft a year ago and it was a GTN 750 issue. He did not swap the GTNs around. We swapped the GMA35 with no impact.

I don't know the specific location of each antenna, will verify.

A complete listing of avionics including the ELT follows.

STEC 55X Autopilot Featuring Altitude Hold
Altitude Pre-Select with Vertical Speed Selector – (Controlled Through G500 PFD)
Garmin G500 PFD with SVT
Garmin GMA-35 Audio Panel with Marker Beacon & (6) Place Intercom
Garmin GTN-750 GPS –
Garmin GTN-750 GPS –
Garmin GI-106A #2 Indicator (Loc and GS)
Garmin GTX-330 Transponder with Mode S Traffic
BFG WX 500 Stormscope
Garmin GDL-69 XM Weather
Garmin GTN-800 Collision Avoidance System
PI EDM-930C Primary Flight Instruments Modification
406 ELT

2005 Piper Saratoga TC II. I have been flying it for two years with zero issues.

 

[bnt83]

Avionics has not done anything with the Coaxes or antenna's. He mentioned having a similar problem on another aircraft a year ago and it was a GTN 750 issue. He did not swap the GTNs around. We swapped the GMA35 with no impact.

So #1 TX and #2 RX = junk?

What about #2 TX and #1 RX = Junk? If not clearly swapping GTNs around and trying again is a logical step to make. If swapping GTNs around changes the junk to RX on #1 then borrow GTN to swap that suspect GTN out and re test.

Also, ELTs are known to cause com radio problems when TX is reflected out of the ELT, in essence, a bad ELT can reflect garbage when excited by your own ship's com radio. Its super simple to remove the ELT or just disconnect its coax for a troubleshoot. I haven't heard of a 406 causing issues but that doesn't mean they can't.

What other things have changed? Portable radio in flight bag. Cellphones?

I was blasted with trash once on both coms while on the ramp trying to leave my home base (Class C) 5 minutes of troubleshooting led me to bad USB charger in the cigar lighter.