GNC355 Weak Transmit

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guzziguy

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guzziguy
Just installed a GNC 355. I'm very happy with it, but the towers I speak to tell me that I am 'weak and unreadable' when I'm only about 10 miles away. The twist is that I can hear them just fine, they are coming in booming.

Any thoughts on this? Is there an internal setting I need to ensure is correct? Kinda stumped on this one.
Understand, this does not appear to be a VOLUME problem, but more a transmission problem.
Antennas are new.
 
I’d try adjusting the mic gain

B6A51764-7910-4B72-835E-A2BCE8E82643.jpeg
 
In case you don’t know how to get into setup....

C415A66B-97A0-46A0-A9C0-D04AE226AE8E.jpeg
 
Actually, YES, thank you. I DO know how to get into setup, but I am unsure what the proper mic gain setting should be? When I started up, it was at -12dB which I believe is max of one end.

Is there an 'optimal' setting?
 
Also:

Question: would this explain that I was perfectly readable at less than 5 miles but unreadable farther out?
 
guzziguy said:
Also:

Question: would this explain that I was perfectly readable at less than 5 miles but unreadable farther out?
Click to expand...
No, it would not. I missed that in your OP
 
A friend of mine installed the same unit and has the exact same problem. Shop concluded it is a bad unit and is trying to get a warranty replacement. Still waiting. I'll pass to him the info on mic gain.

We demonstrated it to the shop by using a handheld. Quality was fine when right next to the plane, but volume was noticiably lower (on the handheld while transmitting on the GNC355) by the time they were 200 yards distant. At 300 yards it was very faint. Switched antennaes, no difference. With his other com, everything was fine.
 
noahfong said:
A friend of mine installed the same unit and has the exact same problem. Shop concluded it is a bad unit and is trying to get a warranty replacement. Still waiting. I'll pass to him the info on mic gain.

We demonstrated it to the shop by using a handheld. Quality was fine when right next to the plane, but volume was noticiably lower (on the handheld while transmitting on the GNC355) by the time they were 200 yards distant. At 300 yards it was very faint. Switched antennaes, no difference. With his other com, everything was fine.
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No kidding!

I went down to the airport today with the installer and just got back now. Hi winds kept me from taking the plane to his shop so we couldn't measure the transmit power (or whatever you call it). All we could do was verify connections were good. They were. Also, Zero impedance on the coax from the radio to the antenna.

We then started messing around in the settings.
Mic Gain was at -12dB
Carrier Squelch was at +98%
Rx Squelch, can't remember the setting.

Our estimate is that Carrier Squelch was set too high and Mic Gain was set too low (though that wouldn't have anything to do with the problem at hand).

He said he had never heard of the term "Carrier Squelch" before but he inferred it has something to do with ouput/transmit. So we bumped it down to 40%.

We attempted to test these new settings, but nobody could hear us from the ground except the tower. This was not unusual.
I won't know if this fixed things until Thursday when I start my trip to Florida (from Oregon).

The good news is that I have a new friend ("over 50 and learning to fly" on YT) who installed a GNC355 and loves it. I asked him to check what his settings were set at on his radio. Perhaps if I just mirror his settings we can determine if the problem is just with settings or with the actual radio itself.

I did get an email back from Garmin which basically said, "we don't know, you'll have to send it in". They couldn't even tell me what the settings SHOULD be set at. Pfft...
 
Just as a data point, I installed my gnc355 and didn’t change any of those settings from the defaults and mine works just fine. I can pull what the defaults are if needed, just ask.
 
Salty said:
Just as a data point, I installed my gnc355 and didn’t change any of those settings from the defaults and mine works just fine. I can pull what the defaults are if needed, just ask.
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Yes, please! That would be great.
 
Great! Thanks. I see your carrier squelch is on 55%. That’s what I think the issue was, mine was at 98%.
I’ll use these settings and let you know. Thanks.
 
"Carrier Squelch" is certainly a misnomer of a term here if it affects transmit. In usual AM radio parlance it's the same as noise squelch, or just what we call squelch on receivers. It's in contrast to things like "tone squelch" where a (somewhat) inaudible tone is necessary to pass the received signal. Oddly, the maintenance manual is mum on what the setting means (though it mentions its existance).

The 225/255 doesn't have that setting (must the rx squelch setting and mic gain).
 
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Interesting. Please come back when you have a chance to test and let us know what difference it makes. Thanks.
 
We may simply swap the box too. Gonna bring it to the shop today for them to work through it. We'll see.
 
Just as a quick update, the smoke here in Oregon has kept me on the ground keeping me from testing these settings. Thanks again. Hoping this works! If it doesn't we're going to blame the box and swap it.
 
I hope the box swap works for you. My CFI installed a 355 in a plane and has had the same issue. Unfortunately they have swapped two units with same issue. Garmin claims it’s a software issue and they expect an update by the end of the year.
 
BrianM said:
I hope the box swap works for you. My CFI installed a 355 in a plane and has had the same issue. Unfortunately they have swapped two units with same issue. Garmin claims it’s a software issue and they expect an update by the end of the year.
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Thanks for this. Your post is timely. I am at the airport now meeting the avionics installer to test connections to the antenna etc. And make sure the basic stuff is good.
I will tell him your experience and Garmin a response.
Can you Email or post Garmin a response? We bought the unit from pacific coast avionics which is local to us.
Thanks!
 
Also: why replace the whole box if the problem is software and updates can be
 
Update:
spoke with Garmin engineering field service. According to them, they have NO RECORD of ANYONE complaining about this issue. Not even the instance where BrianM said that "Garmin says it's a software issue" above. Sorry BrianM, not sure what to tell you here.

So, I made the call to swap the box.
Consider:
1) MULTIPLE people have claimed the same issue of a weak transmit and have contacted Garmin about it
2) Garmin says they've heard no complaints from others
3) My installer has tried and confirmed that all grounding or other potential issues are NOT the cause
4) We have checked all things that Garmin recommended we check, "we did that, we did that too"
5) The engineer thought that it DID sound like an issue with the box itself.

We'll see what happens.
 
My friend's warranty replacement came in so I flew with him to have his unit swapped out yesterday. All works fine, now. Best of luck with yours.
 
guzziguy said:
Update:
spoke with Garmin engineering field service. According to them, they have NO RECORD of ANYONE complaining about this issue. Not even the instance where BrianM said that "Garmin says it's a software issue" above. Sorry BrianM, not sure what to tell you here.

So, I made the call to swap the box.
Consider:
1) MULTIPLE people have claimed the same issue of a weak transmit and have contacted Garmin about it
2) Garmin says they've heard no complaints from others
3) My installer has tried and confirmed that all grounding or other potential issues are NOT the cause
4) We have checked all things that Garmin recommended we check, "we did that, we did that too"
5) The engineer thought that it DID sound like an issue with the box itself.

We'll see what happens.
Click to expand...
This sounds very familiar. The avionics installer had multiple conversations with Garmin who told him the same thing initially that this was the first they had heard about a problem like this. After lots of back and forth, checking and triple checking the install he finally got the ‘it’s software that will be out by end of year’. Garmin won’t take it back. Owner is down with the hassle and wants to swap for a 175 with a separate com.

This isn’t my plane but I know the installer and owner well. It’s been a very frustrating mess for them and Garmin has been little help.
 
The first thing is to do an SWR check - make sure your coax and antenna are behaving. A simple DC continuity check may not always catch the issue. An antenna problem that causes a bad 'match' could cripple the transmitter output (most newer solid-state radios 'fold back' the output for self-protection with a bad load), but usually doesn't affect receive. You may need somebody with a suitable VHF wattmeter; find out if your transmitter is putting out what it's supposed to. I believe all aircraft VHF com radios have some standard transmit output power, around 10-15W?
From the description, it sounds like low RF output. Mic gain would only affect how modulated the signal is (or isn't) - and should not directly affect the radio's transmit range.
 
40YearDream said:
The first thing is to do an SWR check - make sure your coax and antenna are behaving. A simple DC continuity check may not always catch the issue. An antenna problem that causes a bad 'match' could cripple the transmitter output (most newer solid-state radios 'fold back' the output for self-protection with a bad load), but usually doesn't affect receive. You may need somebody with a suitable VHF wattmeter; find out if your transmitter is putting out what it's supposed to. I believe all aircraft VHF com radios have some standard transmit output power, around 10-15W?
From the description, it sounds like low RF output. Mic gain would only affect how modulated the signal is (or isn't) - and should not directly affect the radio's transmit range.
Click to expand...
Thanks for your comment and input. I just received notice from the avionics installer that the box was confirmed to be outputting better than 10w on transmit -> NOT a box problem. This is good news in that it nails down where the problem ISN'T.
Unfortunately, it points the finger at the installer, which I didn't want to do. But of course, it is what it is.
 
+1 for weak transmit on the GNC 355. Guzziguy, it would be quite helpful to know what you find out. Out of sheer curiosity, did your installer re-use an existing comm antenna/cable for your installation? My installation re-used the comm antenna, as well as RG58 cabling. I think Garmin calls for RG400, and maybe the thing is picky about it.

To be specific, my problem seems intermittent. I did get a "loud and clear" from the tower on the initial radio test. Later, after the tower closed, I taxied out to the runway and another plane on the field said they could "barely hear me". No problem when switching over to the KX-155 radio.
 
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ichiphead said:
+1 for weak transmit on the GNC 355. Guzziguy, it would be quite helpful to know what you find out. Out of sheer curiosity, did your installer re-use an existing comm antenna/cable for your installation? My installation re-used the comm antenna, as well as RG58 cabling. I think Garmin calls for RG400, and maybe the thing is picky about it.

To be specific, my problem seems intermittent. I did get a "loud and clear" from the tower on the initial radio test. Later, after the tower closed, I taxied out to the runway and another plane on the field said they could "barely hear me". No problem when switching over to the KX-155 radio.
Click to expand...
I'm sorry I don't know what class of cable he used, but I DO know that he 'suspected' that the cable was bad, so he swapped it out altogether.
Theory: When he installed the box and using the original coax, his creation of a new connector was less than perfect and it was THIS that was causing the issue (h/t Baker Avionics). When he confirmed the box was operating normally, this implicated an installation error, he investigated it and found his own mistake. Instead of fessing up, he blamed 'bad coax' or 'bad insulation' or 'kink.' He replaced the coax altogether and that is that.
Theory 2: As you suggest, he used the wrong class of coax in the installation. Comm1 was originally a KX170b with coax going to a new antenna. I can't for the life of me remember if my A&P changed the coax when he installed the new Comm1 antenna. Nevertheless, it IS possible that the coax used for the KX170b was of a lesser grade than what is necessary for the Garmin 355 and this was merely overlooked when the 355 was installed.
In both instances, replacing the coax as he did SHOULD resolve the issue. I will be heading back to PDX to pick up the plane in a couple weeks and only then will I be able to test it in the real world.
 
guzziguy said:
I'm sorry I don't know what class of cable he used, but I DO know that he 'suspected' that the cable was bad, so he swapped it out altogether.
Theory: When he installed the box and using the original coax, his creation of a new connector was less than perfect and it was THIS that was causing the issue (h/t Baker Avionics). When he confirmed the box was operating normally, this implicated an installation error, he investigated it and found his own mistake. Instead of fessing up, he blamed 'bad coax' or 'bad insulation' or 'kink.' He replaced the coax altogether and that is that.
Theory 2: As you suggest, he used the wrong class of coax in the installation. Comm1 was originally a KX170b with coax going to a new antenna. I can't for the life of me remember if my A&P changed the coax when he installed the new Comm1 antenna. Nevertheless, it IS possible that the coax used for the KX170b was of a lesser grade than what is necessary for the Garmin 355 and this was merely overlooked when the 355 was installed.
In both instances, replacing the coax as he did SHOULD resolve the issue. I will be heading back to PDX to pick up the plane in a couple weeks and only then will I be able to test it in the real world.
Click to expand...

Thanks so much for the info, Guzziguy. I'm sure this will be helpful to many. My issue may have just been another plane giving me a bad radio check. I haven't been able to reproduce the weak transmit since that night, but I'm keeping an eye on it, and if it rears itself again, I think I'll toss the antenna cable and replace it with RG400.
 
This has been a valuable discussion for those of us considering the GNC-355. I am a bit curious as to what the proposed Garmin software update is intended to fix? Please provide some follow-up reports as time goes by. Thanks!
 
Rob58 said:
This has been a valuable discussion for those of us considering the GNC-355. I am a bit curious as to what the proposed Garmin software update is intended to fix? Please provide some follow-up reports as time goes by. Thanks!
Click to expand...
Second that. I am also looking at a GNC 355 and watching this thread is helpful in the event we run into issues.
 
According to Garmin, when I spoke with the engineering rep, he said he had never heard of such a problem. This of course may have been both true and untrue. He stated that in their morning meetings, nobody had mentioned it and it was not in their library of known issues. However, either on this board or on the Piper board, there are people posting that they have discussed their weak transmit issues with Garmin. So...???
I will be picking the plane up in a couple weeks. I'll keep everyone updated.
 
I would try a radio check using a hand mike or different headset to determine if there is a modulation issue. AM broadcast readability relies on output power plus modulation. Next I would check output at coax going to antenna using a watt meter. Even a low power transmitter should be clearly received line of site at 10 miles.
 
My avionics installer sent me a picture of the Comm settings from a known-good GNC-355. I adjusted my settings to match these and the tower could finally hear me just fine!
I then felt confident to depart on my OR-FL trip.

After adjusting the Comm software settings and an entire day of flying, the next morning the tower could no longer hear me

Because I had success the day previous by adjusting the software settings (gain, db, etc) I wondered, "did the settings revert BACK to the old and no-good settings?"

So I went into the Comm settings page of the unit and sure enough, the Mic Gain had reverted back to -12db!!!! All other settings were as they were previously set/corrected the previous day. I then adjusted the mic gain up to 0 or +6 db and boom, the tower could hear me again.

This same scenario played out multiple times (at least 2 or 3) for the remainder of the trip!

I am posting this here for others to learn from in case they have this same issue.

I am hopeful that when I obtain updated databases a sw update will also come and this issue may be resolved. Has anyone else experienced this where settings go back to some incorrect default? How was this corrected?
 
Any chance you can post the picture of the Comm settings that provided the solution to the transmit signal?
 
It turns out it was the mic gain that was the issue. Anything set at zero or above was the trick. I have mine now set at +6. +12 however was TOO much and seemed to be overdriving the audio.

I tried uploading the pic, but it was too big. Sorry.
 
guzziguy said:
My avionics installer sent me a picture of the Comm settings from a known-good GNC-355. I adjusted my settings to match these and the tower could finally hear me just fine!
I then felt confident to depart on my OR-FL trip.

After adjusting the Comm software settings and an entire day of flying, the next morning the tower could no longer hear me

Because I had success the day previous by adjusting the software settings (gain, db, etc) I wondered, "did the settings revert BACK to the old and no-good settings?"

So I went into the Comm settings page of the unit and sure enough, the Mic Gain had reverted back to -12db!!!! All other settings were as they were previously set/corrected the previous day. I then adjusted the mic gain up to 0 or +6 db and boom, the tower could hear me again.

This same scenario played out multiple times (at least 2 or 3) for the remainder of the trip!

I am posting this here for others to learn from in case they have this same issue.

I am hopeful that when I obtain updated databases a sw update will also come and this issue may be resolved. Has anyone else experienced this where settings go back to some incorrect default? How was this corrected?
Click to expand...
This is exactly what is happening to the unit a friend of mine is having trouble with. Still being told a software update is coming to fix it.
 
guzziguy said:
According to Garmin, when I spoke with the engineering rep, he said he had never heard of such a problem.
Click to expand...

Is Garmin hiring out-of-work former BMW Motorcycle dealer service managers? Pretty sure their entire training program is devoted to being able to say, "I have never seen that before!" with a straight face...
 
BrianM said:
This is exactly what is happening to the unit a friend of mine is having trouble with. Still being told a software update is coming to fix it.
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I spoke with Garmin today. His recommendation was that the problem must be that I was running an older software version and that an update to the latest version would likely fix this.
I checked. It is running the latest software version (3.1). (well, crap)
He recommended I bring it to an avionics shop or Garmin dealer to bench test that the settings wouldn't 'stick' after a power cycle (which I already know, and it's intermittent anyway). And then to send it back for warranty replacement!

What a pain.
 
Well....you can try cleaning your mic jacks and plugs just to see if that is your problem. That is easy and costs almost nothing. I did not read all the above posts, but try a different headset or hand mic and listen in your headset while you transmit to see if you can hear a difference in modulation. Try to wiggle your mic cord where it plugs into the jack to see if you have a bad mic cable or plug.
 
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