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Audio noise issue questions

William

Well Known Member
Hi everyone I have a plane I?m working on that just under went a complete G3x panel upgrade with all accompanying boxes including a gtr200 on com1, Sl30 on com2, gtx375, and a pma7000b audio panel. The issue we are having is some humming noise on com1 gtr200. The noise becomes most severe when the auto pilot is engaged. Also is it normal for the gtr200, Sl30, and pma7000b to power up even when all of their grounds are disconnected? I?ve found this happening and I?m wondering if this is a sign of a ground loop or the audio lines shielding completing the grounds?

Some thing to note:

- This aircraft has a elevator trim motor that uses about 5amps in operation so it was required that we install a safety trim servo booster as we are using the auto trim function on the G3x autopilot.
http://www.tcwtech.com/Installation instructions model st-1-12v-10a-bstr_rev1.1.pdf

- the aircraft is 24 volts system

- com2 SL30 is crystal clear with the autopilot engaged or not

- com1 gtr200 has a humming noise that is most profound when the autopilot is engaged


Thanks for any help
Bill
 
Many, if not most, avionics are grounded internally to their cases which are grounded to the airframe.
So yes, it would be normal for avionics to work with the ground power wire disconnected, although not a good idea.
Try disconnecting the following GTR200 input pins to see if the noise goes away:
9 AUX MONO IN 2
18 MUSIC IN RIGHT
19 MUSIC IN LEFT
32 AUX MONO IN 1
 
Many, if not most, avionics are grounded internally to their cases which are grounded to the airframe.
So yes, it would be normal for avionics to work with the ground power wire disconnected, although not a good idea.
Try disconnecting the following GTR200 input pins to see if the noise goes away:
9 AUX MONO IN 2
18 MUSIC IN RIGHT
19 MUSIC IN LEFT
32 AUX MONO IN 1
Joe,

This aircraft has an audio panel, so the internal intercom in the GTR 200 should not be enabled, and thus there should be no audio connections to any of these audio inputs, but it will be good to see Bill confirm this.

The GTR 200 should be quiet except when receiving on the tuned frequency (squelch broken, RX active), so some additional clarification of what exact conditions are present when "humming" is heard would be useful. Is something in the aircraft causing the RX squelch to break and the RX indicator to remain on?

Thanks,
Steve
 
The noise bill is speaking of was present only in Com 1 when the Garmin 480 was installed. We did a complete avionics upgrade to all Garmin products EXCEPT kept the PS engineering PMA7000BT and currently using a Com1 Garmin GTR200 and Garmin SL40 com 2.

Com 2 is perfect, but com 1 is still bad. Can hear the autopilot intermittently and mostly though the GTR200 RX sounds like it?s in a well. Also the transmissions from other aircraft sounds like there is a screeching sound when they speak.
Wish I could delineate the sound better than to say it is only in com 1 and is like screeching noise when others RX.

Bill is doing an amazing job and has basically rewired the entire panel and troubleshot things more times than I would have ever been able to. He?s doing a fantastic job!

Today however, I called PS engineering and the guy basically made me feel stupid, told me that I had done everything wrong, and that there was absolutely no problem with his product and clearly it is a problem with our installation.


Thinking it?s about time to replace the PMA7000BT and go full Garmin 😁
 
Well, the obvious test is to get a pin puller, and swap the audio input pins at the audio panel. e.g., com 1 is now the SL40, com 2 the GTR200. If com 1 (now the SL40) now has the problem and com 2 (now the GTR) is okay, then the problem is in the audio panel. If the problem is now on com 2, then the issue is the GTR and or its set up or wiring.
 
Many, if not most, avionics are grounded internally to their cases which are grounded to the airframe.
So yes, it would be normal for avionics to work with the ground power wire disconnected, although not a good idea.
Try disconnecting the following GTR200 input pins to see if the noise goes away:
9 AUX MONO IN 2
18 MUSIC IN RIGHT
19 MUSIC IN LEFT
32 AUX MONO IN 1

Hi Joe thanks for the response! Those pins are all not connected as we are running a audio panel.

Thanks
Bill
 
SNIP

Thinking it?s about time to replace the PMA7000BT and go full Garmin 😁

You have jumped to the conclusion that the Garmin GTR200 is not the issue. My experience with the GTR200 has been problematic, with crosstalk noise as you describe - and this was a panel done by an avionics shop with a Garmin Audio Panel (comm #1 being a GTN-650).

Carl
 
Joe,

This aircraft has an audio panel, so the internal intercom in the GTR 200 should not be enabled, and thus there should be no audio connections to any of these audio inputs, but it will be good to see Bill confirm this.

The GTR 200 should be quiet except when receiving on the tuned frequency (squelch broken, RX active), so some additional clarification of what exact conditions are present when "humming" is heard would be useful. Is something in the aircraft causing the RX squelch to break and the RX indicator to remain on?

Thanks,
Steve


Hi Steve thanks for responding! The humming occurs almost constantly with or without a transmit or receive. But becomes even worse upon the autopilot being activated. The original antenna wiring was not changed and was run along with a bundle of wires.

Currently I’m thinking com1 gtr200 May be picking up some rmi from exterior wiring. The gtr200 antenna is run with a bundle of wiring including the autopilot wiring. Next plan of action is to install a new rg400 antenna cable to the gtr200 and route the coax cable away from all other wiring. The other thing that has me second guessing this is the Sl30 com2 antenna is run along the same wiring bundle and is virtually silent.

Thing to note as well though is his original com1 was a gtn480 and was the com with the noise. Com2 was a KX155 and was silent in regards to noise.

I’ve read in the past these newer coms are higher powered then the older radios. from my understanding is great for reception distance but also leaves them more susceptible to rmi. If this is so it might explain why the com noise wason com1 on the original panel and in this new panel.

Thanks
Bill
 
Well, the obvious test is to get a pin puller, and swap the audio input pins at the audio panel. e.g., com 1 is now the SL40, com 2 the GTR200. If com 1 (now the SL40) now has the problem and com 2 (now the GTR) is okay, then the problem is in the audio panel. If the problem is now on com 2, then the issue is the GTR and or its set up or wiring.

Thanks Bob for the response! I will see what happens with the antenna exchange and if that fails that is my next course of action.

Thanks
Bill
 
The noise bill is speaking of was present only in Com 1 when the Garmin 480 was installed. We did a complete avionics upgrade to all Garmin products EXCEPT kept the PS engineering PMA7000BT and currently using a Com1 Garmin GTR200 and Garmin SL40 com 2.

Com 2 is perfect, but com 1 is still bad. Can hear the autopilot intermittently and mostly though the GTR200 RX sounds like it?s in a well. Also the transmissions from other aircraft sounds like there is a screeching sound when they speak.
Wish I could delineate the sound better than to say it is only in com 1 and is like screeching noise when others RX.

Bill is doing an amazing job and has basically rewired the entire panel and troubleshot things more times than I would have ever been able to. He?s doing a fantastic job!

Today however, I called PS engineering and the guy basically made me feel stupid, told me that I had done everything wrong, and that there was absolutely no problem with his product and clearly it is a problem with our installation.


Thinking it?s about time to replace the PMA7000BT and go full Garmin 😁



Thanks Jon! We will get to the bottom of this!

Bill
 
My GTR20 has had no hum issues. The RG400 antenna cable is not bundled with other wires, and all avionics use a common ground point.
 
or the audio lines shielding completing the grounds?
Bill

Hopefully you followed the standard recommendation to run a multi-conductor, shielded bundle, for all audio lines, using one of the wires for the ground connection. The shield should be connected to ground at the audio panel but left open at the com radio.
 
Hopefully you followed the standard recommendation to run a multi-conductor, shielded bundle, for all audio lines, using one of the wires for the ground connection. The shield should be connected to ground at the audio panel but left open at the com radio.

Yes all audio wires are 22gauge multi conductor shielded wiring and the shields terminating at the audio panel only. Yes one of the conductors (lo) goes to the audio lo pin on the audio panel.

Thanks
Bill
 
Last edited:
I see no mention of looking at the antenna. Pull it off and verify it is clean underneath and that it is getting a good ground.

Jay
 
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