GTR200 - Screech When Tx

[Piper J3]

A friend has a RV-12 with Garmin GTR200 and dual Dynon HDX. Airplane is five-years old and has 500TT. The comm has developed a screech sound when transmitting. We hear the screech in both Zulu3 headsets. Makes no difference which PPT is used - still have same screech. It’s been reported that aircraft receiving our transmission also hear the screech. Radio was working well until recently.

Any ideas apprenticed….

 

[jliltd]

1. Pull the radio and then re-install it ensuring the connector is fully engaged in the tray. If this doesn't help try swapping the GTR 200 out with a different unit that hopefully you can find locally from a friendly owner. If the problem persists you know it isn't the transceiver.

2. Check ground wires. Both power ground and audio ground. There may be a loose connection. Ground issues cause all sorts of havoc.

3. Check the antenna coax lead. Make sure the BNC connectors are engaged and in overall good shape. I found one once where the shop tech mis-crimped the collar and then covered it up by gluing the connector on. If the coax isn't RG400 consider changing it out to RG400.

4. Check the antenna. The actual antenna you see is really only half the antenna system, with the other half being the metal skin to which the antenna is mounted. That metal skin surrounding and electrically bonded to the antenna base is what is referred to as the ground plane. Make sure the antenna has good ground plane bonding by ensuring the metal base of the antenna has good contact with the aluminum skin panel where mounted. Check for corrosion between the antenna base and skin. If the antenna has a full contact patch gasket between it and the skin try removing this and then stripping, buffing or sanding the aircraft skin in the footprint of the antenna.. Weather sealing is best done on the exterior periphery of the antenna with sealant such as silicon, Right Stuff or Pro Seal and many have moved away from legacy base gaskets. The ground plane is what the braided outside shield wire of the antenna lead bonds to while the antenna itself is connected to the center conductor of the lead wire.

5. Have someone who has an antenna tester check the lead and antenna as a system. Kind of like shown in this builder video: https://www.youtube.com/watch?v=aBtYlL-OLoE&t=269s

If you are uncomfortable doing any of the above find an avionics guy at your local airport.

 

[Walt]

Sometimes a feedback issue, unplug each headset and check it, also check mic gain setting.
Re-racking is also a good idea as Jim said, pins can develop resistance after long periods of time.

 

[GatorsR1]

Tried most of the remedial actions suggested above.
As an afterthought, checked screws thru antenna into fuselage ... bingo (they were not tight).
Clear comm after snugging up.

 

[jliltd]

Please keep us advised

Piper J3,

Let us know what it ends up being. I have an SL-40 in a friend's Husky that started doing the same thing. I won't have access to it for about 30 days but I think I will start with the antenna. GatorsR1 experience is telling.

Thank you

 

[bbaggerman]

I had exactly the same problem with my RV-12 with the Garmin SL40. Since it happens on transmit it is almost certainly RF getting into a circuit somewhere and causing havoc. After plugging and unplugging various systems the problem seemed to center on the FlightCom 403 intercom. That's not to say the problem is with the intercom per se but rather that seemed to be the place where RF was getting in.
The best solution is to find out why RF energy is not going out the antenna and is instead finding its way into other avionics. But that is hard. So on a lark I made an inline filter assembly for the FlightCom intercom. I ran every wire through an RF suppressing ferrite bead. I also put a small bypass capacitor on the audio input lines. I packaged this up in an inline connector housing and installed it. Now my audio system works great!
Keep in mind this is really just a band-aid. It would be best to find the real cause of the problem. But after lots of looking I couldn't find it.