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Need Help w/ Squeal - Garmin GTR200 in RV-12

Piper J3

Well Known Member
We have a four year old RV-12 built by Synergy in Eugene, OR. Airplane has dual HDX, autopilot, ADSB-in/out, Lightspeed Zulu3 headset. No problems until a few months ago when Comm radio started slight squeal when transmitting. Problem has gotten progressively worse and so we are attempting to fix. Squeal was noticed in headset as well as by receiving aircraft.

So, we removed the radio and sprayed contact cleaner on antenna connections and 25-pin D-Sub connector on both radio and tray. Re-inserted radio and now squeal is constant. We tried a known-good GTR200 in our airplane and same squeal and also tried our radio in another airplane and it was good there, so radio is not the problem.

Headphone jackets look to be tight and squeal is same if headset is plugged into either pilot or co-pilot side. Also tried another known-good Zulu3 headset and same squeal. Both PTT switches show lighted Tx on face of radio, so both PTT switches are OK.

We examined the female 25-pin D-Sub thinking maybe a socket pin got pushed back in the housing, but it looks OK. The coax antenna connector on the back of the tray seems to “float” and we think that is for alignment to the radio. BNC connector on back of tray looks good and center pin is straight. BNC is tight. BNC connector on belly antenna looks good.

Need help with suggestions as to what would cause squeal that is external to the radio itself.

As always, thanks in advance...
 
Look again at the coax, especially the braid connections to the outside of the BNCs, the antenna ground to airframe ground (any corrosion?), etc.
 
I was thinking about running a "test coax" from radio tray to belly antenna kind of externally – just in the hanger for a test. I know radio has to have antenna for load. What coax designation would I need for just running a quick test? I would run coax out over the side of the cockpit and connect to belly antenna thru an access hole.
 
I was thinking about running a "test coax" from radio tray to belly antenna kind of externally – just in the hanger for a test. I know radio has to have antenna for load. What coax designation would I need for just running a quick test? I would run coax out over the side of the cockpit and connect to belly antenna thru an access hole.

You'd want 50 ohm coax. Common types are RG-58, RG-400. (also RG-8 but it won't fit into a BNC connector).
While you're at it, why not connect the coax to an external antenna, to check it out? Connect (just use a nut and bolt) the center conductor of the coax to the bottom of a 23" high vertical piece of aluminum tubing; connect the shield of the coax to a horizontal cookie sheet (or something similar), just below the vertical tubing.
 
Question - Can I disconnect the antenna coax at the back of the tray and power radio for a test? I won't push Tx button. Does this make sense to try?
 
I had a very similar problem that dogged me for about 4 years. Thought I had found the culprit several times but it always came back. It turned out to be poor grounding of the antenna coax in the BNC connector at the back of the radio. Mine is an SL40 but I imagine the GTR200 is similar. Try jiggling the coax near the BNC connector and see if it affects the transmission. I fixed it by simply undoing the retaining nut, pushing the cable right up to make sure the shielded wire was making proper contact and re-tightening the nut. Then cable tied the coax so it couldn't move. It's been perfect since.
 
We have a four year old RV-12 built by Synergy in Eugene, OR. Airplane has dual HDX, autopilot, ADSB-in/out, Lightspeed Zulu3 headset. No problems until a few months ago when Comm radio started slight squeal when transmitting. Problem has gotten progressively worse and so we are attempting to fix. Squeal was noticed in headset as well as by receiving aircraft.

So, we removed the radio and sprayed contact cleaner on antenna connections and 25-pin D-Sub connector on both radio and tray. Re-inserted radio and now squeal is constant. We tried a known-good GTR200 in our airplane and same squeal and also tried our radio in another airplane and it was good there, so radio is not the problem.

Headphone jackets look to be tight and squeal is same if headset is plugged into either pilot or co-pilot side. Also tried another known-good Zulu3 headset and same squeal. Both PTT switches show lighted Tx on face of radio, so both PTT switches are OK.

We examined the female 25-pin D-Sub thinking maybe a socket pin got pushed back in the housing, but it looks OK. The coax antenna connector on the back of the tray seems to “float” and we think that is for alignment to the radio. BNC connector on back of tray looks good and center pin is straight. BNC is tight. BNC connector on belly antenna looks good.

Need help with suggestions as to what would cause squeal that is external to the radio itself.

As always, thanks in advance...

It sounds like it could be noisy/whiney microphone audio is getting into the transmitted COM MIC audio. This could be a ground issue or something has lost a shield or ground.

Here are a couple of things to try:

  • Disconnect the microphone plugs from the jacks, check to see if the whine is still present.
  • Turn off all equipment besides the GTR 200, check to see if the whine is still present.
  • Use a 50 ohm load connected to a short length of coax to unit at backplate to determine if changing the RF cable or transmitting into a load changes anything.
  • Check microphone jack connections. Verify the ground is connected as it should be and the wire shield is at it should be per the IM at both the Jack and the 37 pin dsub.

Thanks,

Justin
 
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Question - Can I disconnect the antenna coax at the back of the tray and power radio for a test? I won't push Tx button. Does this make sense to try?

This won't hurt anything (do not transmit) but I'm not sure what you would learn, either.

Note to Justin: Did you really mean 50 watts for a dummy load? Or is this just wishful thinking. -:)
 
The problem is fixed. My friend – 83 YO Navy Crew Chief on A4 Skyhawk’s on two Aircraft Carriers (USN ’58 and ’59) found the problem. He suggested that something external to the radio was the problem so we removed fuses one-by-one until fuse labeled “Audio” proved to be the problem. Outstanding. Thank you, my friend…
 
The problem is fixed. My friend – 83 YO Navy Crew Chief on A4 Skyhawk’s on two Aircraft Carriers (USN ’58 and ’59) found the problem. He suggested that something external to the radio was the problem so we removed fuses one-by-one until fuse labeled “Audio” proved to be the problem. Outstanding. Thank you, my friend…

Uh, so no audio means no squeal (in addition to no audio of any sort)?
 
Chiming in here as I had a very similar problem, and wanted to help out anyone else who shows up at this page by punching "RV-12 GTR-200 audio feedback" into a search engine

I would get a squeal when transmitting, especially on lower frequencies, and when transmitting my voice always sounded muffled/tinny (especially so on a Bose A20).

What resolved the issue was simply removing the radio from the panel and reinstalling it. There was some grey dust at the bottom back of the housing which I brushed off with my fingertip before putting it back in, which looked like it was in the same general area as the grounding pads on the cage. All that's needed is a long 3/32 hex key, and following the directions in the installation manual.

Repeatedly fiddling with combinations of ICS/Sidetone/Radio Volume/Headset volume sometimes mitigated it, but there were still many instances where ATC said they couldn't hear me.
 
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