Need help diagnosing squeal problem with SL40 Tx

I bought a flying RV-12 with Garmin SL40 and Flightcom 403

I get a horrible squeal when transmiting with intercom set to “all”. Transmit works fine when intercom is set to “isolate”. Intercom works well with both headsets plugged in.

Can anyone suggest how to troubleshoot this system?

Thanks in advance…

PX headphones?

If you have a second set of headphones plugged in, some sets will fold with the mic into or near the ear cup.
Check that first
Tim

I'm not following you on this... I am running a second headset for the passenger. I've tried switching headsets (L-R) and also tried different headsets. Seems the problem is in the intercom maybe.

Boy I feel your pain. I had this same problem on my 12. I wish I could tell you exactly how I fixed it but I can't. I did find a bad pin in the factory harness and fixed it but although that fixed my low volume problem I steal had the squeal. The easy place to start is to check that the headset jacks are not grounding against the airframe causing a feedback loop, this was my next step. This did make most of the squeal go away but know I could not hear or transmit from the pilot side. Worked fine on the copilot side and I plugged in there and flew it a few times. Went back to the left side to check everything and found no immediate problem. I disconnected the headset jacks from the airframe and pulled them outside the airframe and again checked it with my headset. Still had the same issue but while checking I accidentally touched the jack to the airframe. Amazingly this fixed it so now my right side jacks do not ground to the airframe and my left side does (removed the insulator washers on the left). This has been a real pain but it's working.

I have had that problem just 3 times in three years with my SL-30. Have not been able to see any pattern to the issue. All three instances occurred when I had a passenger with me. When it occurs it's AWFUL. I'm at a towered airport and communications with ground/tower are virtually impossible when it occurs. I was eventually able to launch in all three cases by just "dealing with it"; within 15 to 20 min of flying the problem was gone - not to reoccur for 6 months or more. I'm no help to you in diagnosing your problem, but misery loves company., right?

Piper J3 said:

I'm not following you on this... I am running a second headset for the passenger. I've tried switching headsets (L-R) and also tried different headsets. Seems the problem is in the intercom maybe.

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Sorry, what I'm saying is, if you have an unused headset plugged in and folded, the mic is up near the speakers in the cup and it can generate a feedback squeal.
My px Bose do this, I just reach over and unplug the mic jack until they are needed.
Tim

I appreciate all the good advice. As soon as the weather warms up I'll start tracing wiring and ground connections...

Ran into a similar problem today. Turn on the aircraft and the intercom is scratchy and has a weird static noise. Get a loud squeal when transmitting through the SL40 and I noticed as well the EGT reading shoots up while transmitting

Welcome to VAF!

Sam, welcome aboard the good ship VAF

I had the same problem with my SL-40. After trying several "fixes" that had no effect on the squeal, I was at my wits end. One of the local tech guys said he had seen that before and it was a SWR problem. I thought SWR was for the "Breaker, Breaker good buddy" crowd. He recommended adding 10 feet of extra coax wound up near the antenna and give it a try. Well, long story short, it worked, so I expect that I'm "Westbound and down, good buddy" Dan from Reno

Just want to close the loop on this and perhaps help someone else suffering same fate. Today I adjusted the sidetone setting on the Garmin SL40 and was able eliminate the squeal when transmitting. Can't believe I fixed this nagging problem. Happy day...

Piper J3 said:

Today I adjusted the sidetone setting on the Garmin SL40 and was able eliminate the squeal when transmitting.

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How about telling us how to access, and adjust the sidetone?

Glad you got it fixed, and thanks for reporting back as to what the problem was.

Mike S said:

How about telling us how to access, and adjust the sidetone?

Glad you got it fixed, and thanks for reporting back as to what the problem was.

Click to expand...

It is done using the front controls in a set up mode. Need the manual for all of the details.

Very easy to do. Just push and hold MON button for 2 seconds to enter setup mode. Large outer tuning knob selects item to be adjusted and small inner tuning knob alters the setting. Push MON again to exit setup.