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Radio issues (RV-12)

rgmwa

Well Known Member
I'm trying to track down a transmit issue with the SL40 and Flightcom 403 intercom in my RV-12. My radio installation has never worked particularly well, and initially had a loud squeal until I adjusted the SL40 settings as others have previously posted, so I know they are OK. However every time I press the PTT button, I get a loud steady tone in the headset (sidetone?). It is possible to hear voice over the tone on a nearby handheld radio, but the reception is pretty bad, and in the air is reported as unreadable. This happens even with just the master and radio on and everything else off. My mic jack has isolation washers on it as per the plans, and both pilot and co-pilot sides have the same problem. Swapping headsets makes no difference. It was suggested the problem may be in the 403 rather than the SL40. Pilot/passenger conversation works OK. The problem only occurs when the PTT button is pressed. Any ideas?
 
Thanks Jim. In my case I can receive transmissions OK and voice gets out when I transmit, but the steady loud tone that I hear when I press the PTT button also seems to get transmitted and pretty much masks the voice. My jacks are as per the plans, so they should be OK.
 
Try removing the radio antenna, cleaning the both the antenna base and the fuselage skin to make sure there is good metal to metal contact when it is reinstalled. Also check the coax cable at each end.
Make sure the radio is FULLY seated in its rack.
 
Thanks Joe. I'll try that. The plane is due for its annual (again!) in a week or two, so it will be a good time to take things apart.
 
You could try removing the power wire that powers the 403 intercom. This will "activate" the failsafe feature of the intercom and connect the pilots headset directly to the radio. This should completely bypass the intercom unit.
 
You could try removing the power wire that powers the 403 intercom. This will "activate" the failsafe feature of the intercom and connect the pilots headset directly to the radio. This should completely bypass the intercom unit.

That sounds like a good idea. Thanks Noel.
 
Not yet Tom. Haven't had a chance to get back to the hangar, but will post here if I can track it down. It's a steady tone, rather than a typical feedback squeal, which I've had before and fixed by slaving the side tone to the volume control. The tone continues as long as either PTT button is held down. It's probably something simple (I hope).
 
Not yet Tom. Haven't had a chance to get back to the hangar, but will post here if I can track it down. It's a steady tone, rather than a typical feedback squeal, which I've had before and fixed by slaving the side tone to the volume control. The tone continues as long as either PTT button is held down. It's probably something simple (I hope).

My reason for asking is that I think we have similar issues. I've played with all of the settings on the SL40, including slaving the side tone to the volume control and I can reduce it, but not eliminate the squeal/hum in transmit. I'm planning to work on the antenna connections next. If I find a fix, I will let you know as well. I have noticed that it is not as noticeable in flight. It is much more annoying during taxi and while parked, even with the engine shut down. ........Tom
 
Another thought: I seem to remember in the back of SL 40 Instalation Manual there is a very simplified wiring diagram of an installation using the SL 40 as the 2 -place intercom.

You could pull the radio out of the plane, wire up a 25-pin D shell connector following the wiring diagram with connections for headphone and mic and PTT connections, hook up a 12 volt battery, connect a small coax and antenna and jumper for the internal intercom function.

This setup is what I fabricated in a role around cooler for a portable ground setup that I use when I communicate with our formation flight team.
 
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Which brings up another thought: on your SL 40 install, be sure the pin position for the selection of "internal intercom" function is not populated. I do not know what symptom would show if the internal intercom function was wired and you also used an external intercom box.
 
Got down to the hangar today, plugged in the headset, turned the master and avionics on and Skyview off, pressed the PTT and sure enough the loud tone was there again. Made sure the SL40 was fully seated and locked as per Joe's suggestion, which it was. Then thought I might as well pull it out and have a look anyway. Turned everything off, unlocked and pulled the radio out, examined the aerial connector and D-sub, and still none the wiser, pushed it back home and locked it again. Master and avionic back on, hit the PTT and made a test call, and presto - no noise. Great result!

Feeling pleased with my apparent mastery of electronics, I pulled the plane out to go flying. Settled in, started the engine, flipped all the required switches and pressed the PTT to make a departure call, only to hear the dreaded noise again, although not as bad as before. Took off and flew around for a hour. Reception was fine. Called up another aircraft that had just landed at a field about 8nm away to ask for a radio check and was told readable but `crackly'. When making the call, the interference at my end didn't seem too bad, but by the time I arrived back at my home airfield the noise during radio calls was as bad as ever.

The plane is now down for its annual, and I will be away for the next 2-3 weeks, so any further troubleshooting will have to wait. My current prime suspect is the antenna cable or its connections, unless any of you electronics experts out there have any better ideas.
 
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Way back when - -

I had an issue with a crimp and the antenna end of the coax cable. Some just did not get a good crimp of the tiny center wire. Fixed and it solved it for a few year now anyway.
 
I wouldn't be surprised if it was something like that John, although I've been known to be wrong before. :(
 
And / or if you turn sidetone off completely on the radio do you still get the noise ? That would determine if it?s introdroduced through the sidetone circuit
 
I had very similar symptoms, in addition to a very touchy headset jack - least bump and I would lose audio. I just finished my annual and while I had the wings off, I tried running a dedicated ground wire from the ground lug of the audio headset jack to a firm ground on the airframe. It made a world of difference. I'm not proposing that exact fix for you, but it would be easy to test while you have the wings off.
 
This is an old thread, but I finally found the source of the feedback noise in the SL40, so thought I'd report it in case it helps someone else.

The loud feedback tone in both headsets has been an ongoing problem since day one, and I'd posted about it several times before and received plenty of helpful advice. I also had others look at the plane. Several times I thought I had fixed it, but then it would return.

A couple of weeks ago I was flying back to the airfield and was reported as `readability two'. Another time my transmissions were "unintelligible", but mostly people seemed to be able to understand what I was saying despite the background noise so I just put up with it. Anyway, I found the problem last week and it was so simple I should have found it much earlier.

The antenna coax cable was supplied pre-wired. At the SL40 end, the BNC connector plugs into the back of the radio support frame. The shielded coax cable feeds into the connector and is secured with a nut. The nut was loose by half a turn. After all this time, that's all it was.

Because the cable wasn't secure, if it was moved slightly it would be fine but then vibration or some other influence would move it back which would affect the shielding and generate the noise. I had checked and re-checked all the wiring and connections I had done, the earthing, headsets etc, but didn't think to check the manufactured cable. Lesson learned!
 
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Thanks for posting the solution, I learn something new on here every day, and glad you got it sorted!
 
As much as I hate taking off the avionics cover, I will be doing it first thing tomorrow - this might be the cause of the problems I've had with my SL40 for the last few months. I sure hope so - I was starting to think about replacing it with a newer radio.
 
There’s a helpful drawing of the connector in the SL40 manual so you can see how it works. Good luck!
 
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