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flightcom 403mc grounding mic to ring or?

charosenz

Well Known Member
I am installing a 403mc in my plane. I also have a IC-A200.

What is interesting to me is that the flightcom manual shows the PTT switch going to ground, but not to the mic return(changed to "Sleeve") like I have seen in most other mic diagrams.

I could connect the mic PTT to ground to pin #8 on the IC-A200 which seems OK on the bench, but grounding the PTT to the (changed to sleeve - mic return) seems to work as well too.

I would like to hear from folks who have wired up a 403mc with a IC-A200 and learn which PTT/Ground you used and has it worked well.

Thanks!
 
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The mic "ring" should be electrically the same potential as any other audio common. It won't make a difference.

The important thing is to make sure that all the audio commons (audio ground wires) are at the same potential and not at various "local" grounds, which can cause "ground loops".
 
Also, I'm not sure if this is a problem with this particular intercom/radio combination. But, because the radio "keyline" is controlled by the intercom, there could be tiny residual voltages present on the keyline from diodes in the intercom circuit. For some iCom radios, even a couple of millivolts present on the keyline can cause intermittent PTT gripes. The only way to solve this is to wire the keyline through a relay and let the intercom control the relay rather than directly controlling the radio keyline.
 
Generally speaking, that whole mic ring thing is a hold over from when handheld mics were common and the PTT was on the handheld.
 
Some poor terminology in the OP is causing some confusion here. Generally speaking, a stereo (3 contact) phone plug has a ‘tip’, then a ‘ring’, then a ‘sleeve’ contact. By convention, on aircraft mikes, the tip is connected to the hot side of the PTT circuit; the ring is the hot side of the mike audio; and the sleeve is ground. On most (all?) older audio panels, the sleeve went to aircraft ground, and you completed the PTT circuit by connecting the tip to ground. Some newer audio panels float the audio input ground off the aircraft ground, to improve noise immunity. In this case the sleeve terminal jack should be isolated from aircraft ground, and brought back to the audio panel ‘Lo’ input. (This is always good technique even for non-floating grounds). What about the PTT return? It depends on the audio panel design. It may need to go to the Lo input, or maybe to aircraft ground. Read and follow the audio panel instructions.
 
Some poor terminology in the OP is causing some confusion here. Generally speaking, a stereo (3 contact) phone plug has a ‘tip’, then a ‘ring’, then a ‘sleeve’ contact. By convention, on aircraft mikes, the tip is connected to the hot side of the PTT circuit; the ring is the hot side of the mike audio; and the sleeve is ground. On most (all?) older audio panels, the sleeve went to aircraft ground, and you completed the PTT circuit by connecting the tip to ground. Some newer audio panels float the audio input ground off the aircraft ground, to improve noise immunity. In this case the sleeve terminal jack should be isolated from aircraft ground, and brought back to the audio panel ‘Lo’ input. (This is always good technique even for non-floating grounds). What about the PTT return? It depends on the audio panel design. It may need to go to the Lo input, or maybe to aircraft ground. Read and follow the audio panel instructions.

Bob, Thank you for catching my mistaken terminology. I should not have written "ring" I should - as you noted - written "Sleeve". I edited that mistake.
 
I appreciate the replies. Honestly I was not looking so much for "should or should not" ideas as much as I was looking for comments from folks who have chosen one of the two methods I listed in the OP and how it worked for them in the actual real life examples.

Just curious as both choices are consistent with the manuals....
 
I connected my PTT return line to my common (single point) aircraft ground buss bar. Home brew intercom uses common ground (not floated). No extraneous noise.
 
As a follow up I connected the PTT to avionics ground, as well as to pin #8 on the radio, as well as to the mic return (Sleeve) listed as #13 on the 403mc. I could not discern any difference in the quality of the audio.

I am not necessarily suggesting anything, I am just listing my observation.
 
What you’ve done is to create a multi point ground, sometimes called a ground loop. Often problems with noise won’t show up for years, after a bit of oxidation causes small changes in resistance. Good practice is to avoid ground loops as much as possible.
 
What you’ve done is to create a multi point ground, sometimes called a ground loop. Often problems with noise won’t show up for years, after a bit of oxidation causes small changes in resistance. Good practice is to avoid ground loops as much as possible.

Bob,

I can see why you came to that conclusion - because I failed to follow up with describing the end results "after" my testing. In other words what I wrote up above was the result of my testing but more importantly here is how the connections ended up.

I connected the audio headphone return "ground" to the audio return pin #7 on the radio, allowing the radio to do its magic to prevent the dreaded ground loop hum.

I "grounded" the PTT to aircraft ground. During the testing I did not discern any audio quality problems when I tested grounding the PTT to the mic return circuit (#13 on the intercom schematic). Which is not too surprising because many radio schematics (PS Engineering/Aeroelectric, etc) have the PTT going to the mic "low/return/shield/sleeve" too.
 
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