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Comm will not transmit with audio panel installed

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claporte75(at)msn.com
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Posted: Sun Apr 12, 2009 7:36 pm Post subject: Comm will not transmit with audio panel installed

Hello C

 I have recently completed the installation of an avionics wiring harness (completed at an avionics shop) C and I have a perplexing problem of trying to get comm 1 to transmit...here are the details:

 Microair 760 as comm1 on KMA 24 audio panel with Sigtronics sport 200 intercom...SL-30 as comm 2.

 The ptt switch activates the SL-30 to TX every time as expected (the only issue is a squeal during transmit...another issue all together...the squeal is nil at 118.0 and loud at 134.95) C and my swr meter shows power and swr coming from this radio during TX.

 The ptt will only activate the red TX indication on the Microair 760 about half of the time C (with the audio panel installed) C and at no time (even with the red TX light on) does the swr meter show power or swr coming from this radio.

 Now C to get creative C I removed the audio panel and located the Comm 1 mic key position on the KMA 24 connector C (pin R) and when I connect this pin to ground C the red TX indicator shows every time....and the swr meter shows power and swr.

 So C I thought that maybe comm 1 mic key and comm 1 mic audio might be in the wrong positions...I switched them C and the radio went to transmit mode (continuous..until the audio panel was removed).  So C with the pins back in there original spots C I am thinking that the Microair 760 wiring might have something preventing it from being told to transmit through an audio panel?  This is also the second audio panel I have tried to no avail.  Also C I checked to continuity of the inner conductor and outer shield of both coax cables going to the antennas (both belly mounted...I have tried removing one of the antennas on the non-transmitting radio of course).  I have also changed out the pin located at position R in the audio panel connector (Com 1 mic key).  I don't know what to try next...any ideas would be greatly appreciated!

 Thanks C

 Craig

  [quote][b]

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Radioflyer

Joined: 04 Feb 2009
Posts: 43
Location: Boston

Posted: Mon Apr 13, 2009 4:35 am Post subject: Re: Comm will not transmit with audio panel installed

Sounds like you have two issues, the microair PTT function and the squeel. I'm not talking from experience, but apparently the MicroAir radio is known for having peculiar "ground level" requirements for the PTT switch to work. Look into the RST (Jim Weir's) website and in one of his past articles he talks about the issues he found re: Microair and the PTT switch. This may help one of your issues, but I can't be sure.

--Jose

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claporte

Joined: 13 Apr 2009
Posts: 1

Posted: Mon Apr 13, 2009 7:22 am Post subject: Re: Comm will not transmit with audio panel installed

OK

1.  I have installed the fiber washers on all headset jacks...I will check that they are properly grounded.



2.  The RST article states "the little rascal wants no

more than 0.4 volts to fully transmit. So, two days before Oshkosh I'm inside the audio panel cutting and

pasting an extra relay onto the board to do a "hard" ground on the PTT line. Just be aware of this requirement

for very low voltage on the key line when you install your Microair."  I am almost certain this is the problem.  However, digging around inside of the audio panel is beyond my capabilities...any ideas on how to do this with the wiring (possibly a variable resistor in the com 1 key wire)?



Thanks for the help!

Craig

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