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It's the little things

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slipstream(at)wavecable.c
Guest

Posted: Sat Aug 27, 2011 8:03 pm Post subject: It's the little things

I had such a great time diagnosing a radio problem, I 

figured I'd share it with the group.  Here goes:



I installed a Z13/8 electrical system in my RV-7, pretty 

much exactly as described in the current iteration.  On 

the radio side, I kept it fairly simple -- One GNS430W to 

handle all my nav/comm needs, and an intercom.  It 

received GPS signals, vor/loc, and received comm.  Good 

sidetone on the intercom.  I figured if it would receive, 

it would send!  It didn't.  Pressing the PTT got me the 

little "TX" indication on the GNS, but nobody could hear 

me.



I looked through the list archives, and did some 

trouble-shooting of my wiring.  I removed the factory cork 

gasket from my Comant antenna.  I removed the aluminum 

antenna screws I'd used and replaced 'em with steel 

(desperation there).  Checked my BNC crimps.  Made sure I 

didn't do anything really dumb like plug the nav coax into 

the comm receptacle.  I discovered after the fact that I 

should have removed primer from under my antenna doubler, 

but I was loathe to start drilling out rivets on another 

goose chase.



I decided to let the "experts" give it a shot.  I received 

permission to fly the RV-7 nordo to the nearby towered 

airport with a full-service avionics shop.  They looked it 

over, bench tested the GNS, took some readings off the 

antenna and checked my amateur BNC crimping (not up to 

their professional standards, it should be useable). 

 Finally got a bill for $100 and still no transmit.



I called Garmin factory support.  In their opinion, my 

Bose headset must be the problem.  Thank you for calling.



I spent some time perusing the GNS installation manual. 

 Turns out there IS a "Comm won't transmit" item in the 

toubleshooting pages.  Answer:  "Make sure it has power." 

 What kind of dopes do they think install these things? 

 It's like the home stereo instructions.  "Make sure it's 

plugged in."



I pulled the GNS out for the nth time and stared at the 

back plate.  The GNS-430 has three D-sub sockets 

connecting it to the airplane -- "Main," "Comm," and 

"Nav."  The Main and Comm sockets both have power inputs 

-- four power pins from two fuses or breakers.  I knew 

that transmitting drew more power by far than any other 

function of the unit. Hmm..  My unspoken assumption had 

been that if the unit received comm, it must have comm 

power, right?  Well, when nothing else pans out, you'll 

try anything.  I whipped out my little voltmeter and 

tested the comm power pins.  3 volts??  Is that possible? 

 No idea, but when I touched the Main power pins, I got 

the full 12 volts.  I checked out the fuses and voila!  My 

comm power fuse had blown.  As I was saying about dopes 

and electronics...



I'm not sure how to explain the 3 volts I did get -- I 

thought fuses were all-or-nothing affairs, but perhaps 

not.  Anyway, I popped in a new fuse and presto, I get 

"loud and clear" reports from everybody I talk to.  SO.. 

 Maybe the "Comm" power pins need to be relabeled 

"Transmit" power.  And there you have it -- The $100 fuse.



Brooks

RV-7 N513BW Phase I ...

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aerobubba(at)earthlink.ne
Guest

Posted: Sun Aug 28, 2011 4:39 am Post subject: It's the little things

Howdy-

Might I suggest you contact the shop owner, tell him what you found, and that a couple thousand people on this list all around the planet are waiting to see how he handles the situation?  We all make mistakes, but hopefully we don't make others pay for them...



Glen Matejcek

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Aug 29, 2011 7:53 am Post subject: It's the little things

My unspoken assumption had been that if the unit received comm, it must have comm power, right?  Well, when nothing else pans out, you'll try anything.  I whipped out my little voltmeter and tested the comm power pins.  3 volts??  Is that possible? No idea, but when I touched the Main power pins, I got the full 12 volts.  I checked out the fuses and voila!  My comm power fuse had blown.  As I was saying about dopes and electronics...



 I'm not sure how to explain the 3 volts I did get -- I thought fuses were all-or-nothing affairs, but perhaps not.  Anyway, I popped in a new fuse and presto, I get "loud and clear" reports from everybody I talk to.  SO.. Maybe the "Comm" power pins need to be relabeled "Transmit" power.  And there you have it -- The $100 fuse.



    The 3-volts you were seeing was probably coming OUT

    of the radio as opposed to 'sneaking' across an

    open fuse.  Garmin has long been fond of the separate

    input power for the transmitter of some models. On

    some models, this input would like to have 28 volts

    supplied even if the rest of the radio is running

    on 14 volts.



    Be sure and check for details on your particular

    radio. You may want to add a 14/28v up-converter to

    accommodate the transmitter. It will give you a lot

    stronger output. 



  http://www.kenneke.com/avionics.html



  [img]cid:7.1.0.9.0.20110829104338.01dd7d18(at)aeroelectric.com.0[/img] 



 

   Bob . . .

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