Incompatibility of Garmin GMA340 and Narco Radios?

[Ned Gravel]

Anyone every heard of such a thing?

I bought a GMA 340 at Airventure 2010 and had it installed in January this year.  I have had nothing but problems.  I have a Narco Mk 12D and a Narco 810.  The 340 replaced a Narco CP 135(?) but there were already some problems with the old setup.

Took it into a reputable avionics shop (who know what an SWR meter is) and they checked stuff from top to bottom yesterday.  They found that my comm antennae were both corroded inside and were replaced. How they got that way is another story.  They also replaced a small bit of coax in the avionics bay and cleaned up some corrosion on the connectors.

When the technician went for the test flight, he found that a local tower (20 nm away) was only receiving 2X5 and he was getting intermittent sidetone anyway (something I have been experiencing since January).  So he bench checked everything:  radios, audio panel, headsets). All OK.  So he took the shop harness out and plugged that into ship's power and ran it again.  Radios were great - but still some niggles on the transmission quality.  And he used two other 340s to test out mine in comparison.  All the same.  If he left the 340s out of the voice modulation circuits, he was received 5X5 everywhere.  This isolates the problem to the audio panel - it works but it is having some difficulty in my set up.

By chance, he had a PS Engineering 8000B (plug and play replacement for the 340) and all the problems went away.  Only two things could cause that - the RF from the radios is messing up the modulation in the 340, or the 340 is having trouble mating with the Narco radios as regards something simple like impedance.

They called Garmin who suggested something, but it did not work.  I suggested wrapping the 340 in tin foil before putting it back in (field expedient fix for EMC problems).

Any RF engineers out there with any ideas?  Having bought the 340, I am not keen on the $2000 to buy a PS Engineering audio panel.

 

[carusoam]

Narcos are noisey.  My 12D+ ?  in my old M20C would knock out reception on my GPS 196 portable garmin.

My solution was a little more expensive.  Out with the M20C.  I still use the 196.

Best regards,

-a-

[Ned Gravel]

Conclusion:

I am buying the PS Engineering Audio Panel and they are giving me something based on the trade in value of the 340.  It goes in on Monday or Tuesday.

Oh well..........

[Piloto]

The noise problems you may be getting may be due to ground sharing. In order to avoid ground noise from the alternator the microphone ground must be carried on a separate wire that is not connected to chassis ground. To reduce the possibility of RF coupling through the wiring the audio panel must have 0.01uF internal capacitors on each line connected to chassis ground. Check on the 340 schematic. The use of shielded cable for microphone wiring also helps on reducing RF coupling, specially on HF equipped aircraft.

You can recognize a classic airliner on the radio by the 400hz whine heard when the pilot transmit. The 400Hz whine is due to ground sharing by the microphone and the generators. The separate microphone ground wire solve the whine problem.

 

José

 

[Ned Gravel]

Jose:

Does this condition exist for aircraft with generators instead of alternators?  BTW:  Both crew jacks are connected with shielded cable.

[Piloto]

Quote: edgargravel

Jose:

Does this condition exist for aircraft with generators instead of alternators?

[Piloto]

Quote: edgargravel

Jose:

Does this condition exist for aircraft with generators instead of alternators? BTW: Both crew jacks are connected with shielded cable.

[Ned Gravel]

Jose:

Yes, I see what you mean.  The noise you refer to is normally easy to distinguish.  It has a whine and/or a crackle distinctive to power generation. 

I was experiencing something a little different.  What I was getting was complete loss of sidetone with the occasional acoustic-like feedback screech during transmissions.  Completely unreadable by any receiver although the signal strength may have been good.

 

 

[Ned Gravel]

Update:

The avionics shop I am using, Brant Aero, tells me that the previous shop that installed the GMA 340 did it correctly and put in the 0.01 microfarad internal capacitors on each line connected to chassis ground.  So that was done correctly.  But this installation parameter is really only for the generator / alternator whine. 

Occasional and complete loss of voice modulation was the actual problem and it was intermittent tending to "mostly" so I know there were conditions where the system was operating OK.

Having worked in the field of electromagnetic compatibility testing, I understand a few of the ramifications of what was happening - but not all.  My mistake was believing that all the parts should work together. 

I have since learned that Narco radios are "dirty" (lots of washover into other frequencies and harmonics) and the Garmin audio panel does not handle electromagnetic interference well (it sat on top of my Narco 810 in the panel).  Whole thing was a recipe for disaster in my panel (and may not be that way in another's).  Wish I had known these things last summer when I bought the GMA 340 in Oshkosh.  I also discovered that PS Engineering units have a lot of EMI filtering in them.

This is the first time I have run into my own EMC issue with my Mooney.  Oh well. Live and learn:  "smart people learn from their own mistakes....wise people learn from the mistakes of others..." Smarts for me.  Wisdom for all of you.

Thanks to those that responded.

[Vref]

Quote: edgargravel

 I also discovered that PS Engineering units have a lot of EMI filtering in them.

This is the first time I have run into my own EMC issue with my Mooney.  Oh well. Live and learn:  "smart people learn from their own mistakes....wise people learn from the mistakes of others..." Smarts for me.  Wisdom for all of you.

Thanks to those that responded.

[N601RX]

No coax, shielded wire or filter is perfect..  Most coax will "Leak" signals in the -105dbm range.  Physical seperation works best, but your somewhat limited when dealing with an aircraft panel. 

RF engineering is kind of like black magic.  Something as simple as rotating a capacitor on a circuit board 90 degrees or moving it 1/2"  will make the radio pass or fail FCC testing.  It would have interesting to see what would have happened if it was relocated in the panel.