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Transmission breakthrough onto adjacent frequencies - 8.33kHz GNS430W/GMA340


I just had a message relayed to me from ATC that when transmitting on 133.440 talking to Farnborough Zone that there was “breakthrough” onto 133.485, which is a London Control frequency. I had no problems outbound (or at least no complaint from Swanwick) but after departure on the return flight I wasn’t able to raise Farnborough. I power cycled the 430 and was able to raise them thereafter but I assume this is when the fault occurred. There was what sounded like ignition interference (clicking/popping) for a few minutes when receiving immediately prior to this.

After landing I confirmed this with a hand-held that my 14V GNS430W, with a GMA340 audio panel, is indeed transmitting on at least 133.485 and 133.440 when 133.440 is selected. I also tried 122.405 and was also transmitting on 122.415 and 122.425.

The radio was worked on a day ago, as previously it was only transmitting a carrier wave. Everything from 430 to antenna was fiddled or temporarily swapped out to track down what turned out to be a failed circuit breaker which wasn’t passing power to the comm side of the 430.

Is there anything I can do to try to debug it before I call my engineer on Monday?



Elstree, United Kingdom

I think the box needs to go back to Garmin for investigation.

How interesting. I have a vanilla 430 in my -161 that has suffered from intermittent ‘noisiness’ since it was new 12 or 14 years ago. Last week at Solent the controller reported Strength 3 and I had noisy reception, just like described here. This lasted about 10 min, and then was hunky dory all the way home, the same behaviour it’s had all its life. The 430 has been swapped (and everything else) but the behaviour is still there. The noise can also be heard on box 2 which is 25khz and a far better radio than the 430.

Up until right now when you mentioned ‘circuit breaker’ I’d never considered that. Neither have any of the so-called ‘engineers’ who’ve investigated this problem over the years. It’s so bad that I went round the LAA show yesterday asking for inputs, price of 8.33 com2 etc. And getting “the wiring’s 40 years old, you’ll need to strip the lot, £30k please”:

But I should have thought of it. Years ago the alternator failed intermittently and the whole lot was swapped out, regulator, everything with no fix. Until one day I happened to catch the cb with my fingernail and it sprang back to life! No amount of pushing it did anything but a very light tap showed the culprit. My then flying partner even heroically brought a spare alternator to Helsinki in his hand luggage after it had failed in Sweden and it worked fine – for another 30 hrs.

I still.have that cb, on my desk as I write. So, next time the radio goes noisy, I shall have suitable implement to hand to give the appropriate cb a little wake up call. You might want to do the same!

EGBW / KPRC, United Kingdom

Same “breakthrough” problems today. No complaints from ATC but I checked it before the flight with a handheld.

Loud and clear the tuned frequency as you’d expect, but also affecting reception on frequencies above up to about 25-30kHz, and below by perhaps 10kHz. Interestingly very loud noise on a couple of frequencies above, but just an initial burst about 10kHz above which only broke squelch when I pushed the PTT.

I did try giving the breaker a bit of a “pump” pre-flight with no difference. @Aveling, I too have the old breaker sitting on my shelf as a reminder. :-)

Elstree, United Kingdom

Checking with a handheld might give false positives by the way, due to how close the handheld is to the transmitting station (I’m assuming you’re sitting in the plane with the handheld in one hand, and pressing the aircraft’s PTT with the other — or at least with a person with a handheld within normal speaking distance of the plane). The filters on the handheld’s receiver can only do so much to attenuate a signal from a very strong nearby frequency, and especially with AM you will likely get bleed through heard on the handheld even if the transmitting radio is absolutely perfect.

To do a proper check with a handheld you want the handheld to be at least a couple of hundred metres away from the transmitter you’re testing.

Last Edited by alioth at 06 Sep 08:16
Andreas IOM

Checking with a handheld might give false positives by the way, due to how close the handheld is to the transmitting station

@alioth Very useful to know. I was indeed sitting in the plane. Our avionics engineer will take a look at it tomorrow and as A_and_C suggested it’s likely it’ll need to go off for some testing. He has a spare 430 so after confirming the problem the first thing to do will be to swap it. If that doesn’t fix it then I really am stumped, but then I’m just a pilot.

I’ll keep the thread updated so we have something useful here for future searchers.

Elstree, United Kingdom

This is very interesting, thanks for posting.

My 430W has recently developed an intermittent issue where I’m told my transmit becomes nearly inaudible. Various on the ground testing of different headsets, connections etc. hasn’t found it yet. I’m beginning to wonder if it may be the 430.

My 430W has recently developed an intermittent issue where I’m told my transmit becomes nearly inaudible.

@Off_Field check that the circuit breaker is working correctly. We were getting 0V on the power pins to the 430 comm board (connector P4002, pins 11 and 12), and then when we checked the circuit breaker it was passing 0V. Very interestingly, after some percussive maintenance, the circuit breaker was passing about 0.75V which would give a very weak transmission. Even with 0V across the breaker (or pulled) the 430 was transmitting carrier-wave-only.

That circuit breaker should be 5A on a 28V system and 10A on a 14V.

Last Edited by Winston at 06 Sep 11:06
Elstree, United Kingdom

Thanks Winston!

I shall have an investigate next time I’m up at the aircraft.

Interesting. I too have been told of an issue where my audio fades away as though I’m turning away from the Mic. I’m not of course as it’s hanging from my head.
I’m keen to hear of a similar issue that finds a resolution. My Avionics have been checked but a fix wasn’t found.

United Kingdom
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