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Garmin GMA 340 Problem? Maybe but I'm not sure.

AndyWW

Active Member
Hi everyone,

The RV-6 that we bought flying has just come out of Covid-19 lockdown and seems while just sitting around to have developed a nasty radio problem that is a devil to troubleshoot. So I thought I'd try the VAF brains trust to see if anyone has experienced something similar, or has any ideas that could help us figure out what is wrong.

Our setup is a Garmin GMA 340 audio box, with 2 radios connected, an old Bendix King KX155 configured as COM1, and a new Icom IC-A220 configured as COM2. Most of the time we use the Icom since it is 8.33 capable and we are in the UK where unfortunately that is required. PTT buttons are on each stick.

The problem is that intermittently, usually very soon after takeoff, both radios indicate that they are transmitting and there is a rhythmic tic-tic-tic sound in the headset with a frequency around 1 or 2 Hz which is not normally there. The other symptoms are that sometimes the sidetone is missing when PTT is pressed, and that if I flip channels on the Icom while the problem is occurring the display shows the channel swap happening very slowly as if the radio is preoccupied with some other task which it prefers!

Nearly all the time there is no problem at all when the aircraft is on the ground, which makes troubleshooting very difficult! Wiggling the GMA340 in the rack does seem sometimes to have an effect, which together with the fact that both radios seem to have the same issue, makes us suspect that the Garmin unit might be the culprit. But it would be annoying to ground the aircraft and send the unit off for repair only to find out that it is a wiring or radio issue instead. So I'm posting this in the hope that someone says "I had that problem and here is what I did!"

Thanks,
 
Clarification

I read my initial post again and realized it sounds like there's just a bit of interference in the headset and a side-tone issue. In fact the radios are transmitting semi-continuously and blocking frequencies. I caused a small amount of chaos on Monday afternoon flying from Lee-on Solent back to White Waltham - the Farnborough controller was switching people onto another frequency to avoid my interference and then when I got to White Waltham apparently there was some consternation in the circuit! So it looks like we're grounded until we get this fixed. I tried re-seating everything today but as soon as I got airborne the problem started up again so I turned off the radios and enjoyed a brief non-radio flight. Any help much appreciated...
 
Check your headset RCA jack connections

I had similar issues with weird comms from my GTR 20 and 200 radios using the GMA 240 Audio Panel when building/testing. I chased this issue for weeks until John Stark (Stark Avionics) tested the audio panel (test jig) and found it to be working perfectly. John advised to check each wiring at the solder connection to the RCA jacks for the headsets to see if there is a nick in insulation for one of the wires, causing an intermittent short. Sure enough, that's exactly what had happened.
 
Thank you Chris - we will certainly check for that. One other idea we had is to try the split comm mode where box 1 is tied to left seat headset and ptt, and box 2 is tied to right seat headset and ptt. This might allow us to figure out if the problem is related to left seat or right seat wiring. But unless we can get the fault to occur on the ground it isn't going to be easy to diagnose. I will be going to the airfield tomorrow to try to find the problem...
 
I realized I hadn't closed this one out. Happily, we eventually discovered that due to a misalignment of the pilot side GMA 340 connector in the rack, there was an intermittent short on the pilot PTT line. Once we relocated the connector correctly the short went away and everything was fine. No problem with the GMA 340. Possibly we should have realized previously that there was an incipient problem because the GMA 340 was wobbling slightly in the rack. This problem has now also gone away.
 
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