Radio making "ignition" noise

[Eric_Greenwell]

I have a Dynon com and intercom in my Phoenix U15 touring motorglider (Rotax 912 powered). Over the last couple of weeks, there's been an increasing amount of what sounds like ignition noise that gets worse (and higher frequency) at higher RPM in the air and on the ground. Originally, it was intermittent, but now is nearly all the time. 

The odd thing is the TX/RX labels in the lower right corner of the com radio display flicker rapidly when the noise occurs, and the LED on the intercom flickers a greenish orange. I haven't had anyone complain about spurious transmissions, but I haven't had anyone flying the times I began to realize it wasn't external interference, so I haven't asked anyone about it. Pressing either PTT switch gives a solid TX label, and solid orange on the intercom LED. It's been suggested there is a short in the PTT wiring, but I don't think the transmit function can be activated that rapidly: the flickering of the TX/RX labels is several times a second, and the noise on the order of 1 kHz.

The situation exists even with all other devices, including Skyview, shut off. I flew the plane for 2.5 years and 300+ hours before this noise appeared, so I think the original installation (professionally done by people that have done identical installations in 10+ Phoenix) was done correctly and something has changed - but what?

 

[jakej]

Normally 'most' of these symptoms are usually due to antenna cabling/connections/ groundplane issues. Or you could have some problems due ignition system & maybe a Dynon problem (last option).
1. What type of coax cable was used ? Eg, RG400 ( or another with high density shield ?
2. Is the BNC connector on the radio end a crimp type or screw nut ?
3. Is the centre pin in the BNC Near the end of the fitting or retracted a bit further in ? A picture is useful here.
4. Should have asked earlier also - are the symptoms evident
with engine off or on or either way ?

 

[Eric_Greenwell]

1. What type of coax cable was used ? Eg, RG400 ( or another with high density shield ?
2. Is the BNC connector on the radio end a crimp type or screw nut ?
3. Is the centre pin in the BNC Near the end of the fitting or retracted a bit further in ? A picture is useful here.
4. Should have asked earlier  also - are the symptoms evident
with engine off or on or either way ?

1. I don't know what type of coax was used, but it did function properly for 2.5 years and 300+ flight hours.
2 & 3. I'll check the antenna connection at the RF module, though I wonder how a poor connection would cause the TX/RX labels to flicker. I'd expect noise getting into the antenna to trigger the RX label, but not the TX label.
4. The noise occurs only with the engine running.

There was one other symptom on Friday, while I was talking to Dynon support: the com displayed the stuck mike symbol ("STUC") after I turned on the radio. I turned the radio off, removed the panel screws, then pulled it forward a few inches - all connectors were tight, and the wiring appeared excellent. The "stuck" indication was gone when I turned the radio on again. The stuck symbol has not appeared again after several power cycles, or when I ran the engine yesterday.

 

[jakej]

Eric, bear with me on this as efficient & effective troubleshooting needs considered questions.
1. Although the 'system' was working very well for the 300 + hrs, it didn't recently & that is the issue - problems like that don't 'fix' themselves, they just stay hidden in the background until next time [ch128578]
2 & 3 - may not be a consideration.
4 - the engine vibration may be causing a damaged,through wearing oh insulation against a sharp edge of say a bracket, wire. A thorough check at the back of panel would be a good start.
Let me know how it goes or pm me & I can call you back ( in at AirAdventure all week) [ch128512]

 

[Eric_Greenwell]

I shielded the open ends of the video cables in the engine compartment, ran the engine, but the difference was barely noticeable. I disconnected the video adapter's USB cable from the Skyview monitor, and that reduced the noise by about half (almost tolerable). My next thought is removing the video cables entirely.

Looking through my notes, I see I was experiencing the static a couple months ago, though intermittently and with less volume, and usually near clouds. Not a clue about that.

I also remember the radio having some issue while I was in Florida in 2014, getting flight training from the dealer. He moved a couple cables around behind the right panel, and that seemed to cure it. I'll check that area for changes, and check the squelch, too.

jakej, since disconnecting the video adapter made a noticeable difference, I think a short is now a less likely source, more likely noise is getting into the system somehow. A poor connection on the antenna connector might do that, so I still need to check that out.

 

[jakej]

Hey Eric, no problems doing that from my perspective. Whatever finally fixes it is what matters, you/we'll get this resolved eventually to get it all running again properly as it should

 

[Eric_Greenwell]

When I turned on the transceiver and intercom today, the "TX" symbol appeared, and the intercom LED turned orange. I removed the power from the transceiver (removed it's fuse), but the intercom LED remained orange. I removed the right panel so I could easily reach the cable going to the intercom; wiggling the cable gently caused the intercom LED to blink green and orange. I disconnected the cable, then measured the PTT microphone inputs on the connector; unfortunately, both read "open". I plugged it into the intercom, but everything was working normally now. All the wiggling, pulling, and tapping I did never caused the TX symbol to appear or the intercom LED to change from green to orange; apparently, the short (wherever it is) has "fixed" itself.

I put everything back together, then added a ty-wrap to support the cable better, as it was hanging from connector previously. I'm closing in on the source of the problem, I think, but will have to wait for the problem to appear again, as I don't want to tear in to the four microphone "transmit" lines (two PTT, and two from the pilot and co-pilot headset jacks), looking for a solder joint or ? that is rubbing against the other lead. That's my best guess about the location of the short.

 

[jakej]

Hi Eric
I'm still travelling home from OSH, will reply on a day or two

 

[Eric_Greenwell]

The problem faded away, but came back in Feb. 2017, though not as badly. Eventually, I discovered the microphone on the Clarity Aloft headset was intermittent by switching it with the identical passenger headset, and a standard ANC headset. I returned the Clarity to the mfg, they refurbished under warranty as it was worked satisfactorily there. I've used it for about 20 flight hours since it's return, and everything works correctly

I'm not certain it was the headset originally, but so far, no more problems.