Intercom hum

[yoloflyer]

Finishing a Van's RV-9A. Skyview suite: SV-D1000T, SV-COM-425, SV-COM-PANEL-D15, SV-INTERCOM-2S, among other things. With everything powered up (engine off) there is a constant hum in the headset. Engaging PTT switch makes it worse. If I turn the intercom off, hum goes away. If I turn the display off using the left button it also goes away. With the engine running there is more noise (hum or whine), reduced to the engine off state by shutting off the alternator. Again, engaging the PTT makes it worse. I did notice that things are quiet until the Skyview display has finished its boot sequence.

I wired the audio in accordance with Figure 181 of the Skyview installation manual and the diagram of page 2-3 of the SV-INTERCOM-2S installation and user guide. Shielded wire used as required. Skyview audio connections (with twisted triplet) also per page 2-3. I was careful to bring shield pigtails and grounds together as close to the intercom D-SUB as possible, where they connect to Pin 1 of the intercom. I'm not an electronics specialist but I'm well familiar with the concept of ground loops so I was careful with that. I did note that Figure 181 implies that there is no connection to ship's ground at pin 1 on the intercom, whereas the figure on page 2-3 of the manual implies that there is (I did ground pin 1 to ship's ground). Per Figure 181 I also have pin 12 on the COM grounded (all ship's grounds running to the usual forest of tabs on the firewall, connected directly to the battery). I was careful to keep power leads and audio signal wire runs far apart.

I interpreted the PTT hookup depicted on page 2-3 literally; I ran wires all the way from the mic jacks to the PTT switches and back, in other words the PTT switches don't ground pins 10 and 16 locally, rather they run back to the audio grounds.

Not really sure where to start troubleshooting this, I did a search here but didn't turn up anything useful. Has anyone else encountered this?

Many thanks in advance!

 

[Raymo]

Based on what you've said, sounds like the SV screen(s) may not be well grounded. There are threads here on the topic but it is probably easier to use a web search engine (e.g. Bing, Google) to find it.

 

[yoloflyer]

Skyview display case appears to be well-grounded to panel and hence to battery ground, verified with my Fluke meter. Tried disconnecting intercom ground, changed nothing, so I reconnected it. Tried creating PTT ground locally near bottom of control column rather than having ground lead run all the way back to the mic socket, also changed nothing, so I reconnected as I had it. Today the quiescent noise (not transmitting, no PTT) is less. Baffling. But when I engage the PTT the noise is as bad as ever, especially with engine running.

I found a thread on makerplane that suggests the audio ground (pin 30) from the Skyview should be left disconnected, contrary to Dynon's instructions. Did not try that, yet.

Any other suggestions?

 

[mrdaud]

Had this problem with my Dynon in the beginning and drove me crazy on long flights. Finally found that it was related to squelch. Set my squelch higher by 10% in the setup menu and the noise disappeared. This is one complaint I have with the Dynon comm in that it (mine) is a bit touchy with respect to noise/signal setting and there is no squelch adjustment on the panel.

Usually associated with engine running (alternator),you can get ripple voltage noise, or sometimes some other "box" is not well shielded and you can get noise. One thing to add is a filter capacitor to the power buss. Reasonably inexpensive and certainly won't hurt to have installed. One source is B & C Specialty:
http://www.bandc.aero/electrolyticfiltercapacitor.aspx