Dynon SV1000 with garmin GTR225 - Interface issue

[Don_Jack]

Hi everyone,

I'm have always had a problem with the post installation of a GTR225 that has affected my Dynon Skyview ever since. Everytime I use the PTT on either side of the cockpit, the altitude reading goes up about 500ft on the scale, and the amp sensor starts climbing also. Releasing the PTT and everything goes back to normal.

The radio itself works fine, as do the headsets, with no interference, just what happens with the Dynon. I fitted filters on the AHARS, and on the GTR225 co-axial, but with no difference.

Previous to the changeout, required for 8.33 spacing, everything worked perfectly. I'm posting this on the Dynon sight, not becasue I think there is anything wrong with my Dynon, but with the interface post installation, with the GTR225, that I hope someone in the community might be able to help me with.

From the Garmin manual, Figure D-6 GTR/GNC - MIC Interconnect, I can see that the PTT is in parralel with pin 11 & 20 of the P0001 connector, and I checked the continunity from one side of the PTT, and had continutity between the barrel of the mic plug and the PTT NO contact, but also from the barrel of the MIC Plug directly to GROUND, but had nothing on the other side of the PTT and with any of the other connections on the MIC plug.

Note on GTR225 Manual on the MIC page.
ALL HEADSET, MICROPHONE, AND MUSIC PHONE PLUGS MUST BE ELECTRICALLY ISOLATED FROM GROUND. THIS MAY REQUIRE THE USE OF INSULATING WASHERS WHEN MOUNTING THE PHONE PLUGS. ADDITIONALLY, THE SPEAKER RETURN MUST BE ISOLATED FROM GROUND. also in the D-6 diagram for shielding of the cables they have the comment that all wiring GROUND DESIGNATION: SHIELD BLOCK GROUND.

I can see the cables are shieded but dont appear to be grounded, although from one end there is a ground connection coming from the control column stick, I just haven't been able, at this time, to get is all apart.

I hope that someone in the community will have some idea where to start!

Best regards to all

Don

 

[Rhino]

All wiring that carries audio in your aircraft should be grounded at one central location. Normally that's the intercom, but some builders choose a different location. You run the risk of ground loops otherwise. See this:

Ground Loops and Noise in Aircraft Systems

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those...

forum.flydynon.com

Your PTT does nothing but apply a ground to a comm panel, intercom, or directly to a radio. It carries no audio, and should not be the cause of interference. Whatever your problem is, it's almost certainly downstream of that function. You very likely have an audio or RF wiring issue. I would suspect RF in your case.

What kind of antenna cables do you have? Cheap RG-58 cables are notorious for causing such problems. It really should be RG-400 or better. And make sure all antenna connections are good. Sometimes they aren't after an upgrade.

I would have to dig to research the GTR 225, but I''m fairly sure there are other threads here dealing with that. I would dig now, but I'm recovering from rotator cuff surgery, and keyboards are not my friend right now. Maybe someone else can dig further for you.

 

[maartenversteeg]

True push to talk is not affecting anything. But it activates the transmit function of the radio which in normal cases makes the radio pull much more current, which might increase the voltage drop over the power lines of the radio, This in turn (depending on how all the system gounds meet at the shared ground point) might affect measurements by the dynon system

 

[Don_Jack]

Thank you both very much for your considered view on this, it is very much appreciated.

I have the installation drawing now available, see attached, and I can see that it wasn't installed exactly as the GTR225 manual mandates, in that it used 2 wires to the microphone plugs, Mic 1 & Mic 2 barrel connection have been joined together and use Pin 20, with then Mic 1 connected to Pin 5, and Mic 2 connected to Pin 6. The shielding cable is attached to the chassis of the GTR225.

The other PTT1 connection goes directly to Pin11, and PTT 2 uses Pin 26, with the barrel connections from each control column tied together, and then grounded. It says it was taken to the chassis of the radio, but I suspect this is the yellow wire connected to the airframe close to the control column, but I have to trace this carefully to confirm this. The drawing also shows that the both PTT connections, i.e. Pin 11 and Pin 26, have no sheilded cable used.

Thank you also for the great article with regards to ground loops, which is great, and helps me understand that whilst there has never been an issue when flying without the autopilot, becomes and issue when it activated, because it wants to follow the false altitude reading!!

Cheers

Don

 

[Don_Jack]

Hi everyone that helped.

As it turns out Evektor knew about this issue first some time I think.
Solution was to fit 2 capacitive filters in series with both Dynon AHAR's units.

The only other thing remaining is the battery amp indication, as it still shoots up when you press the PTT. I just have to where Evektor fitted that. Might just be a a similar filter into the EMS unit.

Thanks for all you help

Regards
Don

 

[jakej]

Good result so far Don with great input from Rhino however you never answered the question on coax cable type - what is it ? RG58 or RG400 etc ?

 

[Don_Jack]

Hi Jake, yes it was RG-400 coax fitted, from the factory.
The filter was a DGFC9MF (D9), and I got it from Digikey in the States.
What I forgot to say that I also fitted a capacitor in the circuit from the GPS, as per Evektor's request, although this had little effect untill I fitted the AHARS filters.
Other thought, was I didn't have any of these issues until we were basically forced by EASA to have the Dynon comms, and transponder removed and replaced with Garmin units, just becasue of the RTC aircraft were coming on the market. at significant cost to the existing Permit to Fly aircraft owners.