No Mode C - SV HDX800 & Gray Code Conv. to GTX320

[buckleywe]

New install with an HDX800 and Dynon Gray Code converter feeding a Garmin GTX320. ATC receives primary target but no Mode C. Xpndr back lighting work and I get a reply light on the xpndr when flying. Things I've tried so far:
1. Verified HDX800 serial port (4) settings are correct per the Dynon manual. It shows data transmitting on the Setup page.
2. Verified the Dynon Gray Code twisted pair connection to the HDX800 Dsub 37 pins (Green to pin 10 for serial-4 Tx and black to pin 23 for ground).
3. Reinstalled all the output wires from the Gray Code converter to the xpndr Dsub25. Triple checked the pins numbers are correct. Checked them again 4 more times.
4. Removed the Gray Code converter power input wire from the xpndr power output pin (xpndr pin #14) and wired it directly to the xpndr circuit breaker to make sure the converter is getting power.
5. Verified the xpdr connector is fully seated.

Question: Is there anything I can check/troubleshoot to verify the Gray Code converter is correctly supplying altitude info to the xpndr? I'm more suspicious of the xpndr at this point but would like to verify the correct altitude data is being provided before I tackle a xpndr repair. Yes, I know the Dynon transponder is the ultimate solution and that is my plan in a year or two. I'd like to fly for a while with this 'old-style' transponder while making a fiscal recovery from the recent completion of my aircraft.
Thanks,
William B., Cozy Mk4 - N494WB

 

[Dynon]

So first, just a reminder that before you can use the transponder before you do the tests prescribed in 91.413. IE, unless it broke, ATC shouldn't be discovering the encoder or transponder not working properly.

All of that said, you can check the actual wires on the encoder converter module. For a given altitude, each of those wires will be either grounded or not, depending on the gray code encoding for that altitude (see http://www.avionictools.com/graycalc.php, remember that you want pressure altitude. Set your Dynon to 29.92, then feed that altitude number in to see what you should see at each wire).

 

[buckleywe]

I took the transponder to an avionics shop and they bench tested it. Its checks good for Mode A and Mode C. The avionics technician gave me the ubiquitous interrogation about how I wired the system. I reviewed the Dynon converter wiring diagram with him and swore multiple oaths that I have re-re-re-re-re-verified the converter wiring pin numbers, converter power connection, converter ground connection to avionics ground buss, SV serial channel 4 (pin 10) to converter green wire, converter serial connection ground common to SV pin 23, and serial 4 TX setup in the SV Setup Menu.

Returning to my hangar, I then used the Altitude Offset feature in the Setup menu to test these altitudes in the SV: 500, 400, 300, 200, 100, 0, and -100. Using the Gray Code generator at the website listed above, I tested the 9 output wires for each altitude. All the altitudes checked good for the correct wires having continuity to ground (or lack of continuity as specified). My test point for each Gray Code wire was the tip of the female pin on the connector that attaches to the back of the transponder. For this range of altitudes (500ft and below) not all the pins are 'activated' so not all of them got tested. But with a 100% success rate for the available altitudes its unlikely that the pins used for higher altitudes would be inoperative (yes?). So I have a transponder that checks good on the bench (Modes A & C), powers up (including backlighting) in the plane, has good Mode A reception by ATC, displays a Reply light, and no Mode C reception by ATC. All while being connected to the converter that demonstrates proper code output for 7 altitude blocks tested on the ground.

I used a standard Dsub 25 female connector for the transponder connection. Having some unused Dsub female pins, I checked the fit of the Dsub pins on the transponder male pins and they make firm contact. Just wanted to check even though other features of the transponder are working - all using the same connector/pins. Could the Garmin spec'd connector really be that different from a 'standard' Dsub 25?

Any other suggestions are greatly appreciated.

William B.

 

[buckleywe]

VFR certification complete for my transponder. Finally isolated the problem to be an incomplete seating of the antenna connection with the backplate. I reconfigured the Dsub 25 connector to allow the last fraction of movement (< 1/16"). Being able to check the output of Dynon converter was a big help to narrow down the source of the problem. Thank you for the info.
William B.