TRANSPONDER PRESSURE ALTITUDE ANOMALY

[Bob_Redman]

G'day,

Dynon D100 EFIS, grey scale converter to a Microair T2000 transponder, connected via Dynon and Microair harnesses.   Worked well for 30 months, and passed last transponder check two years ago.

When vacuum applied, transponder displays NO ALT at multiples of zero, 500, 1000, 1500, 2000 ft etc up to 13000ft PA.   Transponder display shows PAs of x100, x200, x300, until about x450ft on the Dynon alt scale, then NO ALT until Dynon indicates 550ft, then Transponder shows x700, x800, then NO ALT until x050ft and so on as altitude increases.

Recently, during flight, I have noticed momentary NO ALT displayed in the transponder, but no anomalies reported yet during occasional comms with air traffic agencies (secondary surveillance radar).

I would welcome any hints to correct this anomaly.

Regards,

Bob Redman

 

[Dynon]

The way that gray code works is, a single bit (wire) changes every 100 feet. If you stare at http://web.archive.org/web/20120726003224/http://www.airsport-corp.com/modecascii.txt for a good long while and your head doesn't turn into Tron, it you'll see that C2 wire that's not changing states (trust us here).

To test, you'd want to use a voltmeter on that wire while connected to the transponder and everything's powered up. You should see the voltage change near the 500 marks, but yours won't. It's likely a wire issue, but not impossible for the encoder to have failed.

 

[Dynon]

(the C2 wire is the white one going to the transponder, incidentally)

 

[Bob_Redman]

Thank you Dynon,

I will check all the connections but give first priority to C2.

Regards,

Bob Redman

 

[Bob_Redman]

I checked continuity of each wire in each harness (D100 to converter; converter to male/female Dsubs; Dsub to transponder) - all good - so suspected a loose/unseated pin or socket not yet found.

Then realised that update of software (version '8') in the Microair transponder would eliminate the need for the converter - I connected a twisted/shielded pair direct from D100 to the DSub at the transponder, and removed the converter, associated looms and Dsubs.

The transponder has now passed the lab test at the factory, and the installed test in the aircraft.   Now back flying and will re-check the cause when I make time.

Regards,

Bob Redman