Existing Transponder codes reappearing regardless of new code.

AF5600 with ACm and sandia STX165R transponder. AS I entered controlled airspace at altitude and 140 KTs, ATC gave me a 4400 squawk code and i was acknowledged and the reply indicator was flashing. Upon exiting airspace i must have tried putting in VFR, by button, screen, or putting in 1200 by digits, on, alt, stby and it would revert back to 4400 and alt. I tried for probably 10 minutes and just put it in standby and landed and still on the ground it would revert back to the 4400 code.

i reviewed the pilots guide and am operating the transponder as i understand the manual. Just pushing the vfr keyboard button would switch the code for about 2 seconds then revert back to the previous squawk code.

Looking for suggestions.

Thanks

Keith Rhea

Keith,
Sounds like either a broken serial tx wire to the Sandia, or a problem with the Sandia itself. How does the transponder respond after a fresh power cycle? If the wire is broken, suffice to say we would expect this problem to persist. If there was a glitch with the Sandia, it could very well be something that fixes itself after the transponder is shut down and rebooted.

Jonathon: Ok thanks will try and get the stx165R checked out.

Keith

1.Remove AFS custom made harness. (57505 diagram), dis-assemble and remove rs232 wires from d15 connector to d9 connector and inspect d-sub pins, check twist (not consistent) Meg-ohm wire 20M scale (0.0 resistance), ohm wire 0.9 ohms resistance. Inspect wire for any damage or anomalies and pull test to make sure no breaks in wire between pins. Used 2 different multi-meters and magnifying glass. {I am asserting that the wire is intact and functioning}

2. Additional observations is: Default transponder code is 1200 and stby upon powering up avionics. I can enter a discreet squawk code and either (stby, on, alt) then ident depending and the code is immediately accepted by the AF5600. AS i try to enter a VFR code of 1200 the AF5600 code display changes and in a count of thousand one, thousand two, reverts to previous code before another keypress can be activated. IF i am ready and can enter the 1200 code and (stby, on, alt) and do those in under the 2 second count then it accepts the code change and stays until another code is entered. Doesn’t seem to change after power cycling either. Repeated multiple times.

Next procedure is to try and check the rs-232 signals. Either the AF5600 is not sending or the ST165R is not switching.

Will report back results.

Thanks


Keith Rhea

To further test the rs232 signals, i built a d9M to d9fm with a 3 volt led on the TX line and a 3 volt led on the RX line. You could clearly see a heartbeat and when key strokes were entered on the AF5600 to tx to the STX165R would light up but the return signal would not, so i am confident that the STX165R is at fault in it's RS232 circuitry. RMA received from Sandia and unit is shipped. Jonathan had me do a loop back test which shows the Rs232 in the af5600 was working.


Thanks

Keith