Transponder problems....how to troubleshoot?

[flyboy1963]

I have an old KT76a which seems to have been working fine at testing & 'recertification' in 2012.
Yesterday the tower tells me its transmitting the wrong codes.
several iterations of wiggle the knobs, ident, and new codes did not resolve the problem.
For example, when assigned 2673 it would respond 2675....but even 6350 or something would reply as 6360, then 6325 etc....so not just one numeral to blame.
Is this common? Is it worth crawling around in the fuselage to check antennae connections, or is it just due for overhaul/replacement....and if so, with what?

( it was just below freezing, but the cabin was well warmed up, but not hot for the duration of the flight.)

note: there was an AD on these units, for silver migration, corrected by replacing two resistor network modules with glass-coated modules but not affecting my serial #118461.
AlliedSignal KT 76A Air Traffic Control (ATC) transponders; part number (P/N)066-1062-00/10/02; serial numbers 93,000 through 109,999

 

[blueflyer]

swap it out for a test flight

Do you happen to have another transponder that you can stick in there for a test flight? Just go for a short hop with the replacement and see what you get. That way you know if its the transponder or not. Then work from there.

 

[RVDan]

Not likely that the antenna will have anything to do with a wrong code. The code is data encoded in the singal. I would Suggest a bench check of the transponder. It will take a tech 5 min on the bench to verify.

 

[az_gila]

I would suggest a check of the encoder to transponder wiring, and a cleaning of the connectors at both ends first...

Do the easy stuff first.

 

[Walt]

Not likely that the antenna will have anything to do with a wrong code. The code is data encoded in the singal. I would Suggest a bench check of the transponder. It will take a tech 5 min on the bench to verify.

The above is correct, the squawk code is generated internally from the knob inputs, there is a switch or a logic problem here. Reseating connectors won't help in this case, pull it out and let a shop verify the problem on the bench.

 

[flyboy1963]

great advice; thanks guys!

I'll pull the unit and get a bench test; it's just too cold out on the field to do much else anyway!

 

[flyboy1963]

turns out to be dirty switch contacts!!!

Just info for anyone who may have a similar problem......

pulled the unit, local heli avionics shop bench tested....all was ok with the guts, so they cleaned the code selector rotary contacts.
NOW they actually go.....'snick, snick, snick' when you turn them, not....' slop, slop, slooo....'. I had no idea what a new one should feel like!

test flite last week, and it seemed to be stable and sending the right codes.

$200 well spent. (thanks Alpine Aerotech)

thanks everyone who responded to my inquiries to replace!