Problem with ADS-B, transponder squawk and Mode C

[547EL]

We’re having a problem with the (older) KT-76A in our RV-6A, and I was wondering if someone here had any thoughts. ATC has told us on the last few flights that our Mode C is intermittently dropping out, and they have asked if I have two transponders on board. (Answer: no!) We tested on the ground and saw that our transponder was swapping between codes 1205 (which we had put in to test) and 0200 (which we absolutely did not put in). We even bought a different KT-76A unit and had the same issue, which leads us to believe it’s a wiring issue or a connection issue. But has anyone ever heard of or dealt with this issue before? I’d happily buy a Trig TT-31 to swap it out (which is the “plug and play” swap for a KT-76A), but I feel like the issue is some sort of wiring, connection, or antenna issue, not the physical unit itself. Any thoughts?

For ADS-B, we have a (micro)Avionix Tailbeacon, which also doesn’t seem to be picking up the squawk code from the transponder.

 

[lr172]

the swapping code is a mystery, but the fact that the uavionics box doesn't pick up the squawk would point me towards coax integrity antenna grounding integrity or a bad bnc connection on either side. Also be sure there is not something that is preventing the unit from fully seating in the tray's backplate. If it had problems with both units, it is probably not the unit itself.

 

[Walt]

I've worked on a lot of KT76A units, to my knowledge, there is nothing external that could cause a code swap, about the only thing that comes to mind is low voltage, this can cause very strange behaviors sometimes in electronics so check that it's getting a good solid 14v

 

[GalinHdz]

I've worked on a lot of KT76A units, to my knowledge, there is nothing external that could cause a code swap, about the only thing that comes to mind is low voltage, this can cause very strange behaviors sometimes in electronics so check that it's getting a good solid 14v

I have also seen voltage drops cause havoc with transponder output. When transmitting, the transponder probably pulls the highest current of all your instruments and with ADSB it transmits a LOT! This can also affect your Tail beacon from receiving a good signal for it to work properly.