Steve

If it has an adhesive aluminum foil tape type ground plane then it may be more suspect than say a better looking aluminum plate ground plane. I’m not sure by what you mean by “properly grounded”... you don’t want to add additional ground wires /jumpers, what you do want to do is check the condition of the ground plane where it meets the antenna connector and that the outer coax nut is tight and contacting actual aluminum. IIRC the DG 400 fueslage is mostly fiberglas with some carbon but I have no idea about where your transponder antenna is or might be mounted. There are enough gliders of this vintage with really bad transponder antenna instalations in places like the gear doors with undersize and poorly made ground planes.

DG have a TN available for transponder antenna installs, because they live in a closed world other folks including me can’t see what exactly they recommend. I hope it is more like an round aluminum sheet plate, and if it’s over carbon fiber the surface in contact with the ground plane is sanded down to expose carbon fabric.... but follow the TN instructions as close as possible.

I also have no idea how hard it is to reach the antenna, if that needs the engine removed etc. but I would really try a jury rigged new coax test on the ground. And avionics shops may have coax already terminated laying around.m they can use for that.

It’s unclear what technician you are working with, is it’sn actual avionics shop it should be easy for then to exercise your transponder in the bench, worse case they might want to pull out your encoder and wiring harness or make up one to get stuff running on the bench. Some guys who do mobile transponder tests only may not be able to or not want to do this. If it’s a glider repair shop they usually will not have the test equipment to do this. If this is your first glider ownership experience be careful who you ask to work on your glider. Many avionics shops may not be good choices unless they have experience with composite aircraft and hopefully have seen gliders before. Many glider repair shops are not set up to do advanced troubleshooting of avionics. You may need to work with both types of shops to get this resolved or a Trig TT22 installed.

I would be wary of replacing the Transponder and finding the coax or ground plane is faulty. If you do replace with a TT22 that does open up the possibility of installing the transponder body remotely, closer to the antenna which might encourage you to replace that coax anyhow.