On Wednesday, May 2, 2018 at 10:24:27 PM UTC-7, wrote:
"Thanks for equipping with a transponder..."

Darryl- I have been flying with a Becker 3401/5401 Transponder/ Control Unit for over 9 years. It didn't pass re certification this year due to an undetermined problem (spurious altitude deviations according to the test equipment). Rather than throw money (possibly LOTS of money) at an old piece of equipment, I chose to go with a new Trig TT22. As to adding the TN72, that will depend on whether to go to Experimental Category. For ADS-B In, I am using PowerFlarm and my LXNav S80 display.

If I can determine the problem with the Becker, I will probably fix it and sell it, or possibly sell it "as is" with the caveat that it needs diagnosis, repair and a new approval.

You can always add a TN72, just on a type certified glider it can only do SIL=1/TABS.

On the Becker transponder... the encoder and cabling is always suspect there. Ideally have a technician test it with a know good encoder and cabling on a bench... but it's probably not worth much not working. I've seen used working Becker ATC4401 transponders selling for ~$600 with encoder.... that can be a nice deal for folks who have older gliders and want to add a Mode C transponder.