Pretty sure Mylaps does not do any repairs. They just sell you a new one. I've opened up the previous version of transponders to replace wires and such but not the newer hybrid or RC4. So I don't know how difficult it would be to do it yourself. They are epoxy filled though so it's quite a headache to get to the board itself.

You double checked that it was plugged into the RX correctly I assume? Weird as I've never seen a Mylaps personal transponder go bad. It's always been bad wires in my experience. It's bound to happen I suppose.