Tom,
The only one I found in the U.S. is Peninsula Avionics, Mr. Nick
Popovski, which I believe is also known as Nick's Avionics. I sent my
FSG71 there this winter and the service was pretty good. $250 and two
months (had to get a part from Germany).
http://www.peninsulaavionics.com/
Erie Aviation no longer services Dittel, the U.S. rep on the Dittel
site also no longer is licensed.
Regards, Bruce