The GPS in the 340 is fixed. Took it to Rockford and had an expert look at it.
As I suspected, it was the antenna. Likely the amplifier at the antenna had failed. They replaced it after checking the GNS 750 unit out in a test fixture. Either way, the 750 GPS works and I now have not only easier navigation, but also ADSB in and out, which gives me airport data, weather radar, and nearby traffic.
Plus, while I was there, I had the plane undergo it’s 2 year IFR certification since it was due.
More flying soon.