Furuno GPS/Fish Finder - GPS has "NO FIX"

[Troutboy NZ]

it's a Gp-1650DF, everything seems to work, but the GPS side keeps coming up NO FIX, won't acquire ant satellites. Have check all cables and tried another antenna and still no go. I understand I am supposed to be getting +5v when I volt test the TNC femal socket on the back?

There is nothing registering. Took the unit cover off, can't see any loose wires / connections etc. the start up tests all say everything is OK, incl. the GPS side. Anyone have any ideas? It'a approx 7 years old.

Thanks

Andrew