Thanks for that sailing dog, I have looked at the wiring every time I have replaced the units, the wiring is very straight forward, one BNC connector for the aerial, one connector for the 12V supply and an output for the NMEA 2000. This connection goes straight to the plotter.
I agree it does seem unusual for six to ďpack upĒ but every time the supplier canít find out why. The rest of the instruments have worked faultlessly.
I had had several other electricians look at it, they have all come out scratching their heads.
I am now at a loss as to why itís playing up.