Phil,
Have you contacted the manufacturer? I just had a similar problem with my Raymarine SL72 Pathfinder Plus 18" radome. Although the radome was out of warranty, Raymarine has a flat rate for repairing the unit. For my 18 inch radome, the flat rate was $500 and it comes with a 90 day warranty.
I was playing on ebay but found that the used units for sale were going for more than $500, there was no warentee, and most of the sellers wanted no part of returns.
I wound up having it repaired directly at Raymarine and surprisingly, the repair was only $200 (shipping and tax included). If you haven't gone this route already, give Furuno a call.
