Need help with Raymarine Radar C70/RD218

[simio47]

I have a Raymarine C70 MFD with an RD218 raydome scanner which has been developing a fault over the last year or so that has been getting progressively worse and now barely gives an echo at all. If I increase the gain to 80 or more the screen becomes cluttered so I can see the unit is definitely turning and transmitting. Occasionally it may lock on to a target but this will disappear after a few seconds. The tuning indicator when I select manual tune is always stuck on one bar. The CT and FT values on the diagnostics screen change constantly between 20 and 220. All tests on the self test feature show a pass.

I have spoken to the Raymarine technical support but they claim they can no longer help with this model. Before I dig deep and call a radar technician can any body help or has any one had a similar problem to this one and what did it turn out to be?