"IanM" wrote in message
...
If I remember correctly, the speed is a pulse signal and the direction
is two signals with voltages corresponding to the SINE and COSINE of the
angle of the windvane. There is an 8 volt supply fed up the mast from the
display head to power the masthead unit and the direction signals swing
from somewhat above zero to a bit less than the supply. The symptoms you
have indicate a missing supply or ground to the masthead unit. It may be
a failed display head or bad wiring or possibly a failed masthead unit.

The display head and masthead unit are often repairable (although
raymarine wont be any help - you need a good independent, preferably
working for beer money rather than commercially) and the wiring can be
replaced. If you do end up ripping it out, as the ST50 wind transducer is
discontinued, someone here may well be interested in it for spares.

Thanks. This is more or less what I suspected but your detailed comments
encourage me to check the wiring again. It did work intermittently for a
while, which could imply a poor connection. It's such a nice instrument that
I am hoping to repair rather than replace - the modern analogue instruments
are pretty expensive. Unfortunately, the wiring runs through a very tightly
packed conduit up the mast which will have to be dismantled to get the wire
out. In the meantime, I'll check the visible ends.
J