I've got back to excavating into the epoxy filler to try and find the wiring
break. There are 2 coils that are precisely laid down in a rectangular cross
section. The straight parts of the D are bowed requiring a wide central
housing.
I can see why the failure, but not the break site yet. There is a moulded
hard plastic protection cover that clips under the section enclosing the
coils. This can fill with rainwater and then that can wick into the upper
section.
This is a conductive plastic (why?) shaped housing (for the top) , about 50
ohm per cm. Upside down at making, the coils are laid in this and
substantially filled with neutral bulked-out , talc or similar, epoxy and
then a "top" fill layer (becomes lower surface) , neat epxy. But the seal
between the 2 forms of epoxy and to the plastic is poor and this neat layer
can be easily prized away and presumably open to wicking.

Other than expected pair of coils there is a rats nest of copper wire at the
point where the , 2 wire only, signal outlet is, and curiously a loop going
half way along this central wide area , presumably for some sort of
matching/balancing process before the final epoxy filling.
Still not found where the 2 coils are joined but if anyone has any inkling
for these things ?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/