I wish I could offer some advice, but I only know how to operate them not repair them. It sounds to me though, that you got it pretty narrowed down! Looking forward to seeing if it works out for you. That's a great machine if you get it working again!
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Thanks for the comment, I sure hope so. I think it would have been easier if the units were just dead rather than this type of fault. I will do some experiments with the coil and see if I can pick up what happens when metal is passed across the search coil. Whatever happens I am sure it will increase my understanding.