Look for
repairs on the
internet. I had a friends Raymarine
chartplotter go black somewhere in
Italy. We followed the instructions found on the
internet for taking it apart, cleaned all the contact areas and put it back together. Everthing works like a charm since then, 3 years ago. Contact oxidation seems to be a known problem, just try it, nothing to loose it is
winter you have all the time. We were moored when we did it in one afternoon and with some rosé wine.