Have had these for years and last time I pulled them out of being stored for some time, the hinge has lost tension and won't hold in position. Is there a cure for this that I can accomplish or will I have to send them in for service?

Take hinge cap off remove snap ring and install small washer install snap ring and cap.jmho

There is no snap ring.

On the set of Minox I have there is a flat round nut (even though it doesn't look like a nut) on the front of the hinge with two holes in it for a spanner wrench. Simply tighten the nut.