Let us know what conclusions were reached about its fault(s) when you return it to the giver/vendor. I am curious as to what it is that caused the issue when it seems that even after "testing" all the things recommended, you weren't able to see what the problem was.
Originally Posted by Roger2000
Unless all the negs were all greatly overexposed and /or overdeveloped then it strongly suggests a lens which has stuck at a very small aperture at just the point that you switch to colour neg printing and yet if this is the cause then switching back to a known B&W neg which was previously OK and bright would have revealed a similar dark neg projection. I am totally flummoxed.