On Thu, 12 Oct 2017 14:27:20 -0500, wrote:

My thinking is that even though it worked fine for years, one of them
solder joints is touching the case and grounding out. But why it did not
do that in the past makes little sense. If I could, I'd put some
electrical tape along that edge, but it slides into a groove with no
spare room.

Maybe try painting the case and/or solder joints to insulate them?