One of the main circuit boards from an old (~25 years) cathode ray oscilloscope, or "scope". This particular one has intermittently been having problems for years. I'd do some work on it, and  it would be okay for a year or two, but the fault would eventually return. Finally gave in and completely stripped out this board today, which took over an hour, not helped by the manufacturer gluing some of the thirteen connections together.

After much cleaning, tweaking and testing of all the suspect switches, had to put it all completely back together to be able to test it. Worked first test, much to my relief, and hopefully that'll be it for its remaining years of usage... for that fault.