Check the lowest, oldest, layer of electronic things in your storage - -
Back in the days when televisions had mechanical tuners, tarnished contact points (even though gold plated) where a recurring problem - -
And the industry had a "tuner cleaner" spray for fixing them.
You might even have some at the bottom of your oldest pile.
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You may have discovered what Lowrey learned the hard way back in the '70s - -
There are some plastics (as used in their keyboard contact supports) that will 'out gas' over time, a gas that will corrode gold (which their contacts where plated with).
I spent a summer working for a Lowrey dealer = going to people's homes and disassembling their organ, using a tarnish remover on the keyboard contacts, and if the people where nice to me - putting the sucker back together.
(It was about a five hour job - the factory never thought those contacts would have to be cleaned.)