We picked up this gorgeous Philips D8569 boombox from 1982 at a second-hand place recently.
When we got it home and put a tape in it, the sound was really muddy. And worse, every time you moved one of the volume sliders even a tiny bit, it produced an ear-splitting crackle at maximum volume from both speakers. We were almost going to try to return it when I thought "I bet it'd be fun to try to fix it instead".
So I ordered a can of DeoxtIT D5 spray to deoxidise the volume sliders, some tape head cleaner and demagnetiser, and some plastic polish to try to knock a few years of aging off its appearance.
Deoxidising the sliders meant partially disassembling the thing. It was quite exciting to see inside an early 80's music player.
The adjustable potentiometers were on this brown circuit board underneath.
I sprayed them a little tiny bit, moved the sliders back and forth a whole bunch, then dried it off as best as I could. After reassembly I ran the cleaner tape and then gave the whole thing a quick polish.
The result is a working boombox, give or take a few remaining issues. I'm not 100% convinced the tape decks are finished yet. One has a pause button that's stuck, so that one is out of action until I'm feeling brave enough to disassemble that whole button mechanism. The other one sounds a ton better but maybe not perfect yet. The volume slider crackle is gone though.