Yeah I couldn’t deal with that… hope you get the new knobs soon, Knobs. 
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I had a Gamecube controller that went like this. Must be the acid.
Aciiiiiiiid!
sorry
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Another theory: it might be UV exposure.
Sun, or not sun ? 
I smoke, roll my own as it’s a lot cheaper here in Sweden as well. If I’m to smoke inside, an oven fan and open windows is a must. Usually I just smoke outside. So no smoke near my gear. From me that is. My neighbor beneath me is a walking chimney, his balcony right beneath me makes it impossible to keep windows open, his smoke just fills up the place, it’s disgusting even tho I smoke myself, and it gets to my gear should I leave the windows open.
I’ve always trained a lot, running, weights, jj, been on a 8 month vacation from all that. And I’m really starting to feel the negative effects of smoking, the lungs takes most of the beating, but it’s also affecting blood flow and a host of other things. I really should quit. I need to quit coffee as well, can’t have a coffee without a cigg…ugh:(
So I found this thread looking for ‘clean knobs’ with the search function. haha, good fun to read. but I think the yellow-ish button’s are not caused by smoke. If that was the case, then the smoke would have a strange preference for rotary buttons to stick on.
We all have grease on our hands and the material on the knobs, the rubber, get’s polluted by that grease. So I would put my money on a root cause like - somebody who is a car mechanic that was the owner of the unit.
Cleaning the knobs with a solvent is tricky as you could solve the rubber, too, and the knobs get sticky. I used Cif, this can also be used to clean silicone in bathrooms so should be safe for the rubber (was my guess). Works ok, no yellowness anymore.