Push buttons are intermittent and have static, how do you clean them please? NAD SERIES 20 3020 AMP

I’ve had the amp overhauled and it sounds great but when I switch between like tape and auxiliary and tuner, the sound cuts out and I have to just kinda like press the heck out of the button to get it to actually sounds fine so it seems that they’re dirty, but I don’t know how to clean them.
I’ve included photos of the buttons on the outside and the switches on the inside. Thank you!
 

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