How to fix non-working keys with a pencil for free

How to fix M-Audio Oxygen (and all other digital keyboards) non-working keys with a pencil for free.
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If any key on your keyboard does not work or does not respond correctly to your fingertips, you should do the following:
1) Remove knobs and sliders, open the keyboard, remove plastic buttons, and gently remove rubber strips on electric board;
2) With a pencil gently colors the contacts of the sensors located beneath the rubber strips;
3) Replace the rubber strips on the contacts, reset the plastic buttons, close the keyboard and retrieve knobs and faders;
4) Enjoy it and please like this video! ;)