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If you decide to tear down your MC-303, be patient, sober and in a peaceful place.English is not my mother tongue. Please be forebearing, I try to help you out !
Before starting to tear down this machine, I strongly advise to use boxes to separate all sort of screws to avoid mixing or loosing them. the first thing you need to do is to take off all knobs and the wheel as well.
Just pull them, they should come easily..
Once your groovebox has been opened, this is what you should see.
First, you can see three ribbons attached to the rear pannel to the other cards.
Use a small plate screwdriver then lift the small clips (not on the rear pannel, on the other card !).
Start with the side with a red dot. BE VERY CAREFULL, ribbon and its clips are probably the most fragile part of this machine. You will notice one of the ribbon cannot be unplugged since the clip is underneath the biggest card.
You will see ribbon ends with pins...
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On this second pic, you can see the mother board, the brain of your MC-303 with the small battery used for saving your patterns and songs.. and underneath the mother board, the controls/display dedicated card.
after doing the same operation than previously with the ribbons, take off the 4 screws to liberate the mother board...
Sorry for the bad quality of the photo but it was done 'on the fly'...Well now once you will take off all screws away from the controler card. BUT...something will prevent you to take off the controller card out of from the box...it is normal.... 
The wheel is the guilty one...use a #11 box-end/ring spanner (here in France it's #11 but in other country, sorry I am not sure).
When it will be done all buttons will fall away, try not to loose one ! Only the keyboard should remains (at least mine did)....
Here is the naked creature.
Here is a dirty potentiometer, moreover it has backlash, you can use a wrench to fix this : look the potentiometer is inside a kind of claw, tight it carrefully (and not too much and it will fix this problem)
You can also do this with the Bass boost one on the rear pannel
take the most of the fact your MC-303 is dismantled to clean it, get rid of dust...if you see oxydation on some metallic electronic parts, use cotton buds with a little bit of alcohol.
Well once your potentiomenter are fixed, your MC-303 has been cleaned, you can start to tear up your machine.
One of the most merciless part for your nerve is buttons...so be very patient.
I advise to lift your groovebox with something else than buttons used on the photo because it's way too small (I failed with this so I used 4 soda cans and it was way better). and, it's important, dont tight screws too much or buttons may be stuck. Also, don't put the buttons upside down, do as you see on the pic
Actually, here is an exception : 3 buttons NEED to be put upside down because of Led above them !!!!
