I've figured out the problem to my Casio SK-1200 no sound issue.
It is something to do with the MUTE. I never bothered to find how to un-mute mine, I simply bypassed this feature by soldering a jumper on the LA4620 amplifier (big thing connected to the big heatsink). By applying 12Vdc to the pin 13(I jumped pin 11 to 13 since they're next to each other), the mute function is disabled.
Here is the link to the LA4620 manual http://pdf.alldatasheet.co.kr/datasheet-pdf/view/40130/SANYO/LA4620.html