Keys C anf F# don't work in all octaves. Could you tell me what the problem could be?

It could be a problem with the key scanning circuit not getting the signal from the keybed. it could be caused by a bad connection, for example on a ribbon cable between the keybed and the logic board. one "simple" thing to try is to carefully open the keyboard up and see if there are cables going from the keybed to a logic board that could be unplugged and replugged. sometimes oxidation builds up on connections and the act of unplugging and plugging back in will scrape the contacts. i had a similar problem to yours on a roland, but it took fixing a bad connection to a surface mount resistor that was part of the keyscan circuit - this required a schematic and component layout from a service manual to solve. it also required the use test equipment and some careful soldering. be careful if you go inside, you don't want to make things worse and it needs to be properly reassembled, that is if you need to hand this off for professional repair. another consideration is you don't want to discharge static electricity off your finger into the delicate electronics. 50,000 volts can easily fry circuits - even if a discharge is lower, you begin to compromise the electronics which would make it fail sooner. bench techs use anti-static straps for good reason. the bottom line is, with some careful precautions you could get lucky and fix this yourself, IF the problem is that simple (and mine was not that simple.)