So I finally contacted Korg Canada. Their answer was short but not sweet : they don't honour warranties for products bought outside Canada. They could repair my Continental 73, but I would have to send it to them and pay for the work.
Meanwhile, I kept looking into the problem of the notes that won't sound and I am more and more convinced that it might be a loose or faulty connector inside the machine : sometimes the keyboard works perfectly, sometimes only the D's refuse to sound and some other time all of the three faulty notes (D, B flat and E flat) are involved. And it does not matter if the VC has been on for 2 or 20 minutes so I guess it is not heath related.
Since my warranty does not seem to be worth anything here in Canada, I'd be very interested to have a look inside the VC, just to verify if the problem is that simple. However, I don't know for sure how to open it. There are quite a few screws underneath the board and I would not know which ones to remove without causing any damage.
Does anyone would know the right procedure to do so? Like always, any help would greatly appreciated.
Best regards.