OK! I opened the synth only once, but I had taken some pictures (they could have been useful if I couldn't remember how the reassemble the unit...).
After removing a few screws beneath the keyboard, the three segments of the top cover can be removed. There are two printed circuit boards (PCB): the left one is where we connect the power cord, midi cables and amp cables. The other PCB is located below the buttons and LCD.
http://web.ncf.ca/ch865/keyboard/synth1.JPG
Here's a closeup of the left PCB. We also see dirt on the keys...
http://web.ncf.ca/ch865/keyboard/synth2.JPG
Next, I removed the keys.
http://web.ncf.ca/ch865/keyboard/synth3.JPG
There's 2 little PCBs below the keys, with several identical resistors neatly alligned: there seems to be 2 resistors for each key.
http://web.ncf.ca/ch865/keyboard/synth4.JPG
One of the little PCB has been removed, revealing grey rubber strips. One of these strips had some sticky substance in it, I cleaned it.
http://web.ncf.ca/ch865/keyboard/synth5.JPG
And here's the other side of the removed PCB: when we press a key, the dots on the rubber strips are pressed against the grey rectangles (each grey rectangle accomodates 4 keys). I guess these are some kind of pressure sensors (the keybard has aftertouch).
http://web.ncf.ca/ch865/keyboard/synth6.JPG
As I said before, one of the two defective keys is working again, but not the other. All I did is some cleaning, I didn't even disconnect any electrical connexion. In a few days, I'll borrow a voltmeter (or even an oscilloscope) to see if I can learn something more.
Yesterday, something interesting happened: the synth played a note by itself, while I was not playing: it was the note which should be played when I press the dead key!!!