Before the story... yes, you need to remove the keyboard locking cover in front of the keyboard, and gently pry the back of the key upwards... then move the key forward until it disengages with the hammer arm....
I was lucky enough to acquire a discarded Privia 750 keyboard - minus power supply. My Casio WX-7800 uses the same power adapter 12v/2A...
It didn't work - so I took it apart and while fiddling I managed to short-circuit something in the small panel switch board keys and play the demo song....
My first disassembly was not pretty - trying to take out the wrong screws etc. I found and ordered a second hand replacement Panel Switch Board above from Keyboard Kountry - Cost US$30 and US$55 for priority shipping 😢 ....but it came to life Yayyyy and resembled it!!! Unfortunately there was one loud key that needed to be cleaned.... SI took it apart again and tried to clean under the key... but to do so requires taking out the keys... seems like it should be easy until you realize that trying to force the keys out can and will actually break the small rubber hammer head - because I guess Casio don't sell them anymore they are quite expensive to replace (US$8 each!)
I tried to disassemble the 88th key first - incase I messed up - which I did...
So... what you need to do is partially open the front panel in front of the keys... I found that I just needed to undo two screws on the RHS of the keyboard with a very long cross-head screwdriver and the 5-screws under the keyboard to release the front panel enough to slide the white keys forward until the key comes out. You need to remove enough keys to remove the strip and the white keys need to be removed before the black keys
After that... I removed the rubber.... it just required a gentle clean under the rubber and board then reassemble - voila!
To release the keys, I didn't need to take out the whole panel because on the LHS is the headphone socket - so only needed to open it 1-inch max before cleaning the keys, rubber and putting it back....
Now waiting for the replacement hammer to come through the post to fix the end key
I also figured out that when the rubber foot disintegrates inside the keyboard, it causes the key to drop... so if your key has dropped, you will need to replace the rubber foot on the key as described above. Hope this helps.
