I have a PSR 2000 - about 20 years old. I have a C key that is sunken down approx 1/8 of an inch. The key still plays and all keys play well. I took the back screws out and separated the keyboard. Then detached the entire keyboard from the workstation - then took out the 21 small screws holding the black strip and removed the 2 circuit boards by loosening the 15 clips that held them in. I can see that each octave seems to be made up of 2 white key configurations (CEGB) and (DFA) and one black 5 key piece. I cannot figure out how to remove this octave and separate these keys - nor can I find anything on the internet i.e. you tube that shows how they come apart. I can see and have removed the one rubber contact strip for the octave that has the sunken key. Please help if yu know how to do this. I have spent hours and can't figure it out. The service manual tells how to do it but the explanation is poor and no pictures or video. Thanks so much in advance. Tom What usually causes a key to be sunken ??