with the metal bar removed, you can now remove any built up dust and dirt around and under the keys
with your finger, gently push apart the plastic tabs where the key is connected to the board just enough so that you can pop it out and remove the key / it won't take much pressure to do this - be careful not to apply too much pressure and break the plastic tabs
with the key removed, now remove the hammer weight (highlighted in green here) by simply lifting it out of the housing / the same procedure applies to the black keys as well, the only difference is that the hammer weight on the black keys is shaped a little different than on the white keys / you can now inspect the key and the hammer weight for damage and wipe clean any built up dirt and grease
A: the little plastic tabs break off / symptoms of this are lower key height and keys that do not come back up / most of the time the broken piece can be found right where it broke off in the housing as the grease holds it in place/ clean it off and carefully glue it back on the key with Super Glue or the like / this fix usually will be permanent if you glue properly but not a bad idea to replace the key in the future
B: The hammer weight has cracked and/or broken apart / symptoms of this are sluggish response and a "knocking" noise when the key is pressed / you can glue this back together but in my experience this fix doesn't hold very long before it breaks apart again / I recommend replacing this piece
C: In the worst case scenario, if you don't find anything broken on the key or the hammer weight, it may be the plastic "grid" below the keys that is broken where the keys attach to the board / there is a "grid" for every octave of the keyboard / the little plastic "balls" where the plastic tabs attach sometimes break off / symptoms of this can be the same as problem A / this "grid" needs to be replaced / to replace the "grid", the entire keyboard assembly has to be removed and the DIN connection has to be disconnected between the keyboard and the circuit board in order to gain access to the "grids" / you hope that this isn't the case, but it can be done - take note of all the screws that you remove and take note of the orientation of the DIN connection
"grid" where the keys attach and the plastic "ball" that may break
I hope this page has been helpful and saves you time and money in getting your RD600/500 back in working order. Good luck and happy playing.