This one is kind of an odd problem. We noticed on a lot of the 600XL and 800XL keyboards we saw, that the keys felt kind of dead or mushy. It turns out that each has its own spring under it and they apparently wear out. They also are just dropped into a hole under the key and if you ever disassembled one of these keyboards, you may have found out the hard way that if you turn over the keyboard, you will have springs EVERYWHERE and undoubtedly lose a few
These springs are the exact size, shape and tension as the originals.