I have same problem. The weights (that bring the key back up) break off easily on RD-500s. Apparently, the weights can be easily reglued, but I'm trying to figure out how to remove the hammers. To inspect, simply remove the screws on the bottom, then lift up the black cover. Then you can unscrew the action and remove if from the case. Underneath you'll see the broken hammers. If you have the original receipt, Roland will give you a new set of keys, free. I'm still trying to figure out how to remove the hammers -- which I'm told can be easily reglued. I posted a request for info on this on this board.