We have a Kurzwell digital piano and one of the keys sounds louder than the others. Any way to fix this?
Did one key always sound louder than the others, or did the problem develop recently?
When did you purchase the keyboard? Is it under warranty?
Have you checked with Kurzweil to find out where you can take it in your area for Kurzweil-authorized repair work? Where do you live?
It is doubtful that there is anything you can do to repair it on your own; you will need to take it to a qualified repair facility.
If I were you, I would contact Kurzweil's customer service.
Since you have provided the further crucial details that you just purchased this keyboard used, then you should go back to the person or business that sold it to you and demand that they pay to have it fixed for you. And you should demand that the repair work be done only by a repair company that is authorized by Kurzweil.
If you contact Kurzweil, they can tell you the name and contact information for an authorized repair company in your area.
If the person or business who sold you the keyboard is unwilling to pay to have it fixed, then demand a refund.
You don't mention what model you have, but if I read your question correctly, the volume of the sound on a specific key is louder than the sound from other keys. Often in digital midi equipment, one can manually adjust midi triggers such as minimum velocity and gain. If you increase the min velocity on a specific trigger, the device might see a "soft press" as a "full hit."
Note that not all keyboards and devices allow for user-editing of velocity parameters, so the customer support website that Wheat Williams suggested may help you determine this.
Since it is a used device, you should try a hard reset to factory defaults to wipe out any user settings which may be stored. Again, I don't have your model number, but in the RE210 manual (google kurzweil digital piano manual), there is reference to a midi/pref button (press, lights up), and a "hard reset" is then selectable using the "percussion" button with a visual readout of "rst", then press "up" button to reset to factory defaults.