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.
The keypress operates two electrical contacts, one at the top of its travel, the other at the bottom. Key velocity is computed from the elapsed time, and is translated into loudness.
You are describing the classic case where only one contact is operating.
You need to clean or replace the contact strips under the keys.
This will involve removing the keybed and dismantling at least a section of the keys. Not an impossible user job. I have found that after cleaning the existing strips once, I generally have to do it again before long. And cleaning won't help if the rubber strip is torn. So I suggest you obtain replacement strips straight away. They aren't expensive. And they're pretty generic - lots of keyboard manufacturers buy in Fatar keybeds. I have a Kurzweil keyboard but obtain my strips from Korg UK, who are a lot easier to deal with.
As well as the dirty/split contact strip issue, generic to many makes and model of keybed, there was a seperate problem with one generation of Fatar keybeds where a plastic lug was prone to breakage. If yours starts doing this it may not be worth messing with individual key replacement. More will break soon. But I don't think this your problem here.
I had this problem with an Ensoniq (I think) in the late 90s. The weighted keys were made by Fatar in Italy. As I recall Fatar were known for this problem. At first one key started clicking, then other keys did the same. Eventually some of those keys played at full volume even if you pressed lightly. Velocity sensitivity was shot. Long & McQuade repair shop in Toronto replaced the entire keyboard for free, even tho the synth was years out of warrantee, and I didn't buy it there. Maybe there was a recall. I heard at the time that Kurzweil also used Fatar weighted keys.
A couple of years or so after the keys were replaced, one key started clicking. Obviously they hadn't fixed the problem. I traded the Ensoniq in for a Korg before it got worse.
From what I've read, Fatar weighted keys still have the same problems, and Kurzweil still uses Fatar's weighted keys. I found the following clip about how to remove Kurzweil keys and fix these problems yourself - including squeaking and clicking sounds from the keys, and loss of velocity sensitivity in individual keys: (Note, there is a previous comment by Laurence Payne with instructions how to repair Kurzweil keys, so there may be some repetition...)