I have a similar issue, in that the what is basically ‘Middle E’ in terms of position on the keyboard started to play up, and was not velocity sensitive any more (the Loud Key problem you have described). Then it went completely dead. Given that it’s the middle E on the keyboard, this is a bit of an issue, and I’ve actually got out of the habit of using the keyboard, so the KB37 is a pretty massive sound card now.
To be fair, this is the only issue I’ve ever had with this hardware unit, and I have had it for a long, long time (it is one of the red TonePort models).
To answer your first question: Can it be opened for cleaning? Yes - BUT - I don’t think it is design with user maintenance in mind. You have to basically remove every screw that is in any way visible on the bottom of the chassis (there are a LOT, of several different sizes). You also have to release the three-to-a-side Allen/Hex screws: these are the ones that sit at the top of the control surface. I naively though t that they were the only thing holding the ‘clam shell’ casing together. I was wrong. These screws are difficult to work with as they have quite a shallow head, yo may need to hunt around for the right sized hex/Allen wrench, and they seem to be sealed with a strong dose of Loctite. The challenge I found was getting enough pressure and torque onto the screw without stripping the head. Like I say, I don’t think this sort of thing is supposed to be done at home, but like you, my unit is long out of warranty.
Obviously, you have take the control knobs of the various pots (this is easy - they do not have grub screws). Then you have to ‘jiggle’ top of the clam shell and, eventually, it does lift up, with the keyboard on the ‘lid’ as it were, and all the other electronics on the baseplate. I have to say that the inside of my unit was pretty clean, which says a lot for the design quality of the device. There was some dust and detritus in there, but nothing like what I had expected: also, lack of any fan or ventilation ports helps enormously.
Now the bad news: cleaning doesn’t seem to work. I can’t see any obvious mechanical fault to blockage, so I’m wondering if it is a component failure?
The next step is to disassemble the keyboard itself, as it can be slid out from the upper lid of the casing.….
I’m not sure if that’s a step I want to take, as I early use the KB37 as a sound card and for guitar input - the MIDI keyboard was a nice-to-have bonus.
What to do, what to do…?