The EX-800 switches are very repairable.
The link below discusses how to do it.
http://f1.grp.yahoofs.com/v1/UEpnSYD9-8hD_zwM5Thbt0us6W0oP-FyHcJT-iz6oi_FV9ILu251njQ056U-6qA1zErbPhW3TfUDjjeqFNFI/2%29%20EX-800/Servicing/EX-800%20Button%20Cleaning%20Procedure.txt
Just take your take time when disassembling and cleaning each switch as it is very, very easy to lose the little metal disks if you rush things.
I recommend doing ONE switch at a time. And do NOT do the job over carpet or you will be sorry. Use a large, clean workspace.
Many of us have fixed our bad EX-800 switches using this method. It works!
Mike.
From: Gordon JC Pearce MM3YEQ <gordon@...>
To: korgpolyex@yahoogroups.com
Sent: Friday, January 9, 2009 4:22:03 AM
Subject: Re: [korgpolyex] Repairing an EX-800
On Fri, 2009-01-09 at 01:48 -0200, Alexandre Souza wrote:
> > Amazing work. Too bad that button is actually a fun one.
>
> Thanks! But all my buttons are acting up. They need to be very hard
> pressed to work and some don't work at all. I think I'll have to disassemble
> them and clean. It will be a pain in...well, you know where.
You can buy big bags of clicky switches from eBay. It's worth just
replacing them - there's nothing you can really clean.
Gordon