Since that keyboard is well over 15 years old, pushing on 20 it's probably starting to show its age with drying out electrolytic capacitors and it sounds like it's in need of a major overhaul.
My advice is that unless you are handy with a soldering iron and comfortable rebuilding electronics yourself don't bother wasting the money to bring it back to life.
It would likely cost you $250-$300 if you took it to a service tech and even then there's a chance they couldn't fix the problem either.
For that money you can buy a brand new WK-6600 especially after xmas when the big sales hit.
My advice is to get $40 for it on craigslist from someone who's looking for a project machine or a parts machine and start looking around for a newer replacement as well.
That's my $0.02
Gary