Korg DW8000, Mono/Poly, MS-20...

Hi!

I'm restoring a beat-up DW8000 and the biggest part of the work has 
been done (the usual battery change, major keyboard reconstruction) 
but one problem remains:

There's an audible hum (or a low drone) which appears only when a 
note is played AND either the noise generator or the digital delay 
have been turned up. It's not a stuck note as all of the oscillators 
are functioning perfectly and also when the effect level & noise 
level parameters are turned to zero, the hum is inaudible.

Any ideas what could cause this? Help would be appreciated.


Cheers, Mikko from Finland