Korg Mono/Poly

Hi All

Long time MP4 lover here..  turned it on today after a couple of
months storage and it's virtually silent, till I crank the headphones
loud and then....

when I play keys, I can hear at very low level noise like the "gate"
opening.  
 
The VCA envelope controls shape the envelope of this low level noise.
 
If I twiddle the filter resonance, I can hear the noisy pot, but no
actual filter resonance.
 
If I twiddle the filter cutoff I hear nothing, it doesn't filter the
noise.
 
I can tell the VCO's are receiving trigger because their LED's light up.
 
*** SO *** looking a the block diagram, I guess I either have a
problem in the "mixer" (post VCO's pre VCF) or in the VCF itself???

Does anyone have an opinion on this??

Any suggestions graciously accepted,,,,,,

peace & sine waves
Simon
 
p.s. I never even knew there was an MP4 group....