Yamaha CS80

Hi Everyone,

One of the voice boards on my CS80 is noisy when playing patches at  
low filter settings.  The noise is not coming out of the sine wave  
circuit, and it also goes away when I pull down the slider for the VCF  
level.  Because of this I think of course the noise is originating  
before the VCA controlling the VCF output level.  Adjusting the  
sliders for the filters has no impact on the noise, and turning the  
VCO noise, saw or square waves on and off doesn't impact this moderate  
level noise coming out either.  It seems also independent of filter  
envelope settings, but it does turn on and off when the note is  
triggered, presumably because of the volume VCA that comes after the  
VCF level.

I'm thinking either the VCF is generating some noise independent of  
its function, or perhaps there is noise coming from or near the VCO  
circuitry that isn't related to the actual function of the VCO (saw,  
square, noise waves), but is propagating through the whole circuit.

Does anyone have any thoughts on what might be the problem?  Is noise  
in the circuit likely to be degradation of one of the ICs?  Maybe the  
VCF is the most likely culprit.  I do have some spare voice boards,  
but I'm still deciding how to approach this problem.

Any input would be appreciated!

Cheers,

Kent (USA)