Vintage Synth Repair

hello.  I have a Roland System 100 Model 101 keyboard that will switch
between white and pink noise, but when the Noise level slider is at
maximum, the audible noise is significantly lower in volume than the
VCO when maxed.  also, when the Noise slider is at zero, a key
depression will still produce a slight output of noise.  the slider
does raise the volume of the Noise, but only slightly.  also, the
Noise can still be filtered.  do i have a dirty and/or bad slider? 
any ideas on replacements?  thanks! --stephen