Vintage Synth Repair

>Hi, again, group.  Anyone know the ARP Avatar?...specifically the 4075
>filter module?  It's based around the LM3900 quad amplifier.  Is it
>true that the voltage between the inverting and noninverting inputs at
>each of the four stages should be virtually zero regardless of the
>position of the VCF frequency slider? The following are the voltage
>values I measured at each of the four stages; the first of each set
>represents slider min, the second is slider max:
>.10-.10; .66-7.34; .43-.62; .64-7.22

When op-amps use negative feedback (as they usually do in audio circuits), 
the input terminals should have very similar voltages (ideally zero, but 
that's assuming infinite gain).  So this doesn't indicate a problem.