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

It seems to me that I have a bad LM3900 chip based on my limited
knowledge of opamps.  Is there something else I should consider here?
 Audibly there's mainly noise from the unit.  I can faintly hear the
OSC's being modulated correctly underneath the "wash" but the filter
controls have no effect on the OSC's.  The envelopes can be heard to
mod the VCA.  The CA3080 IC of the VCA seems to have correct values. 
I bought the unit with a burnt power supply tantalum cap in the
vicinity of the VCF which I have since replaced.

Thanks for any help! --stephen