Vintage Synth Repair

Any help you need on the Avatar should be directed to the OdysseyPatch
discussion group:

http://launch.groups.yahoo.com/group/odysseypatch/

All of the Odyssey and Avatar gurus hang out over there.

Rob

--- In vintagesynthrepair@yahoogroups.com, "easterleggs"
<milotoll@...> wrote:
>
> 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
>