Vintage Synth Repair

On Wednesday 14 June 2006 05:23 am, Scott Nordlund 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
>
> 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.

The LM3900 isn't an op amp,  though,  it's a quad "Norton Amplifier",  which 
is not exactly the same thing.  You can find the datasheet at

http://www.classiccmp.org/rtellason/chipdata/lm2900.pdf

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