Vintage Synth Repair

> > 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

Yeah but it's still high-gain with differential input (though I understand 
that the input is a differential current rather than voltage), so in this 
application I think it would still be used in approximately the same way, at 
least as far as the summing point constraint is concerned.  I'm by no means 
an expert here (still 2 semesters left on my electrical engineering degree) 
but I'm pretty sure I'm not wrong about this.  Obviously, correct me if I 
have the wrong idea.