Vintage Synth Repair

Thanks for the replies.  I understand, in theory anyway, the idea of a
Norton amp having current-differencing properties as distinguished
from the op amp's voltage-differencing properties.  Extending that
logic, I tested for current difference at the inverting and
noninverting inputs of each of the four stages of the LM3900.  All
measurements were very close to zero.  Should I assume this chip is ok?

Looking to the VCA as the last suspect in the chain.  I tested the
voltages at the ins and out of the CA3080.  14.43V going in at each
input.  14.14V coming out.  The power supply for the Avatar is
+15V/-15V.  Is the fact that the output voltage is so close to the
power supply voltage a red flag?

I have a scope, but I've never used it other than to get familiar with
it.  How could I test these components with the scope?  What would I
be looking for?






--- In vintagesynthrepair@yahoogroups.com, "Scott Nordlund"
<gsn10@...> wrote:
>
> > > 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.
>