Discussion about the Korg PolySix synthesizer

Sounds like you are making progress, maybe slower than you had 
hoped.   IC10 pins 1,2,3 are configured as a "voltage follower".
This is a buffer with gain of 1.   That means whatever you have at
the input should appear at the output, as you are seeing.

There are several simple rules you can use for analyzing op amp
circuits:

  1) If the op amp is in a feedback circuit, the output will try to
make the two input voltages equal.   (This makes the operation of
the voltage follower rather obvious)  

  2) The input current to op amps is very low, and can be considered
to be 0.   In other words, current flows in the components around 
the op amp, but not into the op amp inputs.   This is just for the inputs,
of course.   The output of an op amp must source or sink current to drive
its load.

Using these two rules and ohm's law, you can figure out lots of op amp circuits.

Note that in some cases, like IC14 pins 1,2, and 3, the op amp is not being used
in a feedback circuit.   The 4.7M resistor R80 provides hysteresis, and is typical
for a "comparator" circuit.   In this kind of circuit, if the "+" input is more positive
than the "-" input, the output of the op amp will go as high as it can.   For many op amps
this is maybe 1-2 volts below the positive supply.   This is what you are seeing.
If the - input is more positive than the + input, then the output of the op amp will go 
as low as it can, approaching its negative supply rail.  If the output of the op amp 
behaves this way as the inputs are varied, then it's probably OK.

   Bob


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On Tue, 8/30/16, 7zlwncpbis4x22jwvkd4oc5hzu2njcsjeup42is6@yahoo.com [PolySix] <PolySix@yahoogroups.com> wrote:

 Subject: [PolySix] Re: Trying to Restore a P6.
 To: PolySix@yahoogroups.com
 Date: Tuesday, August 30, 2016, 12:30 PM
       
       So...new IC 23 installed. Trace looks like it
 should now at pin 6, IC 18, the INH input, and I can see the
 outputs reacting as they should on all but two
 parameters. However, the symptoms remain pretty
 much the same.Which leads me to the two outputs
 that are not working correctly. Pin 1 on IC 18 has the exact
 same voltage as the +5 power rail input into the IC (pin 16,
 +4.98V). This voltage shows up on pins 1, 2, and 3 of IC 10
 also. This is supposed to be the ATTACK control. My guess is
 that IC 18 has been corroded, but it seems I should replace
 IC 10 also. 4558s are cheap.And pin 1 on IC 14
 does not show any response from the MG Delay control. I see
 the signal at it's output on IC 19 (pin 1) and it's
 input to IC 14 (pin 2), but all I see on pin 1 is a constant
 +14.51 V. Can't quite figure out where that is coming
 from, but perhaps I don't understand the circuit. I see
 a wavering voltage on pin 3, which I imagine is correct due
 to the Delay function, and there is a negative voltage on
 the far side of D12, as it appears there should
 be.Again, 4558s are cheap. Should I just replace
 all 3 ICs? Looks like Digi-key still has some 16 DIP 14051
 ICs. 
 One other thing
 I notice on the working control outputs (muxes?). Most of
 them get to a certain voltage (somewhere around 70% of their
 potential), and then fall back to zero with an audible
 click, which also seems to change all settings. This can be
 seen on the trace at TP2 also. It seems like this indicates
 a short somewhere once a certain voltage is reached, but
 there are two or three noticeable exceptions (MG SP, RESO,
 and REL) which do not do this.
 All of which makes me wonder if I
 should just be starting with a new circuit board, as maybe
 this one is just too corroded. but I would like to try to
 continue fixing this one, and any help is
 appreciated.Thanks.