From this list of venerable synths, I have decided to focus on the one sold at Radio Shack, the Realistic MG-1. Why? Because my scan of the schematics is the most legible.
The VCA and master volume circuit is shown. I've photoshopped (actually Paint.NET'd) a little so all the labels are visible. The 3080 is in the center. It gets it's differential input from the output of the ladder filter (not shown). The control current IABC is coming from the lower right. The contour generator puts out a 0-4.5V envelope. Q28 and corresponding resistors convert the CG voltage to the control current IABC. The output current of the OTA passes through R102//R103 to give the output voltage. R103 is set up as a standard pot volume control. The addition of R102 in parallel allows the overall voltage gain to be controlled while using a standard pot size for R103.
In the next post I'll show the math behind the labeled values.
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