[sdiy] jupiter 8 VCO1 problem

[geoff james]

HI All , can anyone assist with the following problem. This is on behalf of 
a friend ; never seen a jupiter 8.   This is what he wrote. 
"Thanks for looking the attached schematics over.  Pages 15 and 16 show the 
voice module schematic.  I've replaced IC 2 (3046) and IC 1A (4558) with no 
change in symptoms.

I was glad to see that the problem followed the voice card from voice 1 to 
voice 3 when I swapped the cards just because I assumed that meant I 
wouldn't have to deal with the CPU and I could focus on a discrete VCO (each 
of the four cards holds two voices; each voice has two VCO's; in my case one 
VCO on one voice is not tuning properly).  I've attempted a calibration, but 
I can't quite get the linearity trimpot to tune the VCO correctly before it 
reaches its extreme point.  I've calibrated the rest of the synth correctly 
with a scope.

I feel like I need a description of the role each individual component plays 
in the circuit to troubleshoot further.  I have a book by Delton Horn that 
suggests R12 and C4A may play an important role in tuning, but beyond that 
I'm guessing."
Regards
Geoff James