Vintage Synth Repair

Returning to this one a little bit..

> there's also the low output on osc1 (maybe a bad custom VCA chip?) thus 
> sourcing the custom part(s) for that is most likely going to be as big 
> of a problem as this VCF chip.

I managed to source a replacement VCF chip, but now having installed 
that one it turns out that indeed my CS-15D seems to have a faulty 
IG00151 as well.

As this is a simple OTA coupled to a exp+lin input stage, does anyone 
know of possible replacements? Has anyone done any research or circuit 
analysis towards a replacement circuit OR maybe even constructed one? 
With a bit of digging, I came across this one:

http://members.tripod.com/urekarm/synth/serge_vca.pdf

This clearly points the direction to take, but the amount of parts seems 
a bit excessive. Surely the parts count could be reduced to make the 
circuit fit to the boards? For example, i see no use for U2 (save the 
"funny" band jokes with this one, please), since the end application is 
a in-circuit replacement and not a module for of a modular system. That 
is, in my opinion circuit protection and such could be omitted.

I also came across a pdf copy of the Yamaha IC Guide Book, but this does 
not offer much more detailed info on the chip other than clearly showing 
that the chip indeed is a OTA paired with exp/lin inputs (block diagram) 
and list its pin functionalities.

So, comments, suggestions, anything anyone?? If someone could help 
designing a circuit or adapting the Serge VCA schematic, I could 
possibly create a (SMD?) board design out of this and make it freely 
available so as to save future efforts from other people looking into 
replacing this VCA. As I understood, there are hundreds of these used 
f.ex. in a single CS-80, so the need for replacements should be there ;)

Regards,
.Arto.