Vintage Synth Repair

Hi!

Would anyone have the schematics and/or a service manual for Yamaha 
CS-15D online? I tried contacting Yamaha about these, but haven't 
received any mail besides some automated "..will contact you shortly" 
crap. I tried also briefly comparing CS-15 schematics against the boards 
but have not yet found any resemblance besides the custom Yamaha chips 
used here and there.

I've had one of these waiting for repairs for quite some time and would 
now like to proceed with it by checking the filter calibration / 
functionality for the 2nd osc. With certain low cutoff value, and when 
operated in manual mode, the output just abrubtly cuts out. Here, the 
VCF EG also needs to be about halfway through in order to get any 
output, if zero'ed the plain cutoff control doesn't enable output at any 
setting. There is also a loud thump in the beginning of each note as if 
there was a very short env mod applied to a separate low pass filter. 
This thump doesn't relate to the VCA EG since the thump can be faded 
away by using a longer attack time. When operated with presets, most of 
them produce no output at all. There is f.ex. a "VCF Click" trimmer on 
the VCF board but I'd rather have the whole synth calibrated according 
to factory specs instead of just tweaking the trimmers at random.

Osc 1 seems to work with any preset selected, but the output level 
varies from preset to preset and is in overall rather low when compared 
against osc 2.

So, any help is appreciated. At a glance it sure looks like a damaged 
custom chip, but naturally I'm hoping to get away with just 
re-calibration and maybe swapping some common parts ;)

Thanks in advance,
.Arto.