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.

Hi Arto I have access to the entire range of schematics in the Yamaha range both old and new.  The CS15D is not listed!  There are only a half dozen listing

> etc.it has 65 pages with two 40" X 28" large foldout schematics.
> it is ebay item number 360105300519

Thanks for your suggestions Brian and Timothy! Still no reply from any 
of the Yamaha service centers, so I guess I'll be placing an order to 
Timothy's Ebay item shortly..

.Arto.

> 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 

Thanks to Timothy's Ebay item (extra neat manual btw, well worth the 
price!) I now got a bit further with this case and it indeed seems like 
the custom VCF chip (iC27, IG00156) on osc2 is shot :(

So, would anyone have any suggestions as to where to source these chips? 
Vintage Planet seems to be out of stock at least. Does anyone know has 
there been any attempts at cloning this chip or are there datasheets for 
this chip available somewhere?

Regards,
.Arto.

> Hello:Did you try to google the part number?you might find someone with
> some old stock,Yamaha is a b**** since they don't stock parts for 

Sure did, the only thing I found was some closed Ebay item selling a 
chip for 50\ufffd. Not an option at this point really, need to do a bit more 
googling it seems ;)

On the other hand, digging a bit deeper to the CS-family of synths I 
found this on J. Haible's site:

http://jhaible.heim.at/CSSynth/CS80_VCF_large.gif

and the CS-50 / CS-80 service manuals pretty much explain the pin 
functions in detail:

http://www.therogoffs.com/cs80/manuals/CS80_Service_Manual/42%20-%20M%20Circuit%20ICs.jpg

So I'm left wondering if there'd be enough pointers here to construct a 
clone replacement. There were also some posts from Scott Rider (like 
http://www.the-gas-station.com/messages.cfm?thread_id=46157&lastdays=39758&type=normal&page=2) 
stating that he had reverse engineered the chip for his MOTM-480 module 
but this of course uses V/Oct scaling instead of V/Hz. Not that this 
module would be an option anyway, given the price :). I doubt that Scott 
would like to share the bare chip design of his reverse engineering 
works OR be interested in manufacturing/selling a replacement module, 
but maybe I'll drop him a line or three when my research into this 
subject pulls to a halt.

I really wouldn't want to trash the synth for re-sellable parts, but if 
nothing else comes up I have to give this further thought. After all, 
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.

Regards,
.Arto.

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.