[sdiy] Yamaha CSY-2 solo synthesizer

[Richard Atkinson]

This email may be of particular interest to anyone who has studied the 
first and second generations of Yamaha synthesizer, namely the SY-1 / SY-2 
/ GX-1 synthesizers with epoxy potted NE series modules in the first 
generation and the CS series of synthesizers with IG series chips in the 
second generation.

It turns out there is an intermediate generation of Yamaha synthesizer 
between these two generations. I found this when looking inside my newly 
acquired Yamaha CSY-2 organ yesterday. This is a later version of the CSY-1 
organ, which has been featured in a few web pages and Matrixsynth articles 
and which contains the same epoxy potted NE series modules as the GX-1.

I took the lid off my Yamaha CSY-2 yesterday with a friends's help. The 
insides of my CSY-2 were rather dirty, having played host to a number of 
spiders and other objects over the years. There was evidence of damp damage 
on the base of the machine; it has clearly been kept on a damp surface at 
some point for quite a while.

It turns out the CSY-2 is very, very different internally from the CSY-1.
Whereas the CSY-1 is a fully discrete design with many circuit boards, some
of them held vertically in a rack frame, the CSY-2 has the first signs of
integration.

In particular, the auto bass chord circuit board (ABC CSY-2) has two large
YM chips on it and there is another board with another large YM chip. The
function of the auto bass chord system on this organ is quite different
from the CSY-1, and this would seem to explain why. There is no rack of
vertically mounted boards; there are much fewer boards and these are all
mounted horizontally.

Of particular interest is the solo synthesizer section mounted on the
underside of the top panel of the organ. Like my CSY-1, the solo
synthesizer boards in my CSY-2 are very clean, much cleaner than the rest
of the instrument. This no doubt has to do with being mounted upside down,
gravity being in its favour.

The CSY-2 does not however contain either of the versions of the CSY-1 /
SY-1 solo synthesizer circuits. It's a completely different, much more
integrated version of the solo synthesizer. The boards are not named SM1,
SM2, SK, SP1 and SP2 as in the SY-2. Instead they are named VCO, VCF, VCA,
FEG, AEG, SK and SP.

VCO is the only board that contains any of the old NE series modules. It 
has the VCO-III module as seen in later versions of the CSY-1, SY-1 and all 
SY-2s.

The VCF board has four ICs on it, all by Rohm. Two BA626s and two BA627s. I
have been unable so far to identify what these are but what is most
peculiar about this is that the organ features neither the older NE module
based filters, nor the IG chip based filters of the CS synthesizers and the
later Electone organs, but a generation in between. This is the only
example of this generation of Yamaha synthesizer filter I have ever seen,
and as far as I'm aware it's completely undocumented on the internet.

VCA, FEG and AEG contain standard parts, CA3080 OTAs in metal can packaging
and so on. The VCA board is self explanatory, the FEG board is for the
filter envelope and the AEG board is for the amplifier envelope. No NE
modules here, in particular no VCF-EG or VCA-EG modules. This is the only
example of these Yamaha envelope generators I have seen, again completely
undocumented on the internet as far as I know.

The remaining boards, SK and SP, seem to be the same as in the older
machines. They are for the gate and trigger pulse recovery and for the
presets.

Sound-wise, it behaves much like a cross between the two generations. The 
oscillator definitely has the same timbral qualities as an SY-2, but the 
filters are much, much cleaner. I wouldn't go so far as to say they sound 
like CS series IG chip filters, but they are certainly more well behaved 
than the highly distorting diode bridge filters in the NE series epoxy 
filled modules.

Richard