Yamaha CS80

--- In yamahacs80@yahoogroups.com, "billwolfer" <bill.wolfer@...> wrote:
>
> Someone mentioned modding the CS80 to 7 voices if you have a dead or
> troublesome voice. I just had this done, courtesy of David Rogoff, our
> moderator and list-owner. One of the VCO chips was misbehaving, so
> David asked me if he could change the keyboard circuit to 7 voices. It
> sounded like a reasonable, albeit temporary solution, but when I got
> the machine home again, now the EG for the ring modulator is not
> functioning. I know that this EG is monophonic, and that it was
> working before the modification, so my theory is that by crippling one
> of the voices, somehow we are bypassing the EG circuit for the ring
> modulator. Can anyone verify this?

Here's the details on what's happening.  Based on the KAS chip
datasheet, I changed the MODE pins from a one (-6.5v) to a zero
(8.5v), which puts the chip in 7-note mode. A side-effect of the way
the KAS works is that in this mode it jams the KO8 gate output active.
This caused the 8th voice cards to drone, which I "fixed" by removing
the VCO chips (one of which was bad, anyway). The other side-effect is
that the TR9 gate signal, which is an OR of the other 8 gates, is
always on. This is the gate used to trigger the EG for the ring-mod.  

If this was a longer-term fix, it would be easy to disconnect the 8th
gate buffer (IC8 / TR8 on the KAS board), which would remove both
side-effects.

It is handy that the KAS chip can be set for any number of voices, up
to 8, for emergencies like dead voice cards (and because it was also
used on the 4-voice CS50), although the dead cards have to be the
highest number voices.  You can disable, for example, just voice 3,
like some poly-synths were able to do. 

 David