changing presets on CS-synths

[Haible_Juergen#Tel2743]

Did my first hacking on the CS-50's presets last nigt.
Changed that ugly (sorry) "electric piano" to a high-
res-click-lead-sound.

Procedure is like this: Set the patch you want to "program"
with the front panel sliders. Then measure connection
points "a", "b", "c", ... "v" on the T71 card. (Easy, as they are
in-line at the top of this card.), and write the values down.
On these resistor/diode matrix boards (they are similar but
not identical for all CS-models, but they are always called
"T<something>") the preset control voltages are "stored"
in form of rersistors. For each parameter, there's a
divider to scale down a 10V voltage to the desired value:
Vout = 10V * R2 / (R1+R2).
R2 is common for all presets, with a fixed value of 10k.
Different R1's are switched to a positive supply via diodes.
The supply voltage is about 10.6V to compensate for
the diode drops. So you have only one single resistor
for each preset and parameter. So change a single
resistor if you want to change one parameter in a preset,
or resolder 22 resistors to program something completely
different.
Vout = 10V * 10k / (R1 + 10k) is not exactly linear, and sensitivity
changes over the CV range, so you should approximate the
calculated values quite closely. (I used the next E24 value,
and I heard differences in LPF cutoff, for example.)
For 10V, solder in a jumper, and for 0V omit the resistor.

Now, this was the easy part.

The rest (changing the resistors) took me some hours, because:
* No connectors on the board - you have to solder halfway inside
   the synth
* Resistor leads were bent sharply before they were soldered in
  - not easy to remove
* What seems to be a perfectly organized matrix on the T71 board,
  is actually a *labyrinth*. (Resistors and diodes for one preset
  scattered over the whole board)
* Most of the presets have many of their parameters on extreme
  values. And what did they do? They did *not* use a jumper
  for 10V (though there are many, many jumpers on the board)
  - they just soldered in a diode. If you want to change the value,
   desolder one connection of the diode, and fumble a resistor
   in series.
   Even worse: They did *not* just leave out a free space for 0V.
   If you want to add it, solder in a diode and resistor somewhere
   on the copper side ...
These boards win any award for the most strange pcb layout.
Looks like a standard-layout for very different functions, to be
configured by loads of jumpers. Mindboggling.

But - I got this sound stored. I am not sure if I will store a second
one, though.

JH.