Gordon,
I just popped all of this into a PDF and it is now sitting in the Poly
800 service files section.
Welcome to posterity!
Mike.
--- In korgpolyex@yahoogroups.com, Gordon JC Pearce <gordon@...> wrote:
>
> selloutdesigns wrote:
> > I just posted a PDF of the Poly-800 Mk1 service manual in the files
> > area. I found this
> > scan at http://www.paintingwithsound.co.nz/manuals.htm and got
> > permission to post it here. Enjoy!
>
> Great, thanks for this. Now we can see what the deal is with the
> joystick board ;-)
>
> Look at page 6 of the PDF, in the bottom left of the circuit diagram.
> There's the PCB for the joystick, drawn in a separate box.
>
> Okay so we've got a quad opamp NJM2056D, which is probably something
> like a TL074 or similar - not critical.
>
> R6 and R7 provide around 2V (2.024V assuming the resistors and 5V
supply
> are bang on) which is buffered by IC1d (following the convention on
> datasheets and pin numbers - the actual chip section isn't written on
> the diagram). Note that the 2V supply goes to the non-inverting input
> and the inverting input is tied to the output, forming a unity-gain
> buffer. This means that no matter how you waggle that stick, you won't
> affect the voltage being supplied.
>
> The centre taps of the joystick are held at 2V to provide the centre
> "dead band" - perhaps one of the reasons for a jittery controller is
> dirt on the track, or a noisy signal?
>
> The MG joystick has a 2k2 resistor to +5V at one end, with the other
> grounded. By waving a bit of Ohm's law at it, we find that a 2k2
> resistor in series with a 10k resistor (the total resistance of the
> joystick track), we get 4V approximately at the top of the pot. That
> gives us a swing of 0V to 4V, with a dead band around 2V in the middle.
>
> Now, what about the pitch bender? That's a bit more complicated. Not
> much, but a bit.
>
> The pitch bend pot supplies a signal going from 2V to 4V as the bend
> range is increased, because one end is wired to the 2V supply and the
> other is wired through a resistor to 5v. IC2b, R3 and R4 form an
> inverter (C2 and C3 just stop it oscillating), which has an output
going
> from 2V to 0V as the input increases - the output is relative to the
> non-inverting input which is wired to 2V so it's worked out as 4V -
> Vcontrol. This is then fed to another inverter (which is again
relative
> to the non-inverting input) giving its output as 4V - Vpin7. The bend
> range pot is wired across the outputs of the two inverters. The
> practical upshot of all this is that as you increase the bend range
from
> 0 to 10, the voltage from the pot goes from 2V to 4V and the voltage at
> the ends of the pot "spreads" from 2V at both ends to 0V and 4V.
> Confused? Read it again and look at the diagram. Think of it as the
> electronic equivalent of opening a pair of scissors...
>
> HTH
> Gordon
>