[sdiy] Roland 303 VCO questions??

[Richie Burnett]

> i have been looking at this schematic all day and I would really
> like to understand  how it works.
> anyone want to take a shot at it?
> the schematic I am working from is HERE:
>
> http://www.ladyada.net/media/x0xb0x/mainboard_beta.png
>
> I am interested in parts  in the VCO, basically IC11B to a little
> bit past Q24

IC11B is the VCO CV buffer from the R-2R ladder DAC.  It also forms part of the
portamento circuit with analogue switches IC12C/D and capacitor C35 to limit CV
slew rate during programmed slides.

Matched transistor pair Q26 and it's servo amplifier IC11A convert the linear
pitch CV from IC11B into an exponential current sink.  This achieves the
approximate 1v/octave law for the oscillator.  The expo converter continuously
drains current from capacitor C33 in order to make it's voltage ramp downwards
linearly.  Q24, 25 and 27 form a thyristor.  This triggers when the voltage on
C33 falls below a preset level determine by the 5.333V applied to Q24 emitter
via R101.  When this thyristor implementation fires Q27 and Q25 both turn on,
with each providing the base current to keep the other turned on.  This results
in C33 being rapidly recharged up to approximately 12V via Q27 and Q25.  They
stay in the conductive state like a thyristor would stay latched until the
charging current into C33 falls below their holding current (typically a mA or
so.)  Then both transistors drop out of conduction and the expo current sink
causes the capacitor voltage to ramp downwards again at a rate determined by the
pitch CV.

So in summary, you really have an exponential current sink centred around
Q26/IC11A, and a crude voltage comparator/reset circuit based around Q24, Q25,
and Q27.  This gives you a raw downwards ramping sawtooth with fixed amplitude
and exponentially conforming frequency.

Q28 acts as a high input-impedance unity-gain buffer to buffer the voltage on
the integrating capacitor before passing the sawtooth output on to the VCF or
square wave shaper.  Q8 and it's associated circuitry form an over-driven
common-emitter amplifier and act to shape the sawtooth waveform into something
that is occasionally vaguely square in shape at some pitches!  Then it's onwards
to the VCF...

I hope this helps,

-Richie,