[sdiy] Korg MS-20 VCO woes

[nicolas]

I would start the problem solving by testing the expo current sink separately from the oscillator core itself.  Disconnect the 10k from Q5 and connect the 10k to ground instead.  Current should flow down the 10k and oscillation should happen.  Connect Q5 to +V via a 100k resistor and measure the current flow.  It should flow and the flow should be varied by your control voltage.  Then at least you'll be able to narrow the problem a bit.

Nicolas


--- On Thu, 21/4/11, lanterma at ece.gatech.edu <lanterma at ece.gatech.edu> wrote:

> From: lanterma at ece.gatech.edu <lanterma at ece.gatech.edu>
> Subject: [sdiy] Korg MS-20 VCO woes
> To: synth-diy at dropmix.xs4all.nl
> Cc: awilliamson42 at gatech.edu, nmurdaugh3 at gatech.edu
> Received: Thursday, 21 April, 2011, 10:55 AM
> > If you look here:
> >
> > http://www.korganalogue.net/korgms/images/service/ms20/circ1.gif
> >
> > You'll find the two VCOs of the Korg MS-20. They use
> different size
> > integration caps, but other than that, they're the
> same as far as I
> > can tell, and they're also the same as the VCOs in the
> MS-10 and
> > MS-20.
> >
> > A team of a couple of my students are working on
> building the Korg
> > MS-X0 VCO and getting it working, and they've yet to
> get it to
> > oscillate. :(
> >
> > They took some DC voltage measurements with a
> multimeter at various
> > parts. I'm not sure what we should expect to be seeing
> at various
> > points - and there's a couple of things they're
> observing that just
> > don't make sense to me at all.
> >
> > I should mention they're using a 3906 for the PNP
> (Q4), 2N5457s for  
> > the
> > FETs (the Q1 voltage buffer and the Q5 current buffer
> - many thanks to
> > the folks who helped decode what Q5 was!), and a
> CA3083 for the
> > remaining NPNs, with the extra-matched pair serving in
> the expo
> > converter. They have the substrate pin of the CA3083
> connected to the
> > negative rail supply (-15 volts)
> >
> > I've uploaded a scan of a rough quick sketch, where
> one of the
> > students measured some voltages:
> >
> > http://users.ece.gatech.edu/~lanterma/ems11/korg_ms20_stuck_voltages.pdf
> >
> > You will see several voltages written down, stacked on
> top each other,
> > with a horizontal line between them. That doesn't
> represent division;
> > it represents a voltage range, as the student twisted
> the expo-control
> > pot (pot on far left). I should also mention that the
> Korg has a 22
> > Meg resistor (with additional 100Ks switched in
> depending on the
> > octave setting) with 15 Volts across it to generate
> the reference
> > current for the expo converter. It didn't seem to work
> with 22 Meg or
> > smaller, so I suggested trying 1 Meg to try to make
> extra sure some
> > current was flowing. So these measurements are with a
> 1 Meg in the 22
> > Meg spot.
> >
> > Anyway, twisting the expo control pot, the emitter of
> Q2 (which is at
> > a spot where two 427 ohm resistors split 15 volts and
> ground) stays at
> > 6.4 volts. The bases of Q2 and Q3 (and the bottom of
> D4) stay at 12.3
> > volts. The student measured a 0.65 mV drop across D4.
> >
> > The bottom of R15 ranges from 3.37 to 6.2 volts, while
> the top of R15
> > (bottom of the integration cap) ranges from 3.1 to 6.2
> volts.
> >
> > Something strange is happening with the FET buffer -
> the output of it
> > ranges from 7.11 and 7.27 volts. So something must be
> wrong with that,
> > at least, but we can't figure out where things are
> going astray. The
> > FET buffer in the MS-20 is powered from the +18
> supply, and we're
> > using +15 - but that shouldn't matter here if the FET
> gate is ranging
> > from 3.1 volts to 6.2 volts.
> >
> > The thing is I don't have a good feel for how the
> circuit works, so
> > I'm not sure of what voltages I should expect to see
> in most of the
> > circuit. The FET buffer is certainly acting weird.
> >
> > As always, any advise of what to look for or things to
> try is greatly
> > appreciated! :)
> >
> > - Aaron
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