[sdiy] Korg MS-20 VCO woes

[lanterma at ece.gatech.edu]

> 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