[sdiy] Korg MS-20 VCO gives up below 3.5 kHz

[nicolas]

Yes I agree the issue is probably with the trigger circuit.  If you were getting a sawtooth of over 6V p-p (which is what should be happening) then it would also be running about 10 times slower, which would be a lot closer to the expected freq range.

How I think it should work is that the emitter of Q2 should be at 7.5V 99% of the time.  That it is not seems to indicate that Q4 is a lot more turned on than it should be, which indicates that Q3 is more turned on than it should be.  They should only be conducting momentarily during the reset phase.

As the current gets sucked out of the timing cap the voltage at the emitter of Q3 should drop and when the emitter of Q3 is at a lower voltage than the emitter of Q2, Q3 should start to turn on which will turn Q4 on, which will turn Q3 on even more.  Q3 and Q4 will keep each other turned on while they reset the timing cap.  That stops once the Q3 emitter voltage gets to within 0.7V of the rail, then they run out of steam, turn off and the timing cap can start charging again.  So the sawtooth voltage peak to peak at the emitter of Q3 should be from about 7.5V to about 14V.  But it maybe sounds like Q3/Q4 are not turning fully off for some reason.

On the other hand in order to work it looks to me like perhaps Q4 needs to leak a tiny tiny amount of current even when it is switched off.  But not Q3.  Perhaps that is where genuine 1970's style transistors might be important.

The other approximation in your build is that you are powering the trigger circuit and the buffer together.  Perhaps there is a feedback path from the fet channel to the timing cap which is making things go weird.  Try isolating the power supply for the trigger circuit from the output fet power supply and see if that helps.

Cheers,
Nicolas


--- On Mon, 2/5/11, Tim Stinchcombe <tim102 at tstinchcombe.freeserve.co.uk> wrote:

> From: Tim Stinchcombe <tim102 at tstinchcombe.freeserve.co.uk>
> Subject: Re: [sdiy] Korg MS-20 VCO gives up below 3.5 kHz
> To: synth-diy at dropmix.xs4all.nl
> Received: Monday, 2 May, 2011, 10:23 AM
> Hi Aaron,
> 
> > more I think, with an 850 mV peak-to-peak swing .
> 
> Where's this measured at, as it seems very low? Quick
> simulation of the
> circuit suggests the swing across C2 is going to be around
> 6V pk-to-pk.
> 
> > R16 gets toasty (but not too toasty), as one might
> expect 
> > with 12 volts dropped across such a small resistor.
> It's sort 
> > of at the edge of a 1/4 watt  rating.
> 
> Most of the time the junction of those two 427s (R16, R17)
> should be at
> mid-rail, i.e. 7.5V, so I suspect a wiring error...? It
> does _spike_ up to
> around 13V or so when the cap resets, which will be the
> rail minus the D4
> and Q2 diode drops. 
> 
> > Juergen Haible suggested that the original transistors
> might 
> > be important;
> 
> I haven't quite got my head round how it is working
> completely, but it looks
> like an 'SCR' type of thing, so as C2 charges, Q3/Q4 are
> off, and the
> sawtooth is falling; when it gets to around mid-rail, the
> SCR 'fires', i.e.
> Q3/Q4 come hard on, discharging the cap, sending the
> sawtooth back up to the
> rail and switching the transistors off. I suspect the
> relative values of the
> hfes and things of Q3 & Q4 will be crucial to its
> correct operation, to make
> the 'SCR' effect work at the right point (is it an
> SCR-thing going on here?
> - too lazy to look it up..). Thus I agree with Juergen's
> observation, and
> even if you have it wired correctly, if the transistors
> aren't close to what
> they should be, then I don't think I would surprised if it
> didn't work
> across the full frequency range expected!
> 
> Tim
> __________________________________________________________
> Tim Stinchcombe 
> 
> Cheltenham, Glos, UK
> email: tim102 at tstinchcombe.freeserve.co.uk
> www.timstinchcombe.co.uk
> 
> 
> 
> 
> 
> 
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