[sdiy] Bergfotron triangle VCO debugging

[Manu Raghavan]

Yes, I tried connecting to -15V as well as ground. Neither one made a
difference to not connecting it.

Was it supposed to? I couldn't find any documentation on it...

Manu

On 12/13/06, Tim Stinchcombe <tim102 at tstinchcombe.freeserve.co.uk> wrote:
> Do you have pin5 (substrate) of the CA3083 tied to the negative rail??
>
> Tim
>
> > -----Original Message-----
> > From: owner-synth-diy at dropmix.xs4all.nl
> > [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Aaron
> > Lanterman
> > Sent: 13 December 2006 18:34
> > To: synth-diy at dropmix.xs4all.nl; manu.synth4803 at gmail.com
> > Subject: [sdiy] Bergfotron triangle VCO debugging
> >
> >
> >
> > Manu, one of my students, is trying to get a Bergotron VCO
> > core working,
> > and is having a lot of trouble. He tried posting this to the
> > list a cuople
> > times, but he and I don't think it got through (I'm not sure
> > why). I don't
> > remember seeing it appear.
> >
> > We've both been banging our heads on this, so any help is appreciated.
> >
> > I'll also toss in a few other notes:
> >
> > 1) Twiddling the trimpots makes a difference but not enough
> > to put it down
> > into the audio range.
> >
> > 2) Manu mostly grabbed 5% resistors just to get things
> > running - I know
> > the resistors at the top of the OTA must be more closely
> > matched, but I
> > figured that would just make the triangle asymmetric (which
> > it is), but
> > that shouldn't throw the freq. off so much.
> >
> > 3) Below, when Manu says removing the 3083 didn't make a
> > difference, he
> > means that it still oscillates, but stays locked at 6.8 kHz
> > (clearly once
> > the 3083 is pulled out it doesn't respond to control voltages.
> >
> > - Aaron
> >
> >
> > ---------- Forwarded message ----------
> > From: Manu Raghavan <manu.synth4803 at gmail.com>
> > Date: Dec 11, 2006 6:10 PM
> > Subject: Bergfotron triangle VCO debugging
> > To: synth-diy at dropmix.xs4all.nl
> >
> > I'm trying to build a Bergfotron triangle-core VCO clone (
> > http://hem.bredband.net/bersyn/VCO/VCO%20Buchla%20core.gif)
> > for a synthesizer class at Georgia Tech and am having some
> > troubles debugging my circuit. Would greatly appreciate any
> > suggestions you might have.
> >
> > Basically, the problem is that the triangle out oscilattes
> > between 6.8kHzand 32kHz for a CV in that goes up to 7V. I
> > want it to oscillate, per the original design, within audio
> > range. "LIN FM" and "SYNC" are not connected.
> >
> > Some of my parts are different from the original design, but
> > they should have the same functionality.
> > OTA:                  SSM2220 instead of MAT03 (Matched PNP
> > pair), SSM2210
> > instead of BC550 (Matched NPN pair)
> > Expo converter:   CA3083 instead of BC847(Matched NPN pair)
> >
> > Here are some of my observations during debugging:
> >
> > I've noticed that removing the CA3083 expo converter matched
> > NPN pair does not make a difference to the triangle output at
> > all! The output continues to oscillate at 6.8kHz even after
> > the entire IC is removed from the board.
> >
> > In the expo converter,
> > TL072 pin 2 = 0.694V (as expected)
> >
> > In the oscillator,
> > TL072 feedback cap (confirmed with RCL meter)  = 4.58nF
> > Square out Vpp = 4.3V (as expected)
> >
> > The voltages at the 56k/1k divide-down ladder next to the CV
> > IN op-amp output are logically right.
> >
> > Please offer any suggestions for further debugging that you
> > think might be useful. I am particularly surprised that
> > removing the CA3083 makes no difference, as mentioned above...
> > ---
> > Manu Raghavan
> >
> >
>
>
>
>