[sdiy] SBM VCO CV interference

[harry]

If it is feedback through ground... then ground isn't really ground is it ???

Its possible for this to happen... but it would imply that the ground is not a
solid
low impedance.

EMI is unlikely... it would not change with the oscillator freq.

I think there is a short, bum component, very wrong value perhaps ?

My VCO4d are very well behaved... same circuit.

H^) harry

vitreousflux wrote:

> Hhaah I thought of that, but no....I think...
>
> The +12V and -12V are 'pure' signals according to my scope.....
>
> Here's something... When I tweak the pulsewidth pot, the half-wave part of
> the unwanted harmonic changes phase.... weird. I see the same interference
> tapped off of the 393 pin 3 directly, too.
>
> Perhaps it's some kind of feedback thru ground? The PW pot is on the
> complete other side of the circuit!
>
> Could it still be an emi problem yet?
>
> Rob
>
> ----- Original Message -----
> From: "tomg at efm" <efm3 at mediaone.net>
> To: "vitreousflux" <vitreousflux at wwnet.net>; "`Synth DIY Maillist"
> <synth-diy at node12b53.a2000.nl>
> Sent: Monday, April 16, 2001 9:22 PM
> Subject: Re: [sdiy] SBM VCO CV interference
>
> >
> > It's truly wierd...I've just been going over the sbm on my bench
> > and see nothing like what you are describing. Is your TV on?
> >
> > Tom
> >
> >
> >
> > > On my EFM SBM, I am getting a strange problem.
> > >
> > > On my input CV line which consists of 1 input and 3 tune pots (freq,
> fine
> > > and init freq trimmer), I am getting an odd narrow pulse-wave signal on
> my
> > > scope - it changes frequency along with the signal CV.
> > >
> > > This causes weird harmonics in the output signals (sine, tri, saw, pul)
> > and
> > > is most noticeable at higher frequencies.
> > >
> > > The pulse wave seems to actually be two signals: a full cycle narrow
> > pulse,
> > > and a half-cycle (positive only) at 90 degrees offset. How bizarre!
> > >
> > > I checked all my grounds and traced the problem back from the opamps,
> but
> > > have no where else to check the signal - it's the start of the
> circuit! -
> > I
> > > am using no input on the cv (although have tried using a cv input which
> > does
> > > not help the problem).
> > >
> > >
> > > Anyone else run into a similar problem on their SBM or analog synths?
> > >
> > >
> > >
> > > Rob
> > >
> >
> >