[sdiy] Interesting f vs. t graphs of pitch instability

[Dave Leith]

Have sometimes seen intermittent connections in the jacks of the patch
bay cause problems like this...

On 2/20/10, Andre Majorel <aym-htnys at teaser.fr> wrote:
> On 2008-10-07 08:40 +0200, Antonio Tuzzi wrote:
>
> > On Mon, Oct 6, 2008 at 3:56 PM, Andre Majorel <aym-htnys at teaser.fr> wrote:
>  >
>  > > The wavering on my MS-20 has become worse lately so I've started
>  > > to look into it. Some interesting graphs have been made in the
>  > > process :
>  > >
>  > >  http://www.teaser.fr/~amajorel/ms20/files/waver1.png
>  > >
>  > > Those are four frequency vs. time plots. On the X axis, the frame
>  > > number. On the Y axis, the frequency in Hz. Each dot represents
>  > > one cycle. Each plot is one minute long so should have about 5400
>  > > points.
>  > >
>  > > From top left to bottom right :
>  > > - VCO2 driven by the CV from by the keyboard,
>  > > - the same with preset VR4 shunted,
>  > > - VCO2 driven by an external fixed voltage (no wavering !),
>  > > - mathematically generated sawtooth of the same approximate
>  > >  frequency.
>  > >
>  > > Note how much frequency noise there is. It probably isn't an
>  > > artefact of the frequency counter as the graph for the
>  > > mathematically generated sawtooth is clean.
>  > >
>  > > Also note how the dot density is higher at the extreme
>  > > frequencies. I have no idea why. It might be related to the fact
>  > > that most of the time-domain noise is 50 Hz hum...
>  >
>
> > IIRC when you use the external CV, you are excluding the S&H circuit.
>
>  The keyboard voltage S&H ? Yes. This is how I knew the fault was
>  not in the VCOs or in the normalling.
>
>  > did you examine  IC4 (4558) and IC5(3140) outputs ?
>  > Is the current source to keyboard stable?
>  > Have you tried to exchange q23? (2sk30gr)
>
>  Finally fixed it. It wasn't VR4, it was the CA3140 (IC5).
>  Replacing it made the wavering go away.
>
>  What is very strange to me is the existence of an intermittent
>  fault inside an IC. It could have been a bad solder joint or a
>  broken trace but I thought I had already touched up the joints.
>
>  Too late to find out now, as sincerely yours stupidly fried the
>  old CA3140 by shorting pins 6 and 7. Switching the synth off was
>  too much work you see...
>
>  You'll be pleased to know, however, that the data sheet does say
>  "output short circuit duration (to ground or to either supply) :
>  indefinite". <g>
>
>
>  --
>  André Majorel http://www.teaser.fr/~amajorel/
>
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