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

[Andre Majorel]

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/