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

[Magnus Danielson]

Andre Majorel 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.

The excessive white and flicker noise seems likely from a partially 
fried chip.

Cheers,
Magnus