[sdiy] Cooling in modular synths

Michael E Caloroso mec.forumreader at gmail.com
Mon Dec 27 16:24:25 CET 2021

I have solved tuning problems with corrections in power distribution.  I
learned some tricks in my career as systems engineer.

When I first got my Minimoog (very early serial #) it was hopelessly
unstable.  Things like modern BiFET opamps in the CV summers helped, but
the biggest contributor was modifying the power distribution.  Not just the
oscillator card but the pitch wheel and front panel tuning pots.  I gigged
that Minimoog; I tuned it before the show started, and never had to retune
it the next four hours.

The biggest contributor to Memorymoog tuning issues is also power
distribution.  I restored two Memorymoogs, including my own.  My fix with
replacing interconnects with gold plated contacts is well known (including
the PSU), the last restore I reflowed the flaky solder joints on the DMUX
board which receives the power harness from the PSU.  The DMUX power
harness is obviously a manual process at the factory and was not done very
well.  After my restoration work was done, I demonstrated it to the
customer and was surprised to find that the MM booted up cold and was
perfectly in tune.

Another cause of tuning problems is incorrect dielectric of the charging
cap in the VCO.  The tempco of the dielectric of the cap is a factor in the
accuracy of the expo conversion, and its tuning stability.  Memorymoogs and
Oberheims equipped with CEM3340s use the wrong cap.  The CEM spec calls out
the correct dielectric (it is NOT a polystyrene).  When I corrected the
caps on my 3340-based synths, the tuning was much better.  I bought a Moog
Voyager SE when they first came out and it has been rock solid tuning;
years later I acquired a Voyager RME and the tuning drifted.  Study of the
schematic and comparing the two boards revealed that the RME had the
incorrect dielectric cap; when that was corrected it was much better.  I
bought the RME at the Moog Store where it was a bench unit; it is possible
that at some point of Voyager production there was a supply problem and
they substituted the cap with the same value, but wrong dielectric.


On Mon, Dec 27, 2021 at 8:42 AM rrsounds (null) via Synth-diy <
synth-diy at synth-diy.org> wrote:

> The analog conversion between linear and logarithmic (and vice-versa)
> scaling using semiconductors often depended upon tightly-coupled
> (physically) compensating thermistors that were themselves highly
> temperature-sensitive, and of variable quality. Keeping a steady
> temperature in the equipment’s environment was often vitally-related to
> keeping in tune.
> As Mattias writes, these problems have been more or less solved in today’s
> equipment.
> I suspect that, like many other generally negative anomalies, the
> serendipitous effects of temperature drift were, however, seen as
> beneficial to some people’s art.
> David Reaves
> Sent from my iPad
> On Dec 27, 2021, at 2:03 PM, Mattias Rickardsson <mr at analogue.org> wrote:
> Den sön 26 dec. 2021 20:50cheater cheater <cheater00social at gmail.com>
> skrev:
>> Would you say current designs are less temp sensitive?
> Yes, and put in other words you could say that there have been advances in
> tuning and temperature compensation since 45 years ago. :-)
> Temperature-dependent deviations have become better understood, temp
> sensing and compensation can be done closer to the error source - on chip
> level with less problematic factors, clever microprocessor control can be
> used, etc.
> What was the most temp sensitive part in things like a Jupiter 8,
>> CS-80, or a Memorymoog? Or even an original Prophet-10? The last two
>> especially are said to have been very difficult to keep in tune...
> As said before, exponential converters are the most sensitive (but luckily
> also the most consistent and relatively easy to compensate) in three of the
> above.
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