[sdiy] Cooling in modular synths

Michael E Caloroso mec.forumreader at gmail.com
Mon Dec 27 18:33:55 CET 2021


Yeah, mica caps are not the universal charging element for every VCO.
Other designs use polystyrene for a reason.

Sadly a lot of engineers/techs do not RTFM.

MC

On Mon, Dec 27, 2021 at 12:18 PM Mike Bryant <mbryant at futurehorizons.com>
wrote:

> Careful or you’ll you start a trend for people ripping out perfectly good
> capacitors and replacing them with mica :-)
>
>
>
> The designers of the CEM3340 probably either designed or tested the
> temperature coefficient of their prototype ICs and looked up which
> capacitors balanced this out best.  Other VCOs/VCFs may be more suited to
> other types of capacitor.
>
> As you have both have done, but many others don’t … RTFM !   Bit
> disappointing that Moog didn’t, but possible R&D originally specified the
> correct component but then someone cost-reduced the product.
>
>
>
>
>
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Tom
> Wiltshire
> *Sent:* 27 December 2021 15:55
> *To:* Michael E Caloroso
> *Cc:* synth-diy
> *Subject:* Re: [sdiy] Cooling in modular synths
>
>
>
> For other people who are wondering, the CEM3340 long datasheet says:
>
>
>
> "a low leakage, low tempco capacitor, such as mica, should be used for Cf".
>
>
>
>
>
> On 27 Dec 2021, at 15:24, Michael E Caloroso via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
>
>
> 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.
>
>
>
> MC
>
>
>
> 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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