[sdiy] Cooling in modular synths

Mike Bryant mbryant at futurehorizons.com
Mon Dec 27 18:18:39 CET 2021

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<mailto: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.


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

Den sön 26 dec. 2021 20:50cheater cheater <cheater00social at gmail.com<mailto: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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