[sdiy] Anybody worked with real Moog System 55 ?

[Richard Wentk]

Stability on the 921s isn't too bad. But generally the limiting design flaw was PSU stability. Pot quality was a relatively minor issue. 

901s were famously bad at staying in tune, which was a combination of poor PSU design and minimally effective temperature compensation. 

If you're worried about stability I'd suggest some experiments. But I wouldn't expect a Moog clone to have perfectly stable tuning - and I'd consider this a feature, not a bug. ;)

Richard

On 27 Sep 2010, at 21:57, cheater cheater wrote:

> potentiometer material has changed drastically over the last 4
> decades. No reason to use carbon film anymore, especially for
> something as crucial as oscillator tuning.
> 
> When you've cloned the VCO, see if you can recreate the phase jitter
> of the reset time..
> 
> Cheers,
> D.
> 
> On Mon, Sep 27, 2010 at 15:36, Jean-Pierre Desrochers
> <jpdesroc at oricom.ca> wrote:
>> 
>> Does anybody worked with real Moog System 55 ?
>> 
>> I'm building a VCO driver which will be very close
>> to the Moog system 55's  921A VCO driver design (parts & front panel).
>> I'm wondering about the stability of Allen Bradley pots used in such a design in late 60's.
>> I'm considering instead Vishay 249 series pots for FREQUENCY front adjustment
>> but they only are capable of 295deg mechanical rotation compared
>> to the original Allen Bradley's 320deg mechanical...
>> This would have an effect on the front panel FREQ divisions
>> (smaller span).
>> Since Allen Bradley doesn't exist anymore I'd use PEC pots
>> like this:
>> http://media.digikey.com/pdf/Data%20Sheets/Precision%20Electronic%20Components%20Ltd%20PDFs/MIL%20Style%20RV4.pdf?cshift_ck=1830796695cs506374199&client_id=5042
>> 
>> So my question is:
>> Were the original Allen Bradly pots that bad
>> about freq stability (carbon type) or this would still be
>> a 'fair' choice to me today ?
>> JP
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