[sdiy] ARP VCO drift

[Dave Leith]

Hi Rob

Not sure if it makes any difference but the ARP VCO had mica cap for integrator.

Dave

On 8/10/08, rob <rob at emulatorarchive.com> wrote:
> Hi folks,
>  I need some expert guidance and ideas, a rather boring subject but this one
>  has me stumped.
>  I recently built a couple of ARP style VCO's (Odyssey/2600) with new
>  components but the same design.
>
>  The VCO waveshapes and frequency range are excellent, but the frequency
>  drifts a lot more than I expected or have measured with a real ARP 2600.
>  At 200Hz the ARP clone VCO varies by 1-2Hz, whilst my E-mu Systems VCO
>  varies by less than 0.1Hz, as does the original ARP.
>
>  I have tried - but with no impropvements:
>
>  Different makes of 1K87 3500ppm Tempcos
>  Adjusting the Tempco ppm with a series resistor
>  Odyssey and ARP2600 front end designs before the expo pair
>  2N3906/3904 transistors
>  BC550C/560C transistors
>  Integrator capacitor is 1% polystyrene
>  Comparator voltage is stable to at least 0.5mV
>  Frequency adjustment voltage rails come from a voltage reference chip, at
>  least better than 0.5mV
>
>  The transistor pairs are not matched, but are in contact with the Tempco.
>  There is some improvement by getting this contact.
>  The original ARP 2600 VCO's are of course encapsulated....
>
>  Where am I going wrong?! Any ideas?
>
>  Regards and Many Thanks for any help...I really would like to get these
>  VCO's to be usable.
>  Rob
>  www.emulatorarchive.com
>
>
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