[sdiy] Potting ARP VCOs, was: VCO reference voltages

[Jean-Pierre Desrochers]

One cool way to avoid potting a complete module
(unable to repair it in the future ...) is to put some
kind  of a metalic box with cover all around the grouped  sensitive parts 
with
a heater or a temp controled oven. this way everything stay temp stabilised
and is repairable in the future..

Just my 2 cents.
J-Pierre

----- Original Message ----- 
From: "thx1138" <thx1138 at earthlink.net>
To: <rob at emulatorarchive.com>; "SDIY List" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, April 18, 2010 10:55 AM
Subject: Re: [sdiy] Potting ARP VCOs, was: VCO reference voltages


> On 4/17/10 3:31 PM, "rob at emulatorarchive.com" <rob at emulatorarchive.com>
> wrote:
>
>> Hi,
>> In the non-potted ARP 2600 VCO I found that the integrator capacitor and
>> HFT resistor were sensitive to temperature, possibly also other
>> components. Of course the transistor pair and tempco were...! The
>> transistor pair being 2 transistors, at best glued together, is very
>> sensitive, and not "buried" in a 8 pin DIL "matched transistor pair".
>>
>> So I simply potted the complete VCO core, using the same layout as the
>> 4027-1 but with 2x 5pin SIP plugs/sockets.
>> Modern thermal potting compound is very good. A very simple test is to
>> blow over your VCO and see if its frequency changes.
>> With a potted module it does not and there is lots of thermal mass.
>>
>> Whether its worth potting other modules is a mute point.
>> For frequency accuracy like the E-mu Systems VCC  - yes.
>> For VCF's I really don't think so.
>> I do plan to build future Roland VCO's with a potted core.
>> Just the matched pair and tempco.
>>
>> Regards
>> Rob
>> www.amsynths.co.uk
>>
>>
>> thx1138 wrote:
>>>> Hi Rob! Very interesting. I have read your page on the ARP VCO - very,
>>>> very cool! You mention that "the VCO core has a number of temperature
>>>> sensitive components as well as the transistor pair; the integerator
>>>> capacitor and a few resistors". What were those resistors? Why did
>>>> they need to be potted with the transistors?
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> Hi Rob,
>
> The reason we potted several of the VCF - UAF was to stabilize the Filter
> trackin when we turned up the Q and it became an oscillator. We wanted it 
> to
> be able to track like an osc as well as possible.
>
> Regards,
>
> Terry
>
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