[sdiy] 910 pF caps (Buchla 292C/266-3)

[Peter Grenader]

Did I hear you say Low Pass Gate????

I didn't even try to find the 910 pf. I   I used a .001. worked like a
charm.  As Mark Verbos pointed out, it's within the tolarance of a 910 pf
anyway.

I've built three LPGs like this, no problems.

But I wil give you tidbits will need to substitute if you are planning on
using the gate with anything other than a Buchla:

1) There is a resister coming off the wiper of the 10K pot which is used for
a manual offset (it turns on the vactrolLED).  Play with this value.  I
brought mine down to 15k.

2) There is a 100K resister (marked R4 on the 292C schematic) that runs in
parallel with a 470K/.002uf from the VC input that I had to lower to 10K in
order to get any amplitude at all out of them.

I would also suggest you substitute the anlog switch with a ON-OFF-On
mechanical one,  Tis way, you won't have to worry about bringing the 15 volt
line down for the analog switch and (for me anyway), it's just a bunch
easier.

Two more tips on reading Buchla prints:  One, sometimes he reversed the
inputs on the op amps.  I'm used to seeing - on top and + on the bottom.
Don drew them both ways, so be careful.  Also, lines that cross each other
DO NOT intersect electriclly.  Only lines that end on others should be
shorted. Don didn't used dots to signify a junction.

One last note if you're feeling frisky:  The method Buchla used for turning
on the vactrols was lame (IMHO). The last one I built, I used a matched
transistor pair instead of the op amp madness he did and it worked a bit
better.  Don't have to worry about a a trim pot, the thing shuts down
complety as well.  One problem with these is if you aren't careful, the gate
will not shut down all the way.  The trimmer helps, but I noticed with the
furst one I made (which I ended up strapping) I still couldn't get it to
shut down completely. You could hear the signal bleeding through a little.

Romeo, contact me offline.  I redrew the 292 C schemo using a mechanical
switch and mine is MUCH easier to read than the orginal, plus all of the
computer controlled  components have been removed.

Hope this helps,

Peter



on 9/30/02 11:39 AM, Fahl, Romeo at RFahl at extensis.com wrote:

> Hello,
> 
> I'm having hard time finding 910 pF caps from the usual vendors that I go
> through.  Mouser is out of stock on the one part that they have, Allied has
> none, nor does Digikey.
> 
> What I'm wondering is, if I could substitute the 910 pF caps with 1000 pF in
> any of the projects I'm working on.
> 
> The first one is Bucha's Source of Uncertainty, in the Fluctuating Random
> Voltages module:
> 
> http://www.musicsynthesizer.com/Buchla/266-3.jpg
> 
> I'm not quite sure what C11A and 12A are doing (around IC 3 near module
> inputs).  The second couples IC 5 to 6 (C17A and 18A).  Seems like that one
> is less critical.
> 
> The other project is an updated version of Buchla's LoPass Gate (292).
> 
> http://www.simple-answer.com/lopass-gate.jpg
> 
> C5 Appears to be doing some audio filtering, so my guess is that it's more
> critical  to have an accurate value there.  I suspect that if I change the
> 4.7Meg resistor, I can compensate for a different value.
> 
> So, can anyone tell me if 1000pF caps are acceptable for any of the above
> circuits?
> 
> Romeo