[sdiy] sync circuit woes

Dan Snazelle subjectivity at hotmail.com
Thu Nov 20 18:15:50 CET 2008

the 18pf cap was on the reset circuit of the lm311.

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(or for techno) http://www.myspace.com/snazelle 

> Date: Thu, 20 Nov 2008 08:55:02 -0800
> From: dixon at interchange.ubc.ca
> Subject: RE: [sdiy] sync circuit woes
> To: subjectivity at hotmail.com; synth-diy at dropmix.xs4all.nl
> Dan,
> I didn't see any 18 pF caps in the schematic.  However, the only timing cap
> there is the 2200 pF in the sawtooth oscillator.  This cap sets the pitch of
> the VCO.  All the other caps in the opamp feedback loops (the 22 pF and the
> 100 pF) are there to prevent spurious oscillations in those circuits.
> I'm not a huge expert in this, but it would seem to me that if the only way
> you could get a circuit such as this to work was by replacing a specified 18
> pF cap with a 10 pF one (presumably in an opamp feedback loop), then either
> the original cap was fried or the circuit design has serious issues.
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> 309-6350 Stores Road
> Vancouver, B.C.  V6T 1Z4
> Canada
> Tel 1-604-822-3679
> Fax 1-604-822-3619
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dan Snazelle
> Sent: Thursday, November 20, 2008 4:46 AM
> To: sdiy
> Subject: [sdiy] sync circuit woes
> working with the sync circuit that is HERE (on 2 pages)
> http://www.elby-designs.com/asm-2/vco/vco1-summer-asm2-cct.pdf
> http://www.elby-designs.com/asm-2/vco/vco1-cco-asm2-cct.pdf
> I am getting the sync signal coming IN to the input. i am getting that into
> pin 3 of the Tl081/71
> the resistors are getting gnd and -15v
> i need to figure out how to test from that point on. (pin 6 and on)
> since the output of that first opamp is a comparator, i cant check for
> sound. can i check for something on a scope?
> or a DVM?
> I had to fix the VCO to work with a 10pf timing cap instead of an 18pf..it
> wouldnt work without that change....could that make a difference? maybe i
> need to change some values?
> are there any common issues with sync circuits i could be on the lookout
> for?
> i will start checking for cold solder joints.
> i really appreciate it
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