[sdiy] sync circuit woes

David G. Dixon dixon at interchange.ubc.ca
Thu Nov 20 17:55:02 CET 2008


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
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C.  V6T 1Z4
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)


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

i will start checking for cold solder joints.

i really appreciate it


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