[sdiy] my vco is TOO alive

[nicolas]

Before you confuse yourself with the scope, I would just do some work with your trusty multimeter.  Even better if you have more than one as you can monitor multiple voltages at once.  

Clip it up to measure the +ve supply voltage (as close to the main op-amps as possible).  Run the VCO and listen to it do its weird thing and see if there is any corresponding change of voltage.  There shouldn't be.  Then the same for the -ve supply voltage.  Then the CV where it enters the board.  Then the CV after the summing op-amp.  Then the voltage where the emitters of the transistor pair meet.  At all these points there should be a stable DC voltage (when the VCO is emitting a stable pitch).  

Then with the transistor pair (lm394) and the integrator op-amp (ca3140) and the comparator (lm311), check the voltage at the power supply and ground connected pins.  Put the probe actually on the IC pin to confirm that the voltage the chip is seeing is the same as you measure elsewhere on the board.

If you still haven't found the problem you can always go back and check these points with the scope later to see if there is any spurious AC.  But a digital multimeter is actually better for seeing small and slow voltage changes which might be the case here.

It may be something as simple as a ground connection you forgot to solder or an electrolytic cap soldered in the wrong way round.  So double check power supply related connections and all the solder joints around the log converter, integrator and comparator area.

Good luck,
Nicolas


--- On Thu, 11/12/08, Dan Snazelle <subjectivity at hotmail.com> wrote:

> From: Dan Snazelle <subjectivity at hotmail.com>
> Subject: RE: [sdiy] my vco is TOO alive
> To: kingravine at comcast.net, "sdiy" <synth-diy at dropmix.xs4all.nl>
> Received: Thursday, 11 December, 2008, 6:16 PM
> thanks for the great advice
> 
> this might be a good time to get more used to my scope
> 
> let me ask...i am still VERY new at using my scope.
> 
> what am i looking for ideally on the power rails? and what
> sort of settings are good for this?
> 
> in the last year i have been so busy that my scope hasnt
> seen nearly as much use as i would like and 
> i am still a bit unsure of it at times
> 
> thanks
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc  
> 
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle 
> 
> 
> 
> 
> > From: KingRavine at comcast.net
> > To: subjectivity at hotmail.com;
> synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] my vco is TOO alive
> > Date: Wed, 10 Dec 2008 17:57:20 -0500
> > 
> > ----- Original Message ----- 
> > From: "Dan Snazelle" 
> > To: "sdiy" 
> > Sent: Wednesday, December 10, 2008 2:11 PM
> > Subject: [sdiy] my vco is TOO alive
> > 
> > 
> >>
> >> one of my vco's, a CGS (ken stone) vco, which
> is being run off a +/- 
> >> 12volt supply and has a tempco in place, lately
> has been exhibiting a LOT 
> >> of instability. This started a few months ago and
> it sounds like there is 
> >> a gremlin inside of it that is messing with its
> stability.
> >>
> >> i am all for organic VCO's but this is too
> much. The note you are playing 
> >> almost always starts dipping in pitch after a few
> seconds.
> >>
> >> it's not that it isnt IN TUNE with itself, it
> is, and the last time I 
> >> checked i think i was getting around4 ot 5 octaves
> of tracking.
> >>
> >> The issue is more about WARBLE. the notes droop
> and have odd amounts of 
> >> vibrato added as i hold a note.
> >>
> >> so..my question is...what could cause a healthy
> VCO to start sounding like 
> >> it's dying?
> >>
> >>
> >> I have about 5 or 6 other modules running off the
> same PSU and none of 
> >> those modules are showing any of the same
> problems.
> >>
> >> Could it be
> >>
> >> Bad caps?
> >> Ripple?
> >> (the PSU is rated for at least .7 amps)
> >> Or something I cant even fathom?
> >>
> >> Up till this point, i had been very happy with
> this VCO and found it very 
> >> flexible.
> >> But now that i cant hold a note for too long
> without it dipping in 
> >> frequency, it doesnt work too well with my other
> VCO.
> >>
> >>
> >> ANY advice appreciated!
> >>
> >> thanks
> >>
> > 
> > 
> > 
> > Hi Dan,
> > 
> > I'd start simple:
> > 
> > jack is dirty / corroded?
> > Loose wire to the CV jack / tuning controls?
> > Bad tuning pot?
> > 
> > Use a voltmeter or scope on the CV summer and see
> what's happening. 
> > Also, look at the power rails with a scope
> > 
> > I'd also try disconnecting every other module from
> the power supply to see 
> > if the problem goes away.
> > 
> > Dave Magnuson 
> > 
> 
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