[sdiy] 4069 VCO troubleshooting success

[Scott Gravenhorst]

"Pat Kammerer" <spivkurl at wearerecords.com> wrote:
>Hi,
>
>Just though I'd let you all know how the troubleshooting went.
>
>Everyone had VERY good advice, and all together it hit right on target.
>
>To start with I used the simple finger pressure test, mainly to see if
>anything reacted that would suggest a bad solder joint.  I didn't find that,
>but I found that a certain point did react by returning the pitch to it's
>normal high state.  This definitely backed up my opinion that the problem
>was in the linear current sink.
>
>Without jumping ahead hastilly, I tried the "divide and conquer" method, and
>this also pointed towards a problem in the current sink, as I could get the
>circuit to oscillate at a set frequency.  One thing I noticed was that I
>could skip the resistor to ground and instead take the input of the VCO core
>to Vcc/2, and this would effect on oscillation as well.  Scott, did you mean
>virtual ground, or ground?  Is this wrong to do, or harmful?

Ground - real power supply ground.  Not harmful using a resistor.  I think
either Vcc/2 or ground will work, but a single resistor to ground is simpler.

>Then I focused on finding exactly what point reacted to the touch of a
>finger.  This led me directly to the non-inverting inputs of the opamps.  A
>quick test helped me conclude that these were not connected to ground as
>they should have been.  I connected them together, but forgot to ground
>them.

AHA!  Nails ya every time.  The old "connected them together but didn't
ground them" trick.

>So all of you were pretty correct - it was a ground problem, a floating
>input, and something causing the current sink to change it's behaviour.
>
>Thank you all for your help, again.  Now on to the suboctave!
>
>Take care,
>Pat
>

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