[sdiy] 4069 VCO troubleshooting success

[Pat Kammerer]

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?

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.

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