[sdiy] VCO oscillation problem

[Jay Schwichtenberg]

My 4D and 4E both worked right off the bench at +/- 15V. The 4D had some
rework on the PCB I don't remember the 4E having any.

Do a good visual inspection and make sure all the solder joints look good.

Might check the cap to see if it's the right value. The difference between a
103 and 104 value cap is pretty substantial.

Also look to what voltages the saw works at. If I remember right it should
be about 0V to 1/2 +V.

Then it could be bad parts.

It would help a lot to if you had an oscilloscope to see what is going on.

Jay
  -----Original Message-----
  From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Peng
  Sent: Thursday, March 18, 2004 10:33 AM
  To: synth-diy at dropmix.xs4all.nl
  Subject: [sdiy] VCO oscillation problem


  I'm having a problem with my VCO 4E from EFM. Assembled and looks nice but
the wave outputs all produce the same high-pitched screalling. It responds
to keytracking and LFOs as cv mod. Tried the trimmers: couldn't bring the
pitch down enough. Quadruple checked pcb, wiring, solder joints, etc.
Started poking my finger around the board ( as a beginner this is my
scientic troublshooting method ) , Got to the op amp that sums the cv inputs
and touched the capacitor in its feedback loop and the pitch changed
slightly. Tried attaching a bigger cap but this had only a very small
effect. My best guess is a problem with the feedback loop. Any pointers,
suggestions, or links would be of great help.

  Thanks,
  Peng
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