[sdiy] VCO oscillation problem

[elmacaco]

Make sure your Resistor values are correct.

I had this happen from misreading a schematic and it took ages to figure out.


  ----- Original Message ----- 
  From: Jay Schwichtenberg 
  To: synth-diy at dropmix.xs4all.nl 
  Sent: Friday, March 19, 2004 12:50 AM
  Subject: RE: [sdiy] VCO oscillation problem


  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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