[sdiy] Newbie needs help with VCO

[Tony Clark]

> a) What sort of CV do I input into the first summing OP-Amp for a
> decent audiable tone for test purposes? (Middle C or 1000Hz etc?)

   If you have a trim-pot in there for frequency trim, then it isn't 
necessary to put an extra CV in.  Just adjust the trim-pot (TR1 on 
schematic).

> b) Is there a way of just making sure that the Oscillator (CA2140) is
> oscillating without worrying about all the additional circuitry,
> especially the EXPO and SYNC circuits.

   First test the output of your exponential converter.  I do this by 
putting a 10k resistor from the output to ground.  Then measure the 
voltage across the resistor.  The output is very small, but it should 
behave in an exponential manner as you sweep the CV in from 0-10V.
   Next, check the output of IC6 and confirm that it's at -V (off state). 
If it is near or at 0V, then the FET (Q2) is turned on and is holding 
the VCO in a reset state.  Typically in operation, the LM311 will start 
rising from -V to 0V and at the moment the FET starts to conduct, the 
output of the CA3140 will drop, causing the LM311 to reset back to -V.  
   BTW, the faster the LM311 can turn on the FET, the faster the reset 
portion of the sawtooth wave will be.  I would recommend reducing R16 
from 680 down to 220 ohms (or remove it, I haven't found it necessary) 
and reduce R20 from 3.3K to 2.2K.  You can lower R20 down to 1.0K, but 
at -15V, it'd run about 1/4W, so it will get rather warm!  Having a 
faster reset in the sawtooth wave will give you a truer saw plus will 
let the waveshaper produce a better triangle waveform.

> c) I think there is an error in that circuit diagram - The Sync input
> should go to the Diff-Amps non-inverting input. I believe the inputs
> are the wrong way around on the LM311.

   I believe you are correct.  I show the sync input on the negative 
input and the output of the CA3140 on the positive input.  Of course 
there can be all sorts of varyations depending on how the FET needs to be 
triggered, but from the schematic it looks like it's the same as the 
ASM-1 type, which means it's reversed.
   Someone will undoubtedly correct me if I'm wrong.  :)

   Cheers,

   Tony

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