[sdiy] Newbie needs help with VCO

[Jim Patchell]

Christian wrote:

> Hi
>
> I am a newbie when it comes to VCO's, although I have been working
> with electronics for over 20 years. If it isn't a 555 or a two
> transistor astable, I am stumped!
>
> I have recently (the past week) built a VCO circuit, but the darned
> thing does not oscillate. I wonder if anyone could help me out.
> The circuit is this one:
>
> http://www.evenfall.com/diy/vco2061.gif
>
> I am using a Transistor array for the Expo, and all the IC's are
> socketed so I can isolate or replace them.
>
> I have closely examined my board for errors etc three times, but all
> seems well (although it isn't ).
>
> Could someone help me out with:
>
> 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?)
>
> 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.
>

    Instead of using the exponential converter, Just tie a resistor of
about 1Meg to the negatvie supply rail.  This will help in the debugging
of the oscilator.

>
> 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.

    The LM311 is connected up correctly, as far as the diagram goes.
There is some detail missing, that, if you are not familiar with the
LM311, you might not know about.  If you have the time to download
this:http://www.silcom.com/~patchell/smb1/smb.pdf, there is a schematic
of basically the same VCO, that shows all of the LM311 pins and where
they need to go.  Unfortunately, the VCO is on the last page of about  24
pages.....I remember the first time I used the LM311....I couldn't get
that sucker to work at all....then I put my glasses on and realized what
I thought was an 8 was a 3 :-).....or something like that....well, that's
my story, and I am sticking to it.

>
>
> Any help is greatly appreciated.
>
> Christian.