[sdiy] Need help breadboarding a VCF

[Ian Fritz]

At 11:54 AM 8/11/05, Jeff Farr wrote:
>Hello, I'm working with the "Filter Option 1" schematic in the EN
>Preferred Circuits Collection, and so far, it doesn't work..  Firstly
>I should ask if there is a less complicated filter I should start
>with, this one barely fits on my radio shack breadboard and the mess
>of jumpers makes it a bit difficult to troubleshoot.  What I would
>like is some type of scheme for testing the expo converter, as so far
>it doesn't seem to be working as I would expect.  I've hooked it up
>alone on the board and tried to measure the current it produces, but
>it remains constant where I would expect it to change.
>Three questions:
>I'm using the common terminal on my PSU as ground, is this correct?
>I don't have any 307's so I'm using lm741's instead, perhaps this is
>the problem?

Hi Jeff --

That's a reasonable filter to start with.  It's relatively simple as far as 
filters go, and will get you going with a variety of filter responses.

The first thing you should always troubleshoot is the PS voltages, 
especially if you are uncertain about the PS connection.  Use your DVM to 
measure between circuit ground and the two PS pins on each chip.

Next measure the voltages on pin 5 of the two CA3080's.  It is real easy to 
burn out a 3080 by accidentally abusing pin 5.  The voltage you measure 
should be two diode drops (1.7 V) above the negative rail.

Next do what Harry suggests -- look at the voltages at the top end of R35 
and R36 as you vary the "coarse freq" control.  Theese should be equal and 
vary from something around zero to something around -13V or so.  If you 
don't see this, check the voltage at the base of T1.  It should be tenths 
of a volt and also vary with the freq control.

Report back, and we can proceed from what you find.  It shouldn't be too 
hard to get this working.

   Ian