[sdiy] Need help breadboarding a VCF

[Jeff Farr]

Ok, thanks for the info.  Once I was home for work I started
re-building the circuit (this is the 3rd time).  I found a connection
that was wrong and took the time to solder leads to the pots I would
need which cleaned up the breadboard quite a bit.  Now, by connecting
R35 to negative supply I get 1.9v to 3.2v across it through the range
of the coarse tune pot.  Working through the math (3.2v/18k ?) this
corresponds to a current delta of ~.0008a.  Seems a bit on the low
side to me, however, I realized that 8/1.8 resulted in ~5 octaves so
perhaps I made a dumb error in the math and things are working
correctly.  I should mention that I didn't bother to connect any other
control input (no CV, fine tune, etc)  and I don't have any 100ohm
pots for the CV trim (just using a 10ohm resistor as a voltage divider
with the 560), perhaps this affects things as well.  While looking
over datasheets I realized that the LM301a is prolly alot closer to
the LM307 than the LM714 is.. (duh?).  I also don't have any AD821's
(or any PNP pairs) so I'm working with 2 3906's.  I haven't hooked up
the actual filter yet as I'm going to take this one sub-circuit at a
time.  I would like some really simple circuits to use the 3080 and
3140 in to understand a bit about them before connecting 5 of them
together and then trying to troubleshoot..  It seems that I prolly
blew the 3 3140's and 2 3080's.  I did manage to get a WOOLY
distortion from it for a few minutes tho...

On 8/11/05, Ian Fritz <ijfritz at earthlink.net> wrote:
> 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
> 
>