[sdiy] AN299 VCO (was: SDIY list troubles)

[flight]

Last year I spent a few weeks playing around with this circuit. I used a
TL074, which worked fine. The VCO works nice except for its horrible drift -
I probably should have insulated the CA3046 from stray air currents. I
abandoned the project finally, as I just couldn't get it to stabilize. I ran
it for up to 5 hours and it just wouldn't sit still! It also has a lengthy
warm-up period while the array heats up (5+ mins). Here's the schematic I
drew from the App Note: http://www.flightofharmony.com/stash/AN299 VCO.pdf .

~flight  

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Aaron Lanterman
Sent: Monday, April 28, 2008 7:20 PM
To: SDIY
Subject: [sdiy] Fwd: SDIY list troubles

Howdy - My student Adam is trying a different oscillator now and  
having trouble both with the oscillator and posting to the list, so  
I'm forwarding it for him...

- Aaron

P.S. We're using a TL08x/TL07x series op amp.

------------------

The message:

Hey everyone.  I'm building the exponential V-F oscillator in figure 1  
of this app note:
http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/opamp/AN-299.pdf 
, and once again I'm getting no oscillations at the output, even after  
putting it through a TL072 buffer and inverting amp to boost it.  We  
took DC voltage readings at a bunch of points in the circuit at 0V  
input  and 15V of input to see if they changed right.  One thing I  
noticed is that the Vbe of Q1 is  changing, so the current from Q1  
_should_ be varying exponentially like it's supposed to.  I put an  
ammeter between the emitter of Q1 and the inverting input of A1 and  
saw no current, but  Aaron told me that can be pretty unreliable.

Anyway, listed below are the DC voltages we measured while  
experimenting with the circuit. Can anyone see any reasons why this  
isn't working?

Node                               Vdc at 0V input               Vdc  
at 15V input
A1, inverting input:            0.5 mV                               
1.7 mV
A1, non-inverting:             9.3 mV                              0.3  
mV
A1, output:                       -0.67 V                              
-0.73 V
Q1, base:                         0.52 V                                
0.71 V
Q5, base:                         0.5  
V                                 -0.5 V
Q2, base:                         6.3 mV                               
-23.1 mV
Q2, emitter:                      -0.68 V                              
-0.73 V

Thanks in advance for any help you guys can provide.

-Adam Troyer
>

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