ENS-76 VCO Option 2 question

[Bob Zimmer]

I just finished breadboarding this VCO and have one remaining problem.  When
building, I swapped out the 'old' opamps for TL071's.  This meant replacing
a 307, 2-748's and 4-556's with 071's. Everything works fine except for one
piece of the circuit, the Triangle to Sine convertor.

Replacing the 556 with an 071 causes it to generate a low amplitude sine
(+-2.5V) that does not change frequency regardless of the frequency of the
VCO.  Also, the sine output can not drive headphones!  If I swap in a old
741, suddenly it works great, but the sine shape isn't as good as it should
be (probably why the 556 was used!).

In testing replacements for the 556, both a 741 and 307 work, yet a 301 and
TL071 don't.

This piece of the circuit is as below:

+-5V TRIANGLE O-----/\/\/\----|
        IN           62K      |
                              |    CA3080          +15V
                ----/\/\/\----|____|\        220K   |
                |     1K       -IN | \<-----/\/\/\---
              -----                |  \____
               ---                 |  /    |
                -             _____| /     |      39K
                              |+IN |/      ------/\/\/\--
                              |               |         |
       Sine Trim Pot        -----             |__|\     |
          left out           ---             -IN | \    |   1K        +-5V
       (100K Trim tied to     -                  |  \___|_/\/\/\___O SINE OUT
        +-15V. Connect thru                  +IN |  /
        1M resistor to the          ---/\/\/\----| / 556
        -IN of CA3080)              |   39K      |/  (Trying to replace
                                  -----               with TL071)
                                   ---
                                    -

I have a copy of Yves Lefebvre's board design for this oscillator and he has
used TL082's to replace the 556's including this one.  If an 082 works, why
wouldn't an 071?  Isn't it supposedly the same part with lower noise?

Does anyone have an ideas of why an 071 won't work while a 741 or 307 will?
I know that the 071 tests good. It is working in another part of the circuit
right now.


  >=== Bob Zimmer -- Philadelphia PA ===<
  >=== bzimmer at voicenet.com          ===<