[sdiy] Weird MFOS VCO max CV problem

[Travis Thatcher]

I just replaced the LF442 (found one) and the OPA and still the same behavior.

On Mon, Apr 9, 2012 at 10:42 PM, Travis Thatcher <recompas at gmail.com> wrote:
> This is the version, schematics on the page:
> http://musicfromouterspace.com/analogsynth_new/VCO200604REV01/VCO200604REV01.html
>
> I have some spare lm13700's laying around. I'm probably going to
> replace that and see what happens. Not sure if i have another lf442
> though, which is the summing opamp.
>
> T
>
> On Mon, Apr 9, 2012 at 10:06 PM, Olivier Gillet <ol.gillet at gmail.com> wrote:
>>> There were some discussions about using a TL07x as an inverting integrator  in an OTA filter, many people had hang-up
>>> issues at power up. Using the same part a voltage follower (with the integtator cap to ground) worked perfectly
>>> every time...
>>
>> I have seen this problem, but I think it's the other way round: I have
>> seen frequently a phase reversal "crash" with the "cap to ground +
>> follower" circuit when signal level was getting too high. Can happen
>> when self-oscillation is reached, when cutoff is increased (less
>> high-frequency attenuation -> higher level), or simply when a very hot
>> external signal is fed into the circuit. I have worked a lot with +/-
>> 5V so there's not much headroom and it's easy to hit the rails - and
>> in this context seeing a TL072 with its +ve input not grounded raises
>> a red flag for me. Neil Johnson has compiled this list of op-amps that
>> do not phase-reverse:
>> http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=2&subsubmenu=2
>>
>> Back to the topic at hand, are there schematics of the offending circuit?
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>
>
>
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