Would it make any sense to temporarily remove the 22K TRIM pot, since that
would isolate the TL072 from the rest of the stuff downstream?
Grasping at straws--
--Adam
would isolate the TL072 from the rest of the stuff downstream?
Grasping at straws--
--Adam
> -----Original Message-----
> From: charlesosthelder [mailto:osthelder@...]
> Sent: Monday, June 06, 2005 8:05 PM
> To: cgs_synth@yahoogroups.com
> Subject: [cgs_synth] Re: VCO weirdness
>
> Now I'm getting someplace!
>
> I've got a perfect sawtooth up to the pin 2 of the saw output
> opamp. 15 volts DC at pin 1. I disconnected the 330R resistor,
> expecting to see zero vdc at pin 7. Measured about 10vdc instead!
> +15 vdc at pin 8 and -15vdc at pin 4.
>
> Also, If I plug into the shape CV input I can hear (and see) the
> same signal somewhat "shaped" That means everything is doing what
> I'd expect *except* the TL072 (which has been replaced twice now
> from different stock) for the saw and square ouput. I've put the
> 600K/200K ohm resistors (600K in the feedback/200K to ground from
> the input) on the saw stage for +-5 volt swing.
>
> Everything seems okay in the traces. What am I doing wrong? Is the
> 47mfd cap to be removed? Has anyone else built a rev 3 pcb?
>
> Ken?
>
> Gee, don't tell my employer about this!
>
> Chub
>
> --- In cgs_synth@yahoogroups.com, "charlesosthelder"
> <osthelder@n...> wrote:
> > Now this is wrong. Tested the second VCO (had the correct TL0__
> > device in place!) and get the same result as the first. VCO core
> is
> > fine - actually works very well! The sawtooth from the core
> > completely goes away at the pin 1 of the TL072 for the saw
> output.
> > I checked for a simple short to ground, but no such luck. Cap
> > polarity is correct and the TL072 has correct voltage at pins 4
> and
> > 8.
> >
> > Another oddity - the sub input and output work as expected. But
> if
> > I plug my square wave audio into the square wave out jack, which
> is
> > connected to the sub input when no plug is in the sub in,
> frequency
> > is halved!
> >
> > This is really odd. How can such a simple stage as the saw and
> > square output amplifier be causing me so much grief? Has anyone
> > else built the rev .3 pcb? When I get a chance, I'm going to
> follow
> > the traces. In the meantime, I'm going to try to calm down.
> >
> > Any ideas?
> >
> > Chub
>
>
>
>
>
> See the following URLS for the CGS Modular Synth home page:
> Main: http://www.cgs.synth.net/
> Secondary: http://otherunicorn.0catch.com/synth/
> Yahoo! Groups Links
>
>
>
>
>
>
>