I've the gut-feeling that it could be a lifted / non-connected Ground from what you've just described. Tame your meter and check all of the Ground connection's continuity back to the PCB's Ground input.
On 2012-11-06, at 7:29 PM, Stephen Richards <okayjustalittle@...> wrote:
> Hi Ken
>
> Just wanted to update what i said about the VCO ; if it makes any difference.
>
> With the Tempco disconnected On the GND rail side , the whole thing worked but the Fine Tune was functioning exactly the same as Frequency.
>
> The connections are : Fine Tune wiper to Tune , Freq to 1V/Oct as in the Diophantine Wiring diagrams.
>
> I have reconnected the Tempco and it's back to not working. In this state there is white noise and radio signals when i touch the knobs or panel.
>
> Also persistent Earth Hum.
>
> I have resoldered all of the board connections , there are no damaged pads ; all connection are correct.
>
> Tune : 0.3V
> 1V/Oct Freq : -14.87
> 1V Oct Input : -3.4
>
> This is the fourth one of these i've built , the one next to it is perfect. I'm stumped.
>
> ________________________________
> From: "sasami@..." <sasami@...>
> To: cgs_synth@yahoogroups.com
> Sent: Sunday, November 4, 2012 11:06 PM
> Subject: Re: [cgs_synth] Re: a couple of CGS48 VCO questions
>
>
>
> Usually, it is EXTREMELY unlikely that it is a damaged part. You probably
> have shorted two pads together/broken a pad by soldering for too long/used
> the wrong part somewhere. The tempco forms part of a voltage divider, so at
> a guess you have a wrong part (or other damage) around IC1 (TL072).
>
> First step: always check you have power on the pins of the IC.
> Second step: Make sure the "Trim zero" trim pot is near its center, not
> wound all the way up one end.
> Watch what happens at pin 7 of that TL072. It is the output of the input
> mixer stage.
> Possible help: You have two vcos. With them wired to exactly the same CVs
> (or using their tune pots set to the same positions) start comparing
> voltages between the two.
>
> >Hi Gary and all
> >
> >Just an update on the one i referred to as the 'Second VCO' that had no output.
> >
> >I changed out the TL072's , no change ; then the LM394 which i thought had
> blown.Â
> >
> >Doing that i discovered that it's working , but only when the Tempco is
> desoldered ( i desoldered the pad which is on the track that connects to Pin
> 7 of the LM394).
> >
> >I'm stumped.
> >
> >If it makes any difference , the VCO wiring diagram i followed was from
> this page from diophantine :
> >
> >http://www.diophantine.net/synth/bocgs/bocgs-wiring.html
> >
> >
> >
> >
> >________________________________
> > From: Mongo <zathrus1@...>
> >To: cgs_synth@yahoogroups.com
> >Sent: Sunday, November 4, 2012 2:44 AM
> >Subject: [cgs_synth] Re: a couple of CGS48 VCO questions
> >
> >
> >Â
> >
> >Regarding the first VCO, it sounds like you probably have a bad IC1, or a
> bad solder joint somewhere. Recheck your solder joints, and if everything
> looks ok, measure the voltage at pin 7 of IC1 - it should track when the
> incoming control voltage changes. If it doesn't, try replacing IC1.
> >
> >On the second one, it's possible that the VCO is actually running, but just
> at a very low frequency. I had that happen on a Teezer VCO once. I'd
> definitely suggest replacing the TL072 - you were running it without power,
> so who knows what happened inside that little fellow.
> >
> >Gary
> >
> >--- In cgs_synth@yahoogroups.com, Stephen Richards <okayjustalittle@...> wrote:
> >>
> >> Hi Ken and all
> >>
> >> I'm building up the BOG panel and have done the two VCO's but am
> experiencing a few problems with both.
> >>
> >> One VCO is working fine apart from VC Freq pot ; seems to have
> zero effect on the Frequency when fed CV and when turned ; it also cuts
> out at the very lowest range , shorting to some sort of low droning sound.
> >>
> >> The Second VCO has no output , no change when Freq pot is moved but when
> FM'd , PWM'd or CV is applied the the Variable CV input , i'm getting sound
> from the outputs.
> >>
> >> When i checked this Second VCO , i found i had only soldered Pins 1 and 5
> of the TL072 directly above the unicorn figure.
> >>
> >> Could this have caused damage to other parts of the circuit ?
> >>
> >>
> >> [Non-text portions of this message have been removed]
> >>
> >
> >
> >
> >
> >[Non-text portions of this message have been removed]
> >
> >
> >
> >------------------------------------
> >
> >The CGS Modular Synth home page: http://www.cgs.synth.net/
> >Yahoo! Groups Links
> >
> >
> >
> >
> __________________________________________________________
> Ken Stone sasami@... otherunicorn@...
> Modular Synth PCBs for sale <http://www.cgs.synth.net/>
> Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
>
> [Non-text portions of this message have been removed]
>
>
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