Well I finally found the problem.
The diode that is connected between pin 5 of the TL072 and the 2N3906 was leaking. It was the source of the stray voltage into Pin 5 that was blocking any signal on the "In" jack.
I've never had any problems with diodes before, but on this build I found two leaky diodes. I must have cooked them when I soldered them in.
Anyway everything seems to work now.
Cheers
Dean
The diode that is connected between pin 5 of the TL072 and the 2N3906 was leaking. It was the source of the stray voltage into Pin 5 that was blocking any signal on the "In" jack.
I've never had any problems with diodes before, but on this build I found two leaky diodes. I must have cooked them when I soldered them in.
Anyway everything seems to work now.
Cheers
Dean
--- In cgs_synth@yahoogroups.com, "goldencardigan" <dean.lowe@...> wrote:
>
> Thanks Darren,
>
> I have been searching both forums, but so far I haven't found anything that helps.
>
> I think I'm getting closer though. I think there is some stray DC floating around, but I haven't been able to track down where it is coming from.
>
> I've noticed that there is around +5v on Pin 5 of the TL072 (the op amp that the 'In' drives) which is blocking any signal on the Input. If I add dc offset to my input signal to raise it above 5v, I start seeing the waveform at the output. Because this op amp is sort of acting like a comparator this stray 5v is causing the output to switch low (close to negative rail). My problems appear to be coming from the 2n3906 that is feed by Pin 9 of the LM3900.
>
> Anyway I'll keep searching.
>
> Thanks again
> Dean
>
> --- In cgs_synth@yahoogroups.com, Darren Shorsky <dschnidsky@> wrote:
> >
> >
> > There is two really good threads on Muff's & electro about this build...... sorry, i don't have the link. If you try those forums????
> >
> > d.
> >
> >
> > To: cgs_synth@yahoogroups.com
> > From: dean.lowe@
> > Date: Wed, 18 Jul 2012 01:07:24 +0000
> > Subject: [cgs_synth] CGS75 VCS problems
> >
> >
> > Hi all,
> >
> >
> >
> > I'm new to the group as this is really my first CGS build. I have built lots of diy modules and I have learnt alot doing so, but I am no electronics engineer.
> >
> >
> >
> > When placed in cycle the module appears to work perfectly. It cycles between 0 and around +6v. All the pots work as they should including the EXP switches and Voltage Control.
> >
> >
> >
> > But when I trigger the module, it slews from around -13v up to +5v and then back again. The End out idles at +5v and pulses to ground at the end of the cycle. I can't seem to really slew anything at the 'In' either.
> >
> >
> >
> > I have triple checked orientation of the parts on the board and the wiring. I'm 90% sure the transistors are the right polarity and I haven't been able to find any broken traces or shorts.
> >
> >
> >
> > I will continue searching the board for mistakes I might have made. If anyone has experienced this before or can point me in the right direction, any info would be most appreciated.
> >
> >
> >
> > Cheers
> >
> > Dean
> >
> >
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> > [Non-text portions of this message have been removed]
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