[sdiy] my VCO not oscillating :(

[Jeff Farr]

Hello again, I'm quite interested in following your conversation here,
although I'll not likely have anything useful to add.  If one of you
could email me a full transcript of the troubleshooting process I'd be
thankful.

On 12/7/05, Michael Ruberto <frankentron at hotmail.com> wrote:
> hi Tim,
>
> I agree about the inputs to U2 looking wrong. All other VCOs with this expo
> topology have the error amp inputs opposite of this design. This might be a
> mistake in the original schemo but I'm not certain. I just reversed the
> inputs on mine with no success. Circuit still does not oscillate.
>
> Now I know the R22, R23, R24, R28 & R29 network has the right voltage level
> so it can't be that. Q7 was reversed and I have corrected that, so it's no
> longer a suspect. This leads me back to Q4 and Q5. I have tried grounding
> the sync input also and that has no positive effect.
>
> This weekend I'll make some time to replace Q4 and Q5. If that doesn't work
> I guess I'm back to the breadboard. I'll have have to also rule out the
> 2SC1583. I have a 3046 on hand now so I'll use that in place of the 1583 and
> see what happens.
>
> M. A. Ruberto
>
> >From: "Tim Stinchcombe" <timothy at tstinchcombe.freeserve.co.uk>
> >Reply-To: <timothy at tstinchcombe.freeserve.co.uk>
> >To: "'Michael Ruberto'" <frankentron at hotmail.com>
> >Subject: RE: [sdiy] my VCO not oscillating :(
> >Date: Mon, 5 Dec 2005 22:59:35 -0000
> >
> >Hi Michael,
> >
> >Spotted this as I was typing, check this out first, if so it will screw the
> >expo converter big-time!:
> >
> >Hello! Big error on schemo! U2 inputs are reversed! Non-inverting input
> >should go to ground (probably) via R13, inverting input to TA1 pin 2, 100p
> >goes to inverting too!
> >
> >(Rest that follows maybe academic...)
> >
> > > >... this statement is wrong! With Q7 on, R23 is effectively
> > > pulled to
> > > >ground, overall effect on D cathode is to pull it below
> > > ground, hence
> > > >Q4 base also. My sim gives Q4 base at -3.43V !!
> > > >
> > >
> > > This is .7 below the reading I get.
> >
> >That's probably fair enough - all I did was put R22, R23, R24, R28 & R29 to
> >either supply or ground, depending on which state the Q's were in (either
> >'ramping' or 'reset') and then checked what the voltage at R22-R23 junction
> >was, plus others, like Q4 base. Thus things like R30 were not there, and
> >also a lot of currents missing (and as I look I see I should have included
> >R25 if Q4 is on, as this is virtually tied to ground..etc...).
> >
> > > Q4 and Q5 are wired properly. However, Q7 wasn't and the R22
> > > heating problem
> > > went away after I rectified the problem. Circuit still isn't
> > > oscillating
> > > though. Tonight I will replace Q4 and Q5. Do you think
> > > substituting the
> > > CA3086 in the original SEM with the 2SC1583 could be a problem?
> >
> >On a gross scale like, does it work or not, probably not at all. But on a
> >finer scale like temperature comp, tracking, hf compensation etc., I'm not
> >so sure (but that isn't the problem at the moment...).
> >
> >It probably wouldn't hurt to try and set P1 (I assume it's a trimpot?) to a
> >realistic value though - if it is not, because U2/TA1 gives an exponential
> >current, it is very easy to get waaaay more than intended (like many mA,
> >when in reality, the max current from it is likely in the order of several
> >hundred uA). (This caught another list member out recently, though that was
> >a VCF!)
> >
> >R12 gives a reference current of 10uA, and Iout at TA1 pin 4 is
> >10*2^(Vin+Voff) uA,
> >where Vin is CV1 (rest are assumed 0), and Voff is P1 wiper. So if we
> >assume
> >max Vin is 10V (?), giving 200uA say, we want Voff approx
> >log2(200/(10*1024))=approx log2(1/50)= -log2 50= approx -log2 64= -6V say,
> >so set P1 to that. ?I'm assuming T1 is a real fine-tune, with that 4M7
> >there?? And hopefully T3 and the HF stuff is likewise negligible in terms
> >of
> >it works/it doesn't work.
> >
> >Regards,
> >       Tim
> >__________________________________________________________
> >Dr. Timothy E. Stinchcombe, CMath, MIMA, CEng, MIEE, MIEEE
> >
> >53 Winstonian Road
> >Cheltenham
> >Glos, UK
> >GL52 2JD
> >
> >email: timothy at tstinchcombe.freeserve.co.uk
> >
> >Tel: +44 (0)1242 571747
> >__________________________________________________________
> >
> >
> >
> >
> >
> >
>
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