[sdiy] my VCO not oscillating :(

[Michael Ruberto]

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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