[sdiy] my VCO not oscillating :(

[Michael Ruberto]

Ok well it's finally come down to the buffer. It's simply not buffering 
enough to turn Q7 on thus not completing the feedback for the Schmitt 
trigger. I'm thinking it's a 4302 issue. I substituted several other 4302s 
and also tried swapping the pins around in case i have the wrong pinout. 
Burned a few fingers too. Frankly I'm ready to flush this project down the 
toilet.

Thanks for the help Tim!

M. A. Ruberto


>From: "Michael Ruberto" <frankentron at hotmail.com>
>To: timothy at tstinchcombe.freeserve.co.uk, synth-diy at dropmix.xs4all.nl
>Subject: RE: [sdiy] my VCO not oscillating :(
>Date: Wed, 07 Dec 2005 20:13:52 -0500
>
>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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