[sdiy] MFOS Soundlab Oscillators not oscillating

Tue Jun 2 02:47:01 CEST 2009

David,

I don't have that kind of time, either!  But, I'm just a wee bit addicted to
synth diy of late, so I'm basically stealing the time.

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada

Tel 1-604-822-3679
Fax 1-604-822-3619

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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of David Holt
> Sent: Monday, June 01, 2009 3:09 PM
> To: synthdiy DIY
> Subject: Re: [sdiy] MFOS Soundlab Oscillators not oscillating
> 
> 
> Wow!  This isn't my problem and I'm just butting in, but that was one of
> the finest analysis and explanations I've ever seen online.  I actually
> learned something.  It would be so great if every help resonse were this
> detailed, although I certainly understand that no one's got that kind of
> time, typically.
> 
> Thanks-
> dave
> 
> 
> 
> ----- Original Message ----
> From: David G. Dixon <dixon at interchange.ubc.ca>
> To: James R. Coplin <james at ticalun.net>; synthdiy DIY <synth-
> diy at dropmix.xs4all.nl>
> Sent: Monday, June 1, 2009 4:45:47 PM
> Subject: RE: [sdiy] MFOS Soundlab Oscillators not oscillating
> 
> > I'm having serious problems with a Soundlab here I'm assembling.
> > Everything
> > works perfectly except the oscillators.  Both oscillators are not
> > oscillating.  I've been pulling my hair out for about a month on this
> > before
> > posting to try to minimize the chances that it's me doing something
> dumb.
> > I've checked the components dozens of times (really) and did a
> continuity
> > check between all the points in the circuit and all the points check and
> > supplies and grounds are all connected.  I can't find any bridges
> > anywhere.
> 
> Well, the SoundLab doesn't do much if the oscillators don't oscillate!  ;-
> >
> But seriously, I've built one of these, and the oscillators worked fine,
> so
> take heart.  You will prevail!
> 
> > I had originally built it with TL072 and TL074 ICs but replaced those
> with
> > the LF444 chips as specified in the schematic as I heard some folks had
> > trouble with the TLxxx series.  I had hoped this would solve the problem
> > but
> > still no luck.
> 
> The venerable TL074 works just fine here.  The LF444 has better input
> specs,
> but you really would not be able to tell the difference.
> 
> I hope you used IC sockets!!!
> 
> > Here are the voltages for each of the pins for IC5 on the
> > schematic, a LF444.  Does anyone have any thoughts on what might be
> > causing
> > this fail?
> >
> > Pin / Voltage:
> > 1 / -0.66v
> > 2 / 0v
> > 3 / 0v
> > 4 / 9.25v
> > 5 / 0v
> > 6 / 0v
> > 7 / .02v
> > 8 / -7.62v
> > 9 / 2.9v
> > 10 / -.63v
> > 11 / -9.13v
> > 12 / 0
> > 13 / 0
> > 14  / .05v
> >
> 
> Let's have a look.  Pins 4 and 11 are the rails.  They look about right
> (you
> must be using a wall wart rather than batteries).
> 
> Pins 5-6-7 are the CV summer and 1/50 gain block, so 1V in should give
> 0.02V
> at pin 7.  This is trimmed to 0.018V by the 100-ohm trimmer for 1V/oct
> tracking.  Pin 5 is grounded, and pin 6 will be at virtual ground, so 0V
> is
> expected at both of these.
> 
> Pins 1-2-3 are providing a constant current source (9V dropped across 1M,
> or
> 9uA) for the expo-converting transistor Q1.  The emitter current of this
> trannie is one diode drop, or about 560mV for a 2N3904 (I believe, if it's
> the same as 2N3906 in the other direction) at base ground potential.
> Hence,
> at 18uA (9uA from both Q1 and Q3), one would predict a voltage drop across
> the 10k resistor R64 of about 180mV which, on top of the diode drop, would
> give a voltage at pin 1 of -740mV.  You are reading -660mV, which seems
> about 80mV too low.  This would lead me to believe that perhaps one of
> your
> transistors (probably Q3) is not sourcing any current...?  Again, pin 3 is
> grounded and pin 2 is at virtual ground, so 0V are expected here as well.
> 
> Pins 12-13-14 are the ramp integrator.  Again, pin 12 is grounded and pin
> 13
> is at virtual ground, so 0V are expected.  The voltage of pin 14 should be
> increasing linearly and then suddenly falling.  This is your ramp wave.
> The
> fact that this is not happening means that this integrator is not
> receiving
> any current, which again points to Q3.  My guess is that Q3 has been
> installed backwards.
> 
> Pins 8-9-10 are the ramp reset comparator.  Pin 9 is receiving a reference
> voltage of 9V*47k/147k = 2.9V, so this is correct.  The input at pin 10 is
> -0.63V, which is below the reference voltage, so pin 8 should be putting
> out
> a voltage which is two diode drops above the negative rail, or about -
> 7.6V,
> which it is.  Hence, there is nothing wrong with the opamp at all.
> 
> Check the JFET to make sure it is installed in the correct orientation,
> but
> it won't switch until the comparator flips positive.  If the integrator
> were
> receiving current from Q3, but the JFET was not doing its job, then the
> voltage at pin 14 would increase until it was two diode drops below the
> positive rail (7.6V) and sit there.  The fact that it is sitting at -0.63V
> means that it is not receiving any current, and there is probably nothing
> wrong with the JFET.
> 
> Hence, all fingers point to transistor Q3, which is probably in backwards
> (most likely), or was fried by applying the soldering iron too long during
> installation (a bit less likely, as my experience is that these little
> bastards are actually pretty robust, unless you're a total klutz with the
> soldering iron, which you probably are not).
> 
> > Any help would be greatly appreciated!  Thanks!
> 
> Well, I hope my little analysis helps you solve the problem.
> 
> > James R. Coplin
> >
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