[sdiy] schematic for Moog 901 VCO

[Dave Leith]

Hi Peter

Yes I am still persevering. Any help is greatly appreciated. (As for
contrast and this insane project I am also building a cloned Buchla
259.<http://minimalist.davidleith.com/DeitzModSynth3.html>

As for the missing emitter resistor value I think that it is also
visible on Terry's photos in a different location - as the closest to
the front and in-front of the sine symmetry trim.
<http://mysite.verizon.net/vze6s1hi/sitebuildercontent/sitebuilderpictures/901leftback.jpg>
I did some comparing of the different 901 photos to circuit values

In my prototype I am trying to place the diode arrays, the transistor
pair Q6,7 (NPN) and possibly R7/P1 (may become a tempco) close to one
another/thermally touching). I was also planning on trying a matched
pair for the PNP's so interested in how yours worked out. This was all
going to be on one board.

Thanks Dave

On 3/30/09, Pete Swarbrick <peter.swarbrick at panavision.co.uk> wrote:
> Hi Dave
>  Sorry I only just caught up with this thread (day-job getting in the way
>  again...)
>  I guess what you were asking is what value R is in the emitter path from
>  Q7 to +12V if there is no vernier circuitry. I built an experimental 901
>  on stripboard a couple of years ago using an SSM2220 PNP pair for the
>  mirror (ie Q7 on the osc and Q10 on the expo generator section) I
>  figured that using matched devices would give a better result; it didn't
>  work very well.
>  I will try and dig the board and my notes out and have a look if you
>  like.
>
>  BTW If you look at Cary's 901 pics there is a resistor on the extreme
>  top left corner which I think is part of the answer: I originally
>  assumed it was the 270K from the wiper of the tracking pot but I'm not
>  sure! Perhaps Cary could give us an opinion on this.
>
>  Are you still persevering with this project? Let us know how you get on;
>  for all the well-documented shortcomings of these VCOs I have never been
>  quite able to get the idea of having some out of my mind, there is
>  something very appealing about the simplicity of the UJT core.
>
>  Best
>  Pete
>
>
>  -----Original Message-----
>  From: synth-diy-bounces at dropmix.xs4all.nl
>  [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Dave Leith
>  Sent: 21 March 2009 15:00
>  To: TJ
>  Cc: SDIY
>  Subject: Re: [sdiy] schematic for Moog 901 VCO
>
>
> Hi Terry  and thanks for the response
>
>  hmmm..wish I had that module to look at :-)
>
>  just got pictures and schematics and old catalogs..
>
>  however it seems the 901A has:
>  the switched (semitones) and 0 to 2 variable voltage for tuning
>  901B has
>  switched Lo to 2 foot plus a frequency vernier of 0 to 10.
>  When they are put together as a 901 there are only the 3 controls not
>  4 (which makes sense) of  switched  (semitones), switched Lo to 2
>  foot, and 0 to 2 variable voltage for fine tuning. The additional
>  frequency vernier of 0 - 10 (dual 10K pot) is missing as the
>  oscillator is no longer part of a bank and would not need to be offset
>  from another VCOs within the bank.
>  So it is the circuitry for the 901B frequency vernier that seems to be
>  missing and small changes are made to accommodate this - well that's
>  how it looks on the remakes on the site link above. The schematics for
>  the 901A/B do not indicate how the dual 10K pot is omitted.  Maybe the
>  original 901's were wired different and just left the extra parts on
>  the board? Not quite sure how they would bypass the function of the
>  dual 10K - that's what I'm looking for.
>
>  Dave
>
>
>  On 3/21/09, TJ <daytona at verizon.net> wrote:
>  >
>  > > I'm looking for the schematic for the Moog 901 VCO - not the 901 A,
>  B
>  > > or C - I have these already in an old service manual. The 901
>  combines
>  > > them ( A B and C) and removes some of the redundant circuitry. Seems
>  a
>  > > bit rare.......
>  > >
>  > > Anyone able to help? I saw a remake of these here
>  > > http://www.moogmodular.com/9501/
>  > >
>  > >
>  >  Hey Dave,
>  >   It is difficult to see from those photos, but I have an old 1969 901
>  here.
>  > It is simply just a 901A on the one side mounted upside down, so you
>  can
>  > adjust the trimmers from the left. And a 901B on the other side, and
>  there
>  > is no 901C involved. I'm not aware of any circuit differences, other
>  than
>  > the front panel attenuators for the four waveforms? Perhaps you are
>  thinking
>  > the 921 oscillator that has additional circuitry for the auxilary
>  output
>  > section?
>  >  Terry
>  >
>
> _______________________________________________
>  Synth-diy mailing list
>  Synth-diy at dropmix.xs4all.nl
>  http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>
> checked by CanIt4
>