[sdiy] schematic for Moog 901 VCO

[Pete Swarbrick]

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
>
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