[sdiy] MOOG 901D schematic?

[Terry]

> Does anyone anything about it or  know where to find a schematic for  
> this module?
>

About two weeks late, but always willing to discuss old Moog modules!
I think the schematic of the 901-C may give a clue as to what might have  
been used in the 901-D.
Looking at an early Moog catalog, the descriptions of the 901-C OUTPUT  
STAGE and 901-D  VARIABLE WAVEFORM OUTPUT STAGE are nearly the same, with  
the 901-D being the super deluxe variable model with a few more functions.  
Both modules quote "In conjunction with the 901-B oscillator, this  
instrument produces two outputs of equal magnitude but opposite sign."  
Then goes on to read "Outputs are used as control voltages".  So it would  
seem the intended or designed use was for it's CV modulation output. The  
901-D goes further with  "All of the 901-B waveforms are available and may  
be mixed in any proportion. In addition, variable clipping is provided to  
shape the triangular wave." (Note: this reads "triangular" wave,and may  
reference the shape of the newly formed wave, and not neccesseraly the  
"Triangle" wave.)

Here is the 901-C schematic:
http://tinyurl.com/944erwf

A photo of the left side:
http://tinyurl.com/9jw7ek4

  And photo of the right side:
http://tinyurl.com/9kkta5c


Looking at the smaller version 901-C.  The 901-C is a 1U module with a  
3-ganged, 6-positon rotary on top with the first three selections -- Sine,  
Sawtooth, Triangular. Then in a boxed section labled Pulse, with  the last  
three  selections -- Center, Positive and Negative. Below the rotary  
selector, a potentiometer for the Amplitude of the selected waveform, and  
at the bottom two jacks labled Complementry Outputs (+/-). Looking at the  
schematic, the four waveforms from the 901, after being selected via the  
rotary switch, goes to what is an almost identical circuit as the CP3  
mixer. So looks to be boosting/doubling the 1.2 volt P-P 901 signal and  
giving both a positive and negative output .
The later CP3 module has the effect of doubling a single input, where  
unity gain is around 7 on the mixer dial with the master gain at 10. And a  
mixer input setting at 10 would double the incomming signal. Here is the  
CP-3 schematic to see the similaraties:
http://tinyurl.com/9v7fknw


   In use the simultaneous inverted outputs of a sine or triangle wave  
could be patched into two seperate VCAs for stereo panning, or the now  
available reverse sawtooth for a modulation source. But the circuitry in  
the Pulse section is the interesting part and would seem to include both a  
positive and negative wave clipper. Just two simple transistor +/-  
clippers that are mirrored on both output sections A and B.
   My guess as to why these special pulse settings are needed, is if you  
were to modulate your 901 oscillator(s) with a pulse output from another  
901, the waveform is non-symetrical and will go in both positive and  
negative directions. So the classic modulation trill sound will be  
difficult to tune as a musical interval since as the modulation amount  
increases, both sides go out of tune. This modulation effect on say, the  
later designed Minimoog, is easy to tune as you hold a C you push the mod  
wheel till you get to the interval you want, because the pulse only goes  
positive leaving the base note C in tune. Not so with the 901 and would  
require constant retuning to get a  C to F trill, and every time you  
change the interval would need retuning of the oscillator. So I would  
imagine this would be the reason there is a Center being that standard  
output. And the Positive and Negative setting, clipping the other half, to  
aid in tuning as a modulation source. Remember this was the 60's when  
these were designed and needed. Plus there was the added boost for the 901  
to modulate another signal with more than the stock 0.6 volt swing. The  
901-C module does have the 4 countersunk screws on the panel, indicating  
there is a C frame behind the panel along with a larger  circuit board.  
(Some of the simpler, smaller circuits had an L bracket that mounted  
through a 1/4" jack to hold the tiny circuit board. Such as the CP-3  
mixer, reversable attenuator.)

Seeing that Moog went through the trouble of giving me a Pulse wave with  
either the positive or negative side of the waveform on the "fixed" output  
stage 901-C. I would venture a guess that that would be the function of  
the super deluxe version of the "variable" output stage 901-D.  I'm just  
speculating here. But if the function was a pot that would blend  from   
positive clipped to center to negative clipped waveform, for each of the  
four waveforms, with variable amplitude for each. That would be a somewhat  
interesting module, both as a modulation source or an audio source.

Here is a somewhat blurry photo of the 901-D.
http://tinyurl.com/8hn2o3s


Tough to make out exactly what is labeled on that panel, but my eyes went  
10 years ago, so I have a good bit of practice reading blurry lettering.  
Looks like across the top "901-D Variable Waveform Output Stage". Below  
are four knobs each with three lines of text, with the first two lines the  
same on all four. Looks like "Variable Clipper Sine".  Then Variable  
Clipper Sawtooth, Pulse, and Triangle.  Suprisingly none of the knobs on  
these have any calibration marks, that would aid in guessing the function  
range. Below that, in the middle reads "Input Mixer" with four more  
un-calibrated knobs labled "1, 2, 3, and  4".  Further down in the middle  
left reads "Inputs" with four jacks below labled 1,2,3,4. To the right of  
that, a knob that  reads  "Master Gain". To the far right reads "Outputs"   
with two jacks below with what looks to be labled with arrows , one  up  
and one down.
  So, the bottom half of that 901-D closely resembles the later introduced  
CP-3 mixer with four inputs, four attenuators, a master gain with positive  
and negative outputs. The only thing missing in the 901-C schematic would  
be the four attenuators and resistors like in the CP-3. My guess would be  
the four variable clipped waveform outputs are normalized to the four  
switching jacks below. Giving the added option to override and introduce a  
CV from another 901 or envelope generator for more complex/interesting  
waveforms on any one of the four mixer inputs.
  The question though would be where the clipping circuits are  
implemented?. On the 901-C the pulse is clipped after it is amplified. But  
I doubt that on the 901-D there would be four seperate amp circuits before  
the clippers, then another mixer amp circuit. So might be clipped directly  
 from the 901 individual waveform outputs, then mixed amplified. Probably  
the same or similar transistor clipper circuit on the 901-C times four,  
one to each waveform. Not sure if there would be a special circuit to mix  
together the positive/negative portions of the clipped wave. Could be as  
simple as the Minimoog modulation mix knob, where the noise and osc 3 are  
on each end of a pot , with the output taken from the wiper? Any guesses  
 from the "Synth Gurus", if that would yeild a non-distorted  
sine/triangle/saw at the half-way point?  I could just breadboard my ideas  
to find out. And I'll bet those early 901-D's are on breadboards too.
  Anyhow, if my guesses are right, most of the 901-D circuit is lifted from  
the 901C, and would be a blueprint to try to make one. This could be an  
interesting module if you were able dial in the lower half of a sine wave  
with the upper half a saw with variable amplitude. Since the four 901  
waves are locked together, might be fun to assemble an odd "Triangular"  
modulation waveform that way. Or add in some other mod sources like a  
rising envelope into the mixer or a faster moving sine from another 901.  
The module was probably seen as obsolete in the later modulars. With the  
1968 model I, II, III there were several of the newly introduced CP-3  
mixers on all of them. Those many mixers were absent on the early systems.  
And the CP-3 could do most of the waveform mixing/inverting for more  
complex modulations. The only thing missing is the clipper. But the CP-3  
was more of a modular approach and could be configured many different ways  
 from audio mixing to boosting/amplifing, to modulation mixing.
Here is a photo of Bob with his hand just below one of the 8 or so made  
901-D modules. That early modular really did not have any extra mixers,  
for modulation mixing. So in an old circa 1967 system like that, the 901-D  
would be very usefull. By the time the 1970 Moog modular price list was  
issued, both the 901-C and 901-D had been dropped.
Bob and 901-D:
http://tinyurl.com/8fdn9kh