The Harold Bode schematic here:
The basic modulator circuit is all that would be needed in a modular system when processing Moog oscillators. So I set out to build just the basic modulator. The transformers I used are Edcore audio type 10K/10K with 20-20,000 Hz frequency response. First step was making the frame and mounting the transformers.
Here is the assembled Moog modular frame.
Using the same type Vectorbord that Bob Moog used on modules, I first had to insert the Vector T-28 pins for the component layout. Here is the begining layout copied over from a component drawing. Can see the Vector P-91 tool for inserting pins. A small Vector note from the 60's reads this special tool can be purchased for 50 cents.
For this circuit I only needed a CP3 type amplifier after the ring modulator to bring the output level back to the same level as the input. Since the ring circuit is a passive one the output is reduced to about half the input. There was extra room in this module frame, so I added two additional seperate Moog type reversable attenuator circuits.
Once all the pins are placed in the Vectorbord, soldering in the components is the next step. The board is removed and placed in a Panavise holder for soldering/assembly.
The two reversable attenuator circuits on top and ring amplifier on bottom.
Assembly is easy as component placement and soldering is all done on the same side. No flipping the board over to solder like through hole construction. Never really liked that with a through hole board, and I find these are more enjoyable to put together. Once the component side is done the back point to point wiring is done. There is a color code where all red is +12 volts, blue is -6 volts and green is ground.
Back after soldering all the connections.
Once the board is finished it can be set back in the frame and transformers wired in. Next the front panel needs to be wired/connected.
Keeping this module R.A.Moog correct, the front panel parts used are mostly surplus NOS. Moog used these serrated washers behind each Switchcraft 112. This was probably for ground connection and helps keep the front threaded sleeve flush with the mounting nut. Mine are black oxide finish, but the Moog ones were plated. Moog also used military spec. Allen-Bradley pots and small Alco toggle switches.
Front panel hardware.
For the reversable attenuators, I ended up selecting 10K linear pots with a center tap. Was not able to source the 25K type, and many early modules only used the standard 25K linear pot without a center tap. After testing, the 25K pots worked with a very precise center. However ther was a slight offset at the zero setting, noticable when using oscillators. There is no way to trim this out and just part of the circuit design using a pan pot between +/- sides. Trying the 10K center tapped pot worked much better as there was a ground at zero and a larger dead band. The 10K pots were from old EML stock and used in a few EML designs.
Next I have to label the front panel. Perhaps in the future I will get a proper etched panel from Re:Synthesis. But for now using a Dotcom drilled blank and dry transfers for the panel graphics.
Transfering the lettering here.
This type lettering turns out OK and is DIY.
Hardware can now be mounted on the panel for final wiring.
Circuit side of module all wired up. Top two circuits are the reversable attenuator, the mixer/amplifier below that and the germanium diodes at the bottom. All discrete circuits, very 1969!
Another view of the component side.
And a view of the back of the circuit. You can see it was tricky to mount those transformers and have room for the circuitboard and support brackets.
Mounted next to the 3U wide 901 oscillator. Looks like it belongs there, and it sounds as good as it looks.
So what does it sound like?
As one can imagine, something magical happens to a signal when you pass it through all that wire and iron in those coils. What comes out when two 901 oscillators are ring modulated together is difficult to put into words. While ring modulators are usually associated with bell like tones, and this can do that fine. The sound when the two sources are closely tuned together at different intervals resembles a chorus effect. It sounds bad ass! Interestingly, it gets that almost Minimoog like quality to lead tones I could never get on the modular.
Here is a youtube demo I put together. A little longer than I usually do a demo at just under 10 minutes. Begins with two 901's sine waves then I begin to patch in the different waveforms. I will edit the video later to include captions of what I'm patching. But here is the link.
R.A.Moog Modular old school ring modulator demo.
Really has a unique tone and sounds awesome!
