The following are all of the mods that I could find or create for the Moog MG-1. Many of the circuits below were just taken from existing circuits and adapted for the MG-1. Others were found on the web and were already done (if I didn't link to your original article, please let me know so I can give credit). A few I designed myself with some experimenting. I brought them all together so others didn't have to do an enormous amount of digging. I've tested most of what I've posted, but they should be tested by an experienced tech before a beginner attempts to perform these mods. I will mark each mod as verified once I receive word that it worked for another person.
I am posting this on the Moog forum and Muff Wiggler since I am a member of both.
Disclaimer: I'm not to be held responsible for any damage that may occur while you are modding your synth. I may also not be around to help troubleshoot any issues you may have during the modification process. As always, make sure that your synth is unplugged from any power source before attempting to perform any mods. You should also completely remove the foam from your MG-1 before any mods. I would also recommend replacing electrolytic caps as well.
You should read through and become familiar with the service manual before opening your synth. The MG-1 manual is very detailed. If there are any preexisting issues with your MG-1, they should be fixed before you modify. The service manual has many troubleshooting tips. It can be located here: http://manuals.fdiskc.com/flat/Realisti ... Manual.pdf
You can find the user manual here: http://www.elektrophonik.org/docs/Reali ... manual.pdf
You can find stripboard from Mouser here: http://www.mouser.com/ProductDetail/Bus ... ZKZs8Yw%3d
All of the images below are resized in the forum. To view the full size, right click and select View Image.
Replace wrong resistors back to capacitors according to schematic
It turns out these are axial capacitors that look very similar to metal film resistors. No need to replace them.
Minimoog Filter Mod
Moog engineers set out to improve upon the ladder filter in the Model D. It resulted in something that didn't quite have the right musical result for Moog fans. The following is from a review of The Source - http://www.retrosynth.com/~analoguedieh ... og_source/
In an attempt to correct this without completely redesigning the op-amp based regeneration path, you can simply bring the ladder filter cutoff caps/resistors, the coupling caps and input impedance back to model D specs. You won’t quite get the growl that comes from transistor clipping in the Minimoog, but you’ll get closer to the general sound characteristics of the ladder filter that seem to be a favorite. I've been told that replacing resistors R78 through R81 and caps C16 through C18 won't make that much of a difference. However, I thought it made a difference, so I included it. The biggest difference was made by replacing the input caps (C19 & C20) and lowering the input impedance (R88 & R89)."Nonlinear" means the resonance on a Minimoog filter does not stay constant across the full frequency cutoff range - it disappears as the filter cutoff falls to low frequencies. So when a "spoinky" bass is dialed up on a Minimoog, the filter resonance disappears as the EQ sweeps the filter toward low cutoff - the net effect is the bass sound is full sounding.
Moog subsequently viewed this nonlinear resonance as an engineering "fault" and corrected it in new designs - including the Source. So the resonance on the Source filter stays constant across its full cutoff frequency range and the engineers were happy. But this was also why later Moog instruments also didn't sound like a Minimoog and the customers were not happy. Moog went through a lot of head scratching to find out why and never realized that their "correction" was the source of the difference.
In order to do this, you will need to replace R78, R79, R80 and R81 with 150 ohm resistors; C15, C16, C17 and C18 with .068uF capacitors; C19 and C20 with .22uF capacitors; and R88 and R89 with 47k resistors.
You will need the following parts:
4 x 150 ohm resistors: http://www.mouser.com/ProductDetail/Xic ... NrtAXYk%3d
2 x 47k resistors: http://www.mouser.com/ProductDetail/Xic ... Z2C%2fw%3d
4 x .068uF capacitors: http://www.mouser.com/ProductDetail/Pan ... %252b5o%3d
2 x .22uF capacitors: http://www.mouser.com/ProductDetail/Pan ... Gvpd9kE%3d
Add continuous glide and re-trigger circuit to keyboard tracking
Designed by Ricard Wolf - http://machines.hyperreal.org/manufactu ... ig.mod.txt
Here is an updated schematic:The MG-1, as all other monophonic Moogs (at least the fully analog ones) lack multitrig, meaning that if you hold a low note and play a higher one the envelope won't retrig. (Note: most Moogs are low-note priority - the MG-1 is an exception to this rule.) This can be awkward when playing fast, and especially on the MG-1, with its, er, limited envelope generator, where you don't have a separate release phase to smooth things out when you're using the envelope generator in AD mode.
So, on my unit, I've added a retrigger circuit. The circuit was originally snitched from the E&MM Spectrum synthesizer (I think) and modified for the MG-1. I've used it successfully in another monophonic I built a long time ago as well.
As an added bonus, the multitrig mod adds 'continuous glide', i.e. if you play a note (with glide on) and release it before the glide has completed, the glide will continue after the note has been released. This is contrary to the original MG-1 function, where the glide stops when you release the note.
Although this might be useful in some odd cases, often it's just irritating when you take your finger of the key to quickly and the note remains at some odd point along the way.
The top half of the circuit implements the continuous glide function, but more importantly provides fast voltage changes to the differentiator/rectifier in the bottom half even when the glide control is advanced.
This part of the circuit is simply a voltage follower in connection with a large capacitor to hold the CV, just like the original circuit shown.
The bottom half can be subdivided into two parts. The left part (around the TL072) is a differentiator which converts the changes in CV to large voltage spikes. The rest of the circuit is basically a rectifier, which rectifies the spikes (which occur when the CV changes), so that both ascending and descending CV's will cause a retrigger, and performs a logical AND function with the gate signal, in that if the gate is active and the CV changes, the result will be a break in the gate signal, in effect causing a retrigger. If the gate signal is inactive, it is not affected.
The output is fed on to switch S7, the auto contour switch, which selects between the keyboard gate signal and LFO square wave (for repeat).
I’ve used this schematic to design a strip-board layout for any of you that would rather not piggyback the individual components on the pcb. I will be uploading an image of the strip-board layout shortly. Right now it is a pretty rough drawing on some paper.
As you can see, I took R61 off of the main PCB and placed it into the strip-board circuit. This is so you can just remove R61 instead of cutting any traces on the PCB. Simply wire the Keyboard In to the top half of the pad from R61 and the Glide Pot Out to the bottom.
Instead of cutting the trace from the Keyboard Gate to the Auto Contour Trigger, I simply removed the white wire connected to #9 on the pcb, ran this to the Keyboard Gate In of the circuit, then connected the Auto Contour Out back to #9.
You will need the following parts:
2 x 1K: http://www.mouser.com/ProductDetail/Xic ... V8Ne6F0%3d
2 x 3.3K: http://www.mouser.com/ProductDetail/Xic ... px2qOkY%3d
1 x 47K: http://www.mouser.com/ProductDetail/Xic ... Z2C%2fw%3d
2 x 100K: http://www.mouser.com/ProductDetail/Xic ... i6KEHFY%3d
1 x 1M: http://www.mouser.com/ProductDetail/Xic ... HFAsQsc%3d
1 x .068uF: http://www.mouser.com/ProductDetail/Pan ... %252b5o%3d
1 x 1uF (use film cap): http://www.mouser.com/ProductDetail/Pan ... 2fYA%3d%3d
2 x 1N4148: http://www.mouser.com/ProductDetail/Fai ... bu6z6oTQTL
1 x TL072: http://www.mouser.com/ProductDetail/Tex ... 2xjDiRo%3d
1 x LM741: http://www.mouser.com/ProductDetail/Tex ... HO7fKRQ%3d
Replace 5837 Noise chip with analog noise generator (strip-board layout provided)
The MG-1 uses a 5837 chip to simulate white noise. This was an early chip and only has a few seconds of white noise before it repeats the same pattern. This can be especially noticeable in S&H. The following schematic shows the original circuit on the left and the modded circuit on the right.
Start by de-soldering and removing U16 (5837 chip). Since this chip is pretty high volume, you’ll need to compensate by removing R160 and replacing it with a 2.7k resistor. This will increase the gain of U17B to allow a similar output level to trigger the S&H circuit.
The following is a strip board layout of an analog noise generator circuit:
Note: I included a 100uF cap for power supply filtering, but didn't end up using it. If you don't want to use it, just remove C2 and the jumper that goes from +12V to the + end of C2.
This circuit is more susceptible to power supply noise. Because of this, the +12V input should be connected directly to the bottom PCB on the positive side of C14 (10uF/25V). I had originally connected the +12V straight to the old pad for pin 4 of U16, but this resulted in a clicking sound that was generated along with the LFO. (Thanks to Varice on electro-music.com for the help on this.)
The output can connect directly to the old pad for pin 3 of U16.
The ground should connect to the same ground as R160. The picture below shows where this is:
You will need the following parts:
1K: http://www.mouser.com/ProductDetail/Xic ... V8Ne6F0%3d
4.7K: http://www.mouser.com/ProductDetail/Xic ... OZq4lN4%3d
680K: http://www.mouser.com/ProductDetail/Xic ... Sp3aD20%3d
.1uF: http://www.mouser.com/ProductDetail/Pan ... ndmQqZw%3d
10uF: http://www.mouser.com/ProductDetail/Nic ... 1k%2fHY%3d
2 x 2N3904: http://www.mouser.com/ProductDetail/Cen ... GOknJzA%3d
Tune Osc 1 & 2 one octave lower
I should note that if you don't mind drilling into your synth, adding a switch to lower the octave of your synth will be better than internally lowering it. The reason is that internally lowering it will leave you with only the lower three octaves. Using the switch method will allow you to switch between lower and the stock higher. Giving you an octave higher than this mod.
This is a simple mod that comes from: http://www.sdiy.org/philgallo/MGvcofzadj.htm
Since 70k resistors are hard to find, you can get 2 x 69.8k resistors.Procedure
Unplug the synth and open up the case
Remove the top circuit board – the one with all the panel controls.
Locate R2. You should find it at the top of the board, close to the VCO trim pots.
- For verification, follow the traces from the resistor. One of them should take
you to the center solder pad of R1 (the VCO1 FREQ TRIM pot).
De-solder and remove the resistor
Solder the new resistor in place.
Repeat for R28, which can be located toward the bottom edge of the board.
- If you trace this out, you’ll see it goes to the center pad on the VCO2 FREQ
TRIM pot (R26).
Re-assemble the synth, and check it for proper operation.
Set the front panel TONE SOURCE tuning control to center.
Using your audio files and tuner, adjust the tuning of the VCOs to the desired interval.
Tuning and Scaling
When sitting at the synth in playing position, the far left hole (as mentioned above) will give you access to VCO2 tuning, and the far right will be VCO1 tuning. This is where the small screwdriver comes in handy. You will be able to tune about 2 octaves down if you wish, but that’s about as low as you can go and still get usable tones. Any lower and it begins to turn into clicks and pops as it goes sub audible. You can also tune both VCOs to the same range, instead of having VCO2 an octave higher. I did that on my machine, and I rather like it
You may find that you need to adjust the scaling as well. To check, play a low note on the keyboard and verify that it is tuned correctly. Then play the same note on the high end of the keyboard. If the tuning on the high note is slightly off, adjust your scaling with the following procedure. You may want to set the MG-1 to “continuous” while you do this.
The SCALE trimmer for VCO2 will be the 2nd hole from the left. For VCO1, you will use the 2nd hole from the right.
Play a low note on the keyboard, and verify it is properly tuned.
Play the same note 2 octaves higher.
Adjust the SCALE trimmer for the VCO you’re working with until the high note is properly tuned
Play the original note again, and re-tune it using the FREQ TRIM pot for that VCO
Repeat until both low and high notes are properly tuned.
2 x 68.9k resistors: http://www.mouser.com/ProductDetail/Xic ... S7uA%3d%3d
Osc 2 Detune slide pot to full octave up and down
The oscillator 2 Detune slider does not reach a full octave up or down. This is a bit frustrating because the ability to do this adds a lot more functionality to the synth.
R27 is responsible for the Detune sliders effect on oscillator 2. You simply need to lower the value until you get a full octave up or down. For me, it was 66k. I suspect that it will be similar for most, but because of part variation, you should hook up a pot to be sure. You can temporarily wire a pot as a variable resistor, or install a trimpot in this spot and connect wiper 2 to one of the outer wipers.
After replacing R27 with the lower value, you will need to adjust the Scale Trim Pot and may have to adjust the Freq Trim Pot as well. Instructions for this are found on page 17 of the MG-1 Service Manual.
Note: If you plan to add the Detune Modulation Pot along with this mod, you should do both at the same time. One mod may affect the other.
Increase audio output level
This may be the easiest mod on the list. Moog included a 1K resistor in parallel with the 10K master volume pot. This basically turns the pot into a 1k pot and decreases the overall volume. The mod is as easy as taking R102 out of the circuit. You can de-solder it, you can clip one end, or you can clip both ends.
Change Ring Mod input from Osc 1 square only to post-switch/pre-mixer Osc 1
The Ring Modulator (Bell Tone) on the MG-1 consists of the square wave from oscillator 1 into the input and the pulse/square from oscillator 2 into the carrier. It does not change if you change the wave shape to saw. It’s very simple change the input (or the carrier if you want).
The Ring Mod input wire is the orange wire connected to P5. Shortly after this connection is a 100k resistor (R24). After the resistor the signal goes to the square wave. Fortunately, located right below R24 on the PCB is a jumper wire that connects the signal from oscillator 1 (post switch ) to the mixer slide pot.
When looking at the top of the top PCB, locate R24. De-solder the left side of the resistor (the side closes to P17. Bend the resistor down and solder the left side to the jumper located directly below it. And that’s it!
ADDITIONAL FUNCTIONALITY MODS
Add Sustain pot
The Envelope on the MG-1 only has Attack and Decay controls. The Sustain is controlled by the by the Sustain switch. The switch gives you the options of Sustain fully up or fully down. Adding a Sustain pot increases the usability of this synth tremendously.
To do this, we need to modify the existing circuitry. R136 and R137 control the sustain when the switch is set to on. We’ll need to remove this and add a circuit that will allow us to modify the sustain with a potentiometer. The picture below shows the original circuit next to the modified circuit.
Instead of using a strip-board for this circuit, I just chose to add the components individually to the pcb. Start by removing R136 and replacing it with a 10K resistor. Next, remove R137 and solder in the emitter and collector according to the schematic (emitter connected to the 100k and collector to ground). Leave the transistor high enough to solder more components to it.
Take the base of the transistor and pull it outwards. You’ll need to solder a 10k resistor to from the base to ground. You’ll also need to solder a 56k resistor from the base to a -15V point. A convenient -15V point is to the jumper that is located in between R125 and R111. It is directly to the left of the Filter Cutoff pot and to the right of the filter tracking switch.
Next you’ll want to wire the pot. Wire wiper 1 to the +12V that R136 connects to (you can use the jumper directly to the right of R122 if you want). Solder a 10k resistor to wiper 2 of the pot, then solder a wire on the other end that connects to the transistors base. Wiper 3 needs to go to ground. You can use the collector of the transistor again for this.
There are two things you will need to know about this circuit. 1) The sustain switch needs to be set to on for it to work. 2) The new sustain pot goes above the level that a normal sustain will go. I chose to leave it this way because I liked the results. With the sustain all the way up, when you play fast, the envelope level will begin to rise higher and higher until it reaches its limit. This gives a nice crescendo effect when you start playing more intricate parts. To get normal sustain, simply back the knob off a little. You can see where the max normally is by holding a note and flipping from Contoured to Continuous.
You will need the following parts:
4.32k: http://www.mouser.com/ProductDetail/Xic ... PQxxW3g%3d
10k: http://www.mouser.com/ProductDetail/Xic ... KR8aMJE%3d
56k: http://www.mouser.com/ProductDetail/Xic ... H2uxbDE%3d
100k: http://www.mouser.com/ProductDetail/Xic ... i6KEHFY%3d
2N3906: http://www.mouser.com/ProductDetail/Cen ... JlsiBMQ%3d
10K Linear Pot
Osc 1 and 2 PWM controls
Oscillator 1 already has an internal PWM pot. It is the Square Trim pot (R16). Simply remove R16 and replace it with a 10k potentiometer. When viewing the trim pot pads from above, wiper 1 needs to go to the top right pad, wiper 2 goes to the bottom pad, and wiper 3 goes to the left pad.
Oscillator 2 has a preset pulse wave. This preset is determined by R46 and R47. In order to mod this circuit to have an external PWM pot, you'll need to make it the same as Osc 1. Start by removing R46 and replacing it with a 10k resistor. Next remove R47 and replace that with a 10K resistor. Next you'll need to wire the pot. Wire wiper 1 to the +12V side of R46 (left side of the resistor). Wire a 33K resistor to wiper 2, then a wire from the 33k resistor to the other side of R46 (right side of the resistor). Wire wiper 3 to the ground side of R47 (left side of the resistor). And that's it! Now you have two external PWM controls.
You will need the following parts:
2 x 10K: http://www.mouser.com/ProductDetail/Xic ... KR8aMJE%3d
33K: http://www.mouser.com/ProductDetail/Xic ... cq38eKQ%3d
2 x 10K Linear Pots
Osc 2 Detune/pitch modulation pot.
This is one of my favorite mods. You can use it to replicate a subtle drift, chorusing effect, or an auto-wah like effect with the sync on. You'll need a 200k pot and a 4.32k resistor. Keep in mind that you will need to adjust the scale of osc 2 according to the manual after this mod is done. You may also have to adjust the frequency.
You are going to tap into the LFO signal that is sent to the filter and tone sources slide pots. The LFO signal runs from pins 1 and 2 of U15 to wipers 1 of each slide pot (R70 and R105). When viewing the PCB from behind, these are the top right pads for the filter and tone sources modulation slide pots. I will take some pictures the next time I open the synth.
Wire wiper 1 of the 100k pot to one of these pads on R70 or R105. Wire the 4.32k resistor to wiper 2. Then wire the other end of the 4.32k resistor to the side of R27 that connects to R29 and U4 pin 2. When viewing the PCB from above, R27 is to the left of the Detune slide pot. You need to connect the wire from the 4.32k resistor to the bottom of the resistor. Wiper 3 of the pot will connect to ground (I used P4).
Note: If you plan to add the Octave Detune mod along with this mod, you should do both at the same time. One mod may affect the other.
You will need the following parts:
4.32K: http://www.mouser.com/ProductDetail/Xic ... PQxxW3g%3d
200K linear pot
LFO speed switch (fast, normal, slow)
The details for this mod can be found here: http://www.sdiy.org/philgallo/MG-1_lfo_rate_mod.txt
This document contains instructions for a modification to the Concertmate MG-
1 synth. This mod gives the user a choice of 3 speed ranges for the LFO rate:
normal, 1/2 speed, and 1/4 speed (approximately). It requires soldering and
drilling a hole to mount a switch.
I don't have any way to make schematics or take pictures, but this is a
pretty easy mod and I don't think you'll need anything like that.
Go to Radio Shack (or your parts supplier of choice) and get the following:
spdt center off toggle switch
10uf capacitor (polarized, rated at least 16v)
22uf capacitor (ditto)
wire (if you don't have any just lying around)
The size of the capacitors can vary depending on how much you want to slow
the LFO down. I chose the 10uf and 22uf caps because they were the closest
readily available values for what I wanted to do. The value of the original
cap in the MG is 6.8uf. When the 10uf is added to the circuit in parallel,
the total capacitance is 16.8uf, which reduces the speed to 41% of normal.
When the 22uf is added, the total is 28.8uf, which gives a speed of 24% of
- UNPLUG THE SYNTH!!
- Open up the MG and remove the top circuit board (where all the
sliders and knobs are located).
- Locate capacitor C25 on the board you just removed. It is located a
bit to the left of the Auto Contour Trigger switch. This is the
discharge cap for the LFO circuit.
- Solder a wire to the negative (-) end of C25. Solder the other end
of this wire to the center contact on the spdt switch. Make sure
you have enough wire to reach your intended mounting location
for the switch! Insulate the connection with electrical tape or
heat shrink tubing.
- Solder the negative(-) end of one of the capacitors to either of
the open contacts on the switch.
- Solder the negative(-) end of the second cap to the remaining open
contact. Insulate the connections with electrical tape or
heat shrink tubing.
- Solder a wire to the positive(+) side of C25. Solder the other end
of this wire to the positive lead on BOTH caps at the switch.
Insulate the connections with electrical tape or heat shrink
That's all the connections that need to be made. Check for shorts and bad
connections and put the top circuit board back into place. Then give the
synth a test run to make sure the mod works as it should.
The final step is to mount the switch. The location is really a matter of
personal preference. I was going to put my switch right next to the rate
control slider, but I wasn't sure if there was enough room there to install
the switch without having it touch the circuit board. I ended up mounting it
on the flat ledge to the left side of the keyboard. There's plenty of
clearance underneath, and it's not used for anything else, so it made an
ideal location. The switch is out of the way, but easily accessible. Just use
a low speed drill so you don't burn the plastic.
Re-assemble your MG-1, and bwoooooooOOOOOOOOOOOOoooooooooowwwwwwww....
Internal or External switchable Saw to Triangle to Sine convertor
This is what I'd consider a pretty advanced mod. Please make sure you can successfully complete some of the other mods before attempting this. I haven't completed this mod yet, so consider it completely unverified.
The MG-1 offers sawtooth and square waves. If you'd like to add additional waveforms to your VCOs, you can use a convertor to complete this. You will need an oscilloscope if you want to get the waveforms properly calibrated. You can use your ears if you'd like, but don't expect the waveforms to be precise. This is per oscillator, so you will need two if you want this on both VCOs. I left the connections open so you can wire it however you want. You can use a switch to bypass (giving sawtooth), to only use the triangle or to use the triangle into the sine. Instead of using a switch, you can use switching jacks to select the waveform you want. It can be an external or internal effect.
The first half of the circuit takes a sawtooth and converts it to a triangle. The second half takes a triangle and converts it to a sine. I don't know what would happen if you connect the sawtooth directly to the sine convertor.
Note that we are going to use +/-12V. Not +12V and -15V. The MG-1 PSU does send -12V to a few select areas. I haven't traced them down yet.
Here is the schematic:
Here is the strip-board layout I created for it:
You will need the following parts:
390: http://www.mouser.com/ProductDetail/Xic ... Qo0f0lk%3d
2 x 2.2K: http://www.mouser.com/ProductDetail/Xic ... rMdgjFY%3d
9 x 10K: http://www.mouser.com/ProductDetail/Xic ... KR8aMJE%3d
13K: http://www.mouser.com/ProductDetail/Xic ... cURXCVc%3d
18K: http://www.mouser.com/ProductDetail/Xic ... LrpjIso%3d
20K: http://www.mouser.com/ProductDetail/Xic ... 9h1xixQ%3d
100K: http://www.mouser.com/ProductDetail/Xic ... i6KEHFY%3d
2 x 15pF: http://www.mouser.com/ProductDetail/AVX ... %2fUqSw%3d
2 x 1N914: http://www.mouser.com/ProductDetail/Fai ... H0FF7uM%3d
TL074: http://www.mouser.com/ProductDetail/Tex ... MTuUhxw%3d
2 x 2N3904: http://www.mouser.com/ProductDetail/Cen ... GOknJzA%3d
Increase filter modulation level – Coming Soon
I haven't experimented with this yet, but R106 should determine how much the LFO affects the filter modulation. You should be able to lower this from 36K to find what value works best for you. I have a feeling that Radio Shack was trying to keep it safe on this one.
Poly pitch modulation pot - Coming Soon
I haven't experimented with this yet, but R70 should determine how much the LFO affects the filter modulation. You should be able to lower this from 200K to find what value works best for you.
FM pot - Coming Soon
I haven't experimented with this either. I believe you should be able to use a 50k pot for this by connecting wiper 1 to Osc 1 or 2 just before the mixer slide pot, wiper 2 to to a point between R107 and R108 (Filter Slide Pot), wiper 3 to ground.
VCA modulation pot - Coming Soon
Once again, haven't finished yet. You should be able to tap the LFO from pins 1 and 2 of U15(A) and insert them before or after R104. Additional circuitry will probably need to be added to change the LFO single to +6V to 0V.
Osc 1 octave switch - Coming Soon
R4 and R5 are matched 1M resistors. These control the Octave switch. When it is at -2, the switch is connecting 2M, when at -1 it is connecting 1M, when at 0, it is bypassing these resistors. Since I decided to tune the entire synth down using the Lower Octave Mod, I haven't used this approach yet. If you're okay with drilling a hole for your switch, it makes more sense to add this mod than to just tune the entire thing an octave lower. This way, you don't lose the highest octaves on the synth. I'll include a wiring diagram as soon as I can. Basically, you'll need to add two more matched 1M resistors in series of R4 and R5 by using a switch. So, one position would add them, the other position would leave the normal stock values.
Ring Mod carrier from LFO (instead of Osc 2 Square) switch - Coming Soon
Internal or External VCA ASDR (strip-board layout provided) - Coming Soon
Separate headphone and master volume pots - Coming Soon
BACK PANEL CONNECTIONS
Note: I haven't completed all of these yet because I don't have a modular setup. Many work for me, but I have only tested them with my Werkstatt and Moogerfoogers. There will probably need to be buffers and diodes added to get the signals correct. I am hoping from some input on this from more experienced people. I will add pictures of the connection points later.
First of all, you should remove the RCA circuit from the back of the MG-1. This was a Radio Shack design idea. Everything they carried used RCA jacks. The RCA jacks simply included an audio output and a useless audio input. The audio input is there so you can play along to music without needed additional adapters to connect it to your stereo. The original audio out uses a 47K resistor in series to the jack and a 1K resistor from the audio to ground. You'll want to keep these values if they already work for you.
The ground wires in the MG-1 are all white and green. You'll want to make note of this in order to connect all of the grounds together and two the MG-1 circuit on your rear panel jacks.
I've already discussed the audio out above. The audio signal comes from P25 on the top PCB, has a 1k resistor to ground, then travels through a 47k resistor, then to the tip of your 1/4" jack. The sleeve connects to ground. I haven't personally messed with these values. The stock values work for me and provide plenty of signal strength to send to my amplifiers, DI and directly to my preamps. You'll need to keep these in mind if you're installing a 1/4" jack.
Pre-Filter Audio In
For this you'll want to find the negative side of C14. You'll simply want to solder a wire to -C14, place a 10K resistor in series, then to the tip of your Filter Audio In jack. Sleeve goes to ground.
Noise In/Out + S&H In
The Noise Output comes from the junction of pin 7 of U17(b)/R162/R161 and the hot side of R85 (noise slider). Simply wire one of these points to the tip of a 1/4" jack. Sleeve goes to ground. Use a switching jack if you want to disable the internal noise generator when connecting an external. This is also where the S&H input is located. I will include some different wiring diagrams over time.
Osc1 - Pre-mixer
Since you may want to manipulate the saw waveform independently, I've included a in/out for the sawtooth before the switch. Simply solder a wire to one of the points that connect together R20, R14 and pins 6 and 7 of U2(B). Solder the other end to the tip of your 1/4" jack. Sleeve to ground. I will include a wiring diagram for a basic jack or an in/out switching jack shortly.
Osc1 Post-Switch In/Out:
It will be useful to have a direct oscillator output. For oscillator 1 you will connect to the hot wiper of R51 (Tone 1 mixer slide pot). This goes to the tip of your 1/4" jack. Sleeve to ground. I will include a wiring diagram for a basic jack or an in/out switching jack shortly.
Osc2 - Pre-mixer
Since you may want to manipulate the saw waveform independently, I've included a in/out for the sawtooth before the switch. Simply solder a wire to one of the points that connect together R42, R44 and pins 1 and 2 of U5(A). Solder the other end to the tip of your 1/4" jack. Sleeve to ground. I will include a wiring diagram for a basic jack or an in/out switching jack shortly.
Osc1 Post-Switch In/Out:
It will be useful to have a direct oscillator output. For oscillator 1 you will connect to the hot wiper of R60 (Tone 2 mixer slide pot). This goes to the tip of your 1/4" jack. Sleeve to ground. I will include a wiring diagram for a basic jack or an in/out switching jack shortly.
Poly Pre-mixer In/Out
You'll probably be noticing a trend by now. The Poly output will connect to the hot wiper of R54 (Poly mix slide pot). Solder a wire from that to the tip of your 1/4" jack. Sleeve to ground. Sleeve to ground. I will include a wiring diagram for a basic jack or an in/out switching jack shortly.
Ring Mod Audio In
We will consider the connection that normally comes from oscillator one as the audio in. This connects to oscillator 1 through a 100k (R24) resistor into the orange wire that goes to P18. I haven't experimented with the difference between connecting before or after the 100k resistor. I would imagine that this input would certainly want to be a switching jack that takes oscillator 1 out of the circuit when a new connection is made. I will include a wiring diagram for this jack shortly.
Ring Mod Carrier In
This is the exact same thing as the Audio In, but it comes from oscillator 2 instead. The 100K resistor is R50 and the wire is the violet wire that connects to P14 on the PCB. You'll also want to wire a switching jack.
Post Filter/Pre-VCA In/Out - Coming soon. I wanted a way to add some external overdrive or processing into the filter's regeneration circuit and into the VCA. I'm not sure how useful this will be.
The VCA CV input is between R104 and the Tone Sources Contour switch. I connected the tip of my 1/4" jack directly to R104 (15K) and did not have any issues using a CP-251. Used the CP-251's LFO through the mixer (boosted all the way up) to create a tremolo effect. However, there may need to be a diode added for protection. I'm hoping for somebody more experienced with modular gear to chime in on these.
The VCF's CV input should be located between R107 and the center wiper of R108 (Cutoff Slide pot). Once again, there may need to be a buffer or circuit protection added here.
Envelope - Coming Soon
V-Trigger/Gate - Coming Soon
LFO In/Out - Coming Soon
Sync In - Coming Soon