Some notes on building the MFOS Eurorack modules

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Some notes on building the MFOS Eurorack modules

Post by lolzdoom » Wed Jul 01, 2020 2:34 am

TLDR: I built some modules and had some obstacles, wanted to share for the next person that tries to build these. If you are thinking about building these, hopefully this helps.

This is my first post on these forums, hi everyone! Hopefully it is okay to make a thread like this. Over the memorial day weekend sale at Synthcube I purchased PCB/Faceplates for 4 MFOS Eurorack modules. During the build process I encountered some issues and I wanted to put this out there for the next person that builds these in hopes that it helps them. I bought the ADSR, Noise, MMVCF and VCO. Some of these notes may not be relevant if you buy the kit. I opted to source the parts on my own and led to some extra struggles that I probably would not have had if I opted for a kit. I have not built the VCO yet, but once I do I will come back and post any notes on that.

These are my first SMT builds ever. Before these I have built about 10 other modules that were through hole, so I am fairly new to both modular synthesis and DIY. SMT was not really that hard, but there were a few lessons learned which I mention below. I think I kind of prefer it to through hole now for resistors and caps. There are no leads to cut off!

if I have any notes on the VCO I will add it to the build thread for the VCO. The other modules do not have a build thread so that is why I made this thread.

A few notes relevant to all of the modules:

The MB1S orientation on all the modules was a bit tricky and I initially did it wrong. On the version of the MB1S I had, pin one is marked by as a plus sign. The +/- seem to be the outputs and when you check continuity they should be connected to the big 60ohm ferrite bead things and NOT the 12v rails on the power header.

Those surface mount electrolytic capacitors are a real pain. Make sure you don't use to much solder. I used to much on my first one and the pads on the bottom actually bridged together under the cap where I could not see it. I recommend checking continuity on both ends of the cap to make sure you didn't bridge them on accident.


The capacitor at C7 is another 0.1uf. This is missing on BOM, so the total 0.1uf caps you will need is 8. You should order extras of the SMT parts anyways cause if you drop one it will probably fall into a pocket dimension and will never be seen again.

At R100 there is a LED that is not mentioned in the BOM. This is a red SM LED. I bought a variety pack off amazon and it seemed to work fine.

The B1M pots are on VR16, VR17, and VR22 while the 100k is on VR18 (sustain). I couldn't find this in the BOM or schematic for the module. I had to find the original MFOS module guide which had the schematic.

For the switch I ordered the exact part number from Newark but found that the opening on the faceplate was actually to small for it. I used a stepper drill bit to widen it a few 1/16ths of an inch, no big deal. I did find there is another part number 2MS1T2B2M2RE which is the unthreaded version of the switch which may fit because it looks like its a bit smaller but I am not sure. I also got a few micro switches from Tayda (SKU: A3643) a few days after drilling the faceplate and it looks like those would have fit without modification but I already drilled so I am not entirely sure.

There are two parts on this module that I couldn't really find any information on. On the board there are two sections of 5 holes labeled "Short" and "Long". It's not obvious to me what these are for but the module seems to work without it. I couldn't find anything in the old documentation for this either. If anyone knows please post.


The 10k resistors at R5, R34, R9, R20, R14 and R35 are missing from the BOM, but they can be found on the schematic.

Calibration information is explained fairly well on the MFOS Noise Cornucopia page.

I am still unclear on the jumper configuration and what that does. I don't have a jumper right now so I can't experiment to try and figure it out yet.

This one came together smoothly. Though while calibrating I noticed that the original build guide suggests cutting one of the leads off for the 2N3904. I did not do this, but the module seems to be working now. Not sure if I should go back and this or not.


The schematic is not available under downloads on the store page, but on most of the other module store pages, there is a zip file containing all of the schematics and you will find it there.

I had a hard time tracking down the correct MFOS module that matched this one, but I think it is "MFOS VCF STATE VARIABLE 12DB SYNTH MODULE". All of the calibration information can be found on the original MFOS site for this one.

The bipolar transistors on Q1 and Q3 are not specified in the BOM and the schematic for the eurorack module does not label them either. By referencing the schematic of the original module I found that DMMT3906 goes to Q3 and DMMT3904 goes to Q1.

I did not check the connections well enough before plugging this one in and it nearly caught fire when I powered it up. Burned out one of my precious b100k pots :( after looking at everything closer I found a few suspect areas. After re-flowing those and swapping out the pots everything worked.

If anyone has any questions please let me know. Hopefully this helps someone. I will post and notes on the VCO when I finish that one, but I will probably use the official build thread when I do.

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