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Some notes on building the MFOS Eurorack modules

Posted: Wed Jul 01, 2020 2:34 am
by lolzdoom
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.

MFOS EURO ADSR SMT Notes:

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.

EDIT: I thought this was working because the gate button on the front triggered it it properly. I later found when I plugged a keyboard into the gate/trigg inputs, that the module was not working. Went through the BOM and the schematic and found yet another mistake. The resistor at R7 is a 220r on the BOM, but the schematic shows a 20k. Plus the resistor at at R6 shows a 220r on the BOM but is a 100k on the schematic. Replacing these components to match the schematic fixed the issue. Seems that 220r resistors should not be on the BOM at all.

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.

MFOS EURO NOISE SMT Notes:

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.

EDIT: Mostly works, but doesn't calibrate

MFOS EURO MULTIMODE VCF SMT:

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.

Re: Some notes on building the MFOS Eurorack modules

Posted: Sun Aug 02, 2020 1:34 pm
by EATyourGUITAR
where are the build docs for MFOS noise sold at synth cube. I got nothing in the mail for docs. no email for docs. nothing on the synthcube website. nothing on MFOS official website. the other MFOS website is down. the PCB has no component values. anyone?

EDIT: I found it on the synthcube website.

Re: Some notes on building the MFOS Eurorack modules

Posted: Tue Aug 04, 2020 7:52 pm
by EATyourGUITAR
the new synthcube MFOS noise has an error in the BOM line 19 the unit of measure is mOhm but the part listed is MOhm. I can remind you that 1000000 mOhms = 1 MOhms. If I ever get my mouser cart finished I will share it here. I believe the voltage or the values may be wrong @ 50v for all ceramic capacitors. I am using 100v to prevent capacitance dropping with DC bias. I haven't spent much time on the schematic but the price difference was very small.

Re: Some notes on building the MFOS Eurorack modules

Posted: Sat Aug 08, 2020 12:03 pm
by EATyourGUITAR
the new synthcube MFOS noise has an error in the BOM line 33 has the qty is in the value data field and the qty field shows 1 which I'm guessing should be 3 from the value field. arguably every value marked with 1 with a unit of measure "each" is an error. each is not a unit of measure. "each" is an english word that describes how to apply a given unit of measure to an atomic object. it would be better to have N/A in the unit column. N/A in the value column if you firmly believe that it is not relevant to the purpose of this document to put some value or rating about the part. my concern is that by putting "1 each" you are creating a redundant column that is easily both conflated and conflicted by the qty column. it should be easy for beginners to shop and build without any previous experience.

Re: Some notes on building the MFOS Eurorack modules

Posted: Sat Aug 08, 2020 12:17 pm
by jwm
i also tried to build an MFOS noise from synthcube and yeah, i had all manner of problems. i've done plenty of smt and am confident it wasn't my skill. i def had parts missing, had no guide, etc. eventually i got the random gate to work, but that was it.

took me a while to realize what the jumpers do, but it basically just changes the range of the random gate speed/occurrence...but i had to figure this out by digging through a bunch of old MFOS through hole stuff, and yeah i dunno...i think synthcube did a pretty bad job at giving someone what they need to really get this stuff done right and i'm not inclined to screw with anything else in the future.. and i have build experience, so i can imagine that someone who is just starting might not even get as far as i did if they didn't know where to look.

Re: Some notes on building the MFOS Eurorack modules

Posted: Sun Aug 09, 2020 8:54 am
by EATyourGUITAR
I have experience so I can work around it. what I find slightly concerning is that I spent money on this thinking it was a finished project that I could build in a day after clicking some mouser cart or something. for the price I paid for this PCB, I think I overpaid. for something that was poorly designed and many mistakes in the docs and PCB, I would have given them $5. this kind of PCB design and project management is what any 18 year old punk on muffs can do. synth cube can do better with quality control. they stocked at least 100 units of what is essentially a non-working project as-is. still selling it too.

look at the bottom left where the power caps are. how am I expected to get my soldering iron in there with the caps so close together? you can't. you can only use paste. so without stencils you expect me to apply little dots of paste to 200 pads. in the end I went with half paste and half soldered by hand. I have to run it through the oven then finish it. so much time wasted to setup every machine I own and work on someone else's project without getting paid after I paid them a little too much.

P.S. Synth cube I still love you. would buy again but not MFOS.

Re: Some notes on building the MFOS Eurorack modules

Posted: Mon Aug 10, 2020 2:01 am
by Bodo1967
EATyourGUITAR wrote:
Tue Aug 04, 2020 7:52 pm
the unit of measure is mOhm but the part listed is MOhm. I can remind you that 1000000 mOhms = 1 MOhms.
I've read this a few times now but still can't quite follow :hmm: .

"mOhm" would refer to MilliOhm, which is 1/1000 of an Ohm. So 1,000,000 mOhm would be 1 kOhm, not 1 MOhm.

Anyway, I believe this to be a typo (m vs. M) in the BOM. At least In practical use I've never seen mOhm before in practical use. Anything below the 1 digit Ohm range is extremely rare; I don't even think I have any resistors below 1 Ohm in my collection :D.

Re: Some notes on building the MFOS Eurorack modules

Posted: Mon Aug 10, 2020 7:06 am
by EATyourGUITAR
Bodo1967 wrote:
Mon Aug 10, 2020 2:01 am
EATyourGUITAR wrote:
Tue Aug 04, 2020 7:52 pm
the unit of measure is mOhm but the part listed is MOhm. I can remind you that 1000000 mOhms = 1 MOhms.
I've read this a few times now but still can't quite follow :hmm: .

"mOhm" would refer to MilliOhm, which is 1/1000 of an Ohm. So 1,000,000 mOhm would be 1 kOhm, not 1 MOhm.

Anyway, I believe this to be a typo (m vs. M) in the BOM. At least In practical use I've never seen mOhm before in practical use. Anything below the 1 digit Ohm range is extremely rare; I don't even think I have any resistors below 1 Ohm in my collection :D.
yes you are correct. I was wrong. 1,000,000,000m Ohms = 1M Ohms

Re: Some notes on building the MFOS Eurorack modules

Posted: Wed Sep 02, 2020 2:46 am
by lolzdoom
I see you guys had issues with the noise module too. In my original post I said that it was working, but I realized sometime after posting this that the module does not calibrate per the calibration instructions on the original noise cornucopia article. The voltage is to high, still generates noise but the random gate is just full speed all the time. Doesn't matter what I have the jumper set to. I ended up building a different noise module and maybe some day I'll come back and try to troubleshoot this but for now it's just on a shelf.

I also finally completed the VCO. I believe there was one minor mistake in the BOM, I think it was a missing a resistor that I had to refer to the schematic to find it. When I find my notes I will post it in the build thread for the VCO. Everything came together fine for that one, except it wouldn't calibrate. It would only do half an octave per volt. Eventually I figured out I got sent the wrong value for the tempco resistor. That was super frustrating but glad to have got it taken care of.