My first DIY modules - starting a Kosmo format build

So…I didn’t write up the PCB etching post yet because I decided to build this up instead and hopefully have a working circuit to show off at the end of that post.

It…didn’t quite go that way. Assembly went off fairly easily:


That one iffy spot on that one trace I patched up…but in the process created a bridge to the nearly ground pour. Fixed that but it left a bit of a “scar” since you can never get ALL the solder off of copper once it’s on there:

Drew up and panel and started printing it…but didn’t want to wait for it so re-used the first Steiner panel instead for now:

Plugged it in…and…I got nothing. Hmmm. Power traces checked out…maybe it’s like my #1222 and the fine tuning is just way out. Turned it a bunch in both directions. Still nothing.

Turned the volume way up and it was doing a bit of clicking. So…it’s not totally dead.

Took a look at the schematic again…and…found a mistake on the CV input circuit. Had a resistor in the wrong place. Whoops. Fixed it on the schematic and on the PCB design…then did the same fix on my board with some bodge wires:

Better. I’m getting SOMETHING at least now. It’s REALLY low frequency though and turning the adjustment trimmer just makes it get lower. The coarse tune and pwm knobs do make a difference…but something still isn’t right.

Took another look at the schematic. And…Yeah…I made some bigger mistakes on the CV circuit :frowning:

Big enough to requite moving components and more than I think I can patch up with just some bodge wires. So…looks like I get to etch a new board tomorrow and move the parts over :frowning: But I’m going to sleep on it first. I might be able to bodge this board into working first to confirm things. Just the changes are big enough that will be awfully confusing to implement.

And I want to take another look at the schematic and make sure I’ve got the CV inputs sorted out.

So big PCB post will hopefully happen tomorrow.


oh so I also made the mistake when building mine :confused:

Well, woke up to a nice panel on the 3D printer so decided to see if I could figure out a few bodges to bring this up to spec instead of etching another board right away:

It’s getting ugly…but…I’ve seen worse on commercial stuff still so I don’t feel too bad:

And I didn’t make things worse at least :smiley:

Well, for the most part. I now get a nice spectrum of sounds out of all three outputs. The coarse tuning pot give me everything from deep rumbling up to squeaky dolphin talk.

The PWM knob is a bit touchy…not sure if I have a mistake or if that’s just how this circuit is. But it only really works in the center for about 1/5th of the travel of the pot. Any further in either direction and it goes silent. My 1222 isn’t like that but the circuit on there is a bit different…so I’m guessing that’s an improvement over this basic circuit (looks like a few additional resistors to keep things under control.)

So before I roll a new board I may try and pull that in.

On the downside…my CV in now doesn’t do anything. It could be a bad bodge…or a deeper issue with my schematic since I still have a gut feeling something isn’t right in the CV circuit. So…need to take a closer look at that.

But - have Sunday chores to deal with first.


Enjoyed my day away from the bench.

Started on a new case with 3D printed rails:

Unfortunately the Ply I got this time wasn’t nearly as flat and kept wanted to warp on me…so not sure how well this case will come out. Need to drill the holes for mounting the rails next - they do fit…but…since the case keeps wanting to warp I’m not sure how well it will work out.

And finished a 3D printed printing press - and the low tech fun of pulling prints off plates made from the side of a box of goldfish crackers inspired me to try my hand at some LMNC style artwork:

Didn’t quite get the K for Kosmo right :frowning:

And the press does kind of fit in here since it was etching circuit boards that reminded me I wanted to try making this. I’m going to try making a nicer plate by etching a PCB to use instead. Will see how that does in a few days.


your small printing are cool!
how are your module rails made?
do you drill them according to the modules or do you have predefined holes?

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Thanks, the rails are 3D printed - I posted details about them here: 3D printed Rails

They have slots that hold thin M3 square nuts to secure the modules.


Are you not worried about the music falling out of the cabinet if the modules are all on their sides like that? Do you need to use a special side-to-side power supply instead of the usual +12V and -12V? Imaginary voltages? +12iV, -12iV? How do they work? Is this useful for producing complex music?


So…sitting here working and this 3340 VCO is sitting on my desk. Reach across to grab a pen…and something about the VCO caught my eye.

I see why my CV inputs aren’t working.

Swapped signals and grounds on the jacks and now the CV in is working.

Something still seems off with the PWM control though. It’s VERY sensitive and only works at a very specific setting now. My board seems to match the schematic though and my schematic seems to match the stripboard.

My schematic:

What’s odd is that it was working better before I made the earlier fixes to the CV circuit. But those changes didn’t affect any of the components or traces for the PWM circuit. It was still overly sensitive then…but it did have some range.

But…I see Sam changed the PWM circuit quite a bit on the 1222:

And the CEM3340 datasheet just shows PWM as 0-5v with no suggested circuit.

So it looks like the stripboard circuit isn’t very good at staying within 0-5v - the 1222 circuit looks like it should be more correct here. But…it’s a bigger change than I can just bodge into this board. So probably will roll a revised board at this point.


I’ve been thinking about this for 4 days. I keep meaning to come back to it but got a little distracted typing up that big post on etching PCB’s. And the more I thought about it the more I felt I really wanted to test this change before etching a new board.

And the changes aren’t that bad…so…

This is getting kind of ugly.

But…I really want to confirm this fixes it before I etch another board.

And…it didn’t :frowning:

Something still seems off with the PWM. The output just seems lower than it did initially and there’s an even smaller zone where PWM actually works now.

I’m just not sure what’s going on now. So I pulled out the scope and the DMM.

I’m only seeing a 0-1v swing on the PWM pin of the 3340. The datasheet says that pin wants 0-5. So…I’m not sure without doing more math than I’m up for at 11PM on a Friday if that’s what Sam’s circuit from the 1222 (which is what I’m now using) is actually designed to do…or if there’s an issue with my bodge job.

And…looking at the square wave output on my scope. I’m seeing a decent (not great it’s got a bit of a spike at the start) square wave and as expected it’s width does change as I change the PWM knob with no jumps or surprises. So I’m not sure why the audio breaks up entirely except for a VERY small range of widths.

So I took a look at my 1222 instead. It looked the same on the scope…so I gave it a listen…And it was doing the same thing. Which…I don’t remember it doing before! I was swinging that PWM knob from one side to the other having fun just the other day. What the hell?

So I gave a listen to my DIY board again…and suddenly the PWM swing was just fine across the whole range.

What?! Now suddenly my 1222 is doing the wonky thing and my diy build is acting like the 1222 used to.

I…I just don’t know now. The good news is my DIY seems to be working quite well - both inputs work, all three outputs work. They sound the same and have the same levels as my 1222. Just…the PWM on both sometimes works and sometimes just gets funky and only works in a very narrow range.

Maybe I just never noticed it on the 1222 before and it’s a matter of being at certain frequencies?

I’m going to have to sleep on this. I’m tempted to call this bodge a success…but there’s just to much that doesn’t make sense for me to fully put this module in the win column just yet.

And I still want to roll another board with the fixes to my layout this weekend.

I also started drawing up kassutronics version of the 3340 - man that’s a lot more resistors! I also decided that since I’m using 75mm wide panels for my oscilators with 70mm wide PCB’s…I should try and use board mount pots to mount the board. So on the kassu3340 I’m trying to design it that way…but…man it’s looking like it will be TIGHT. Pretty sure it won’t be possible with my .5mm traces and .4mm clearances.

We’ll see. I’m really not great at board layout so this is looking like quite a challenge to me. I may just try and redo my simple 3340 to use board mount pot’s before I etch a new PCB since it’s a much more forgiving layout.
I have a feeling I’ll have a tough time fitting the kassu circuit on to a 70x100mm board even without the board mounts pots. I see he uses smaller 1/8w resistors to pack it in. But I really want to give his circuit a try because I like sine output and I like the look of how he’s doing his outputs. HIS PWM circuit is also simpler…but he’s using 5v regulators to add +/- 5v on his design and just doing a simple voltage divider I have an easier time wrapping my head around. I also like the way he uses the switch in the socket to switch one PWM knob between PWM control and pwm CV level control.

We’ll see if I keep going on laying that out or not. I’ll probably put another hour or two into it and if it’s not looking like I can make it all fit I’ll reevaluate my choices.


Mount the resistors vertically… if they still fit between the PCB and panel this way…

Did you by chance change power supplies or power cables in your testing, causing the wonky pwm device to swap?

You know, that should have been obvious to me…one of the bodges on the VCO (not visible since it was on the component side) was to replace a jumper with a resistor and I had to mount it vertically for it to fit.

I think they’d fit vertically…but physical space isn’t looking like quite the issue I was afraid it would be so far:

The big question will be if there’s room for all the traces. Will dig in on that later today and see how things go. And this is just a rough first go at getting everything on the board. I’m sure I’ll find lots of things to shift and reposition once I start adding traces.

Nope, only have one power supply right now (have a second but haven’t finished soldering it up) and I only have one power cable for testing modules with. In fact I have 3 modules just sitting on top of my case right now because I’m out of room to plug them in. Need to get that second power supply going!

The 1222 never even came out of the case. I’m just totally stumped why I’m seeing this behavior sometimes now. I’m guessing it’s been possible all along and I just never noticed. But definitely seems odd.

It could be power supply related though…I do think I’m pushing the limits of my supply. Sometimes when I power it up just the LED on my LFO comes on solid and nothing else…then I have to unplug mains and plug back and then everything comes up normal. Not sure just what’s going on there but I suspect it could be a sign I’m pushing the limits on my supply. I did also have a shorted rail when I first breadboarded up the transistor matching circuit and the supply ran with one of the 12v rails shorted for a few minutes…so there’s a possibility I could have damaged one of the regulators.

Guess my first project today should be finishing that other supply…


The PWM control range is 0−Vcc/3, see e.g. here. The values in the datasheet are for 15 V supplies.

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Ahhh! That makes sense, and I think I remember reading that in the sheet now that you point it out.

I was just looking at the reference schematic on the Electric Druid site which labeled it as 0-5v. I should have paid better attention to the datasheet itself instead of a schematic derived from it!

I did a bit of playing with my layout to convert it to board mounted pot’s…and have something that looks like it should work, has the correct measurements as far a I can tell, and passes DRC checks:

Except…the PWM CV pot is on the wrong side for my panel layout. So either I have to change my panel to put the PWM CV on the left and frequency CV on the right…which just feels backwards to me. Or I have to find a way to move that pot over to where the inputs are now…and that creates a major mess.

I also played with the kasu3340 layout a bit more…started routing it…and…ugh. That’s getting ugly faster than I expected. Might be just a bit more advanced than I’m ready to tackle just yet. Need to do a few more simpler designs first.

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So I realized…I also had the pots on the wrong side of the PCB in that last revision. Sigh.

So…swapped them all around, moved the PWM pot which was as big of a pain as I expected. And then realized…the PWM pot was too far from the edge and the board would interfere with the PWM jack. Double sigh.

So…had to move the I/O connectors as well, and the power to the other end…reshuffle and reroute a bunch of little things…and I think it’s about good now:

Only 4 jumpers…but 3 of them are longer and more ackward than I’d like. But I don’t see any way to improve on them.

This SHOULD work though…Should give me a pot mounted version of my simple 3340

I’m tempted to add the kassu sine output to it as well…but that would require reworking my panel to fit another output jack. But…there probably IS room for the circuitry…hmmm…


If I didn’t know any better I’d say I’m getting better with practice :smiley:

Well…mostly. I still managed to make some dumb mistakes.

Namely I forgot to put my etchant in a heated bath…so it took longer than normal…and didn’t etch quite as well. It was hard to tell when to pull it as a result and while it looked fully etched. There were a lot of “whiskers” bridging various traces :frowning: Also - I didn’t quite position the board on the negative correctly and one trace almost got cut off on the right side. enough of it was still there to have continuity though so I ran with it.

Oh - also I damaged the mask along the bottom…but since it was all ground I wasn’t too worried. I gave it a bit of sharpie to help…but it still got a bit etched.

Not my best board…but should be good enough!

Ok…maybe this one is cursed. Broke 3 drills drilling it. Well, re-drilling a few of the holes by hand because I needed them a little larger.

But…not going to let that stop me:

Alright…one other mistake. I had planned on putting the fine tuning trimmer on the other side so I could adjust it with the module installed. But…not going to happen. I forgot there’s no way to solder it when it’s installed that way since the pads are hidden under it. Plus I forgot to add a hole for it in my panel anyway.

Speaking of the panel - it took just over 2 hours to print…and finished at almost exactly the same time I was ready to install the board in it :slight_smile: I started the print just before I printed the negative to etch the board. So etching and assembling the board took almost exactly the same time as printing the panel to go with it.

It definitely makes for a more sturdy module!

I still had some debugging to do on it though. I stole the jacks from my other DIY 3340 to save time but the outputs are in a slightly different order this time so I had them mixed up at first. Super easy fix.

Bigger problem was once again PWM wasn’t doing anything. Oh nuts…not this again!

But I was able to track it down to a solder bridge due to the lack of solder mask (Soldering the pots is a bit tricky with no pads on the back…and getting the iron in there I accidentally caused a bridge.) And one trace that was still shorted out to ground due to the incomplete etch.

With those two issues fixed it works perfect!

So - now I’ve got 3 VCO’s for my case. I definitely need to finish that other power supply so I can get all these in there. I think the mega drone is going to move to it’s own case for now until I finish up the other case I’m working on.

And I need to wire up this ARP4072 and see if it works still.

So I’ll probably hold off on the Kasu3340. Try a few other boards first and work up to it. I still want a nice snare. Or a 808 clap to go with my 808 kick…of course I still need to finish the trigger outputs for the sequencer and finish the case and power supply for the sequencer so I can use it without boards laying out and janky wires to a bench supply!


Two hours for a build definitely beats waiting two weeks for PCBs from China…

You can use “longer” pads for the pot-pads, like those on ICs footprints.
Should make soldering easier.


Very impressive!
Side chain the triangle and make a mini strip for sine wave.

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Yeah. I had more time into doing the PCB layout changes than making and assembling the board. Though having a proper two sided board with plated vias would solve the issue of the trim pot soldering to get it front facing. I’m not sure longer pads would really help with it…the way the trimmer is shaped it’s just hard to even see under it to where the pins are. I thought about heating the pins from the back but I couldn’t even get solder in from the front because the body of the pot just shields everything.

Longer pads would help with the normal pots. If I had seen existing footprints with longer pads I’d have used them - I use them everywhere else they’re available. But I’m still figuring out how to edit footprints so didn’t want to take the time to make custom ones with longer pads.

Before I even dug in on making more VCO’s I started looking at making a simple tri-sin module by itself. But didn’t want to use up another power connection and space in the rack just yet :smiley: Might do it once I remove the megadrone though…

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@juggle These look super good! Do you print the face plates with 2 colors of plastic or paint the letters afterwards?

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