Just want to check the inductor value as I’m having a tough time sourcing it (most places only go up to 10mh). In the BOM its 100mh but in the photos on the project page it looks like it uses code 686C which comes out as 68H using this inductor code tool and then @SYNTHGUY57 said it is 100uH (which might have been i typo)
Can it be a different value from 100mH?
And with a tolerance of 0.25 nH. A 68 ± 0.00000000025 H inductor would be quite the thing. If it were correct, but surely it isn’t.
Anyway, in the Practical Electronics article this is based on, the BOM and schematic agree it’s 100 mH. Should not be hard to find; DigiKey has 23 100 mH inductors in stock including this one RLB0913-104K Bourns Inc. | Inductors, Coils, Chokes | DigiKey (8,895 in stock) which costs $0.78 and looks like it should be fine.
Thanks for clearing that up
Oh damn I just bought some on Amazon for AU$7
Nothing too helpful here, but I like to point out that I also have no noise. I’ll try to get a bunch of 2N3904 H331 and come back to it.
Do you think I have to replace all three?
All three transistors are involved in the noise generator, but in different ways; from the original article:
TR1b, TR2b, and TR3b correspond in the Kosmo module to Q1, Q3, and Q2. Q2 is the one that actually produces the white noise, by being used in a non standard way, and is most likely the one and only one you would need to replace since some transistors are good at making noise and some are not.
Q1 and Q3 are doing more standard transistor things and are less likely to be not doing their jobs. But if either of these is defective or has a soldering error it could also prevent getting noise.
If you have a scope you can check to see if each transistor is producing the expected output.
Oh that was quick! Great, thanks a lot. Incredible as always. I had a hunch that not all of them would make the actual noise, but only looking at the circuit didn’t convince me fully. I already removed the Q2 one and (sloppily) put a socket there. I research a bit if I have something similar and maybe try some tomorrow.
Okay, so…
i now have noise, but it is incredibly quiet. Can i just switch one or two resistors without it going poof as it was mentioned earlier? I would like a suggestion as to what is save.
Oh yeah, i switched the TL074s, that didn’t do it.
Hey all, I’ve been gathering my components for the twin t and I noticed that the inductor has one leg shorter than the other. Normally I’d associated this with a polarized cap or an LED. I don’t see anything on the PCB screen to suggest a particular orientation, but now I’m questioning myself 
I’ve had a look at the datasheet but it didn’t clarify it for me.
Any idea if there’s a specific orientation required for this part?
edit
Somehow when I posted this last night it didn’t occur to me to search “Are inductors polarized?” After having a strong cup of tea this morning, I found this as maybe the second search return?
The second most common parts we are asked about are inductors. Inductors are somewhat similar to resistors except they are specifically designed for storing electrical energy in the form of magnetic energy. Inductors can be simplified to fancy coiled wires around ferromagnetic material (or other magnetic material). Inductors do not care which way they are placed just like resistors due to the physics, design, and construction. I believe the confusion, again, comes from practices in analysis and certain applications where polarity is necessary when it comes to direction of current. Even though inductors aren’t polarized, the particular direction the current flows will change the magnetic poles present in and around an inductor. For example, a coil present on a relay has particular polarity markings on circuit diagrams simply to indicate that is the intended direction of current to properly acutate the relay, it does not mean that the coil itself is polarized. Magnetic poles may be effected by electrical poles, but are completely separate concepts.
I figured I’d put this here in case the question ever comes up again.
Moral of the Story: If it’s late at night, wait until the morning OR Google is your friend 
I found this a clearer explanation — the question is about an SMD inductor but I assume the thinking is the same with through hole.
Ah, cheers! Definitely clearer I did happen to buy some extras just in case …
hi all! i’ve read this whole thread but i’m still not quite sure where to start. my board is starting to get tired of my shenanigans so i figured it’s time to come, hat in hand, and ask for help.
as far as i can tell, the bongo circuit isn’t working for me. i’ve tuned out all the feedback, but everything just sounds like a click for everything but snare and noise. snare and noise sound identical, so i’m fairly certain the bongo circuit just isn’t working.
problem is, i have no idea where to start. i did try swapping out the ics for new ones in case, but no dice there. reflowed everything, all to no avail. i have an oscilloscope, a multimeter, and a can-do attitude, but unfortunately that’s where my expertise stops. any breadcrumbs toward the solution are much appreciated.
thank you, i did read that and follow it to the best of my ability, i don’t think anything obvious is out of place, but i will continue to check. i saw higher in the thread that someone had a similar result when first building it so i was hoping the answer was maybe already available. but i will continue to poke at it with my trusty multimeter!
Welcome to the forum bretgs. 
thank you! and thank you too @analogoutput
through staring at the schematic and thinking really hard, i found the one common unit of the broken drums but not the fixed ones; the 5v voltage regulator. took a close look, and sure enough, it was a BC558.
one part and three new ICs later, and we’re in business…sort of. each drum has some kind of note coming out of it when not triggered. off to try to calibrate this bad boy, but i wanted to make sure to update the forum in case someone else has a problem in the same spot!
edit to save people notifications: that was obviously just a matter of setting the trims, ladies and gentlemen we have DRUMS
Excellent.
These circuits are inherently noisy and tend to pick up stray signals via their trigger inputs.
Make sure every input has its own ground and use shielded cable or 2 wires twisted tightly toghether these measures should reduce stray pick up and hopefully get rid of that background noise.
thank you! i got the noise out of all of the previously broken drums, just gotta figure out something going on with snare/noise leading to a crazy high pitched whine that hurts to listen to. i’ll track it down!
update here to be a good boy:
i … didn’t do anything? i didn’t change a single thing, i turned it off and walked away for a few days while i was at work, and now it’s fine? i’m sorry if anyone else finds feedback in the noise circuit. maybe it was holdover feedback from before an adjustment? i have no clue but it works
Could be picking up crosstalk from something else. That seems to be a common thing with these kinds of noise circuits; I had a kind of similar problem recently: Noise source circuits - #7 by analogoutput complete with the now-you-hear-it, now-you-don’t. No guarantees but if the problem recurs you might consider filtering the +12 V rail by replacing R30 with two 470 kΩ in series and then adding a 1 µF capacitor from their junction to ground. I don’t think that would do any harm (certainly nothing permanent, just unsolder the cap if you don’t like it) and might be a fix?
I finished building my #2700 today, and, yes, this thing is super sensitive to PSU noise, particularly in the noise circuit. Switching between several of my wall wart style PSUs with various other modules on each one gave drastically different amounts of hum out of the noise portion of this module. Maybe I will add the filter on the +12v rail to see how that helps.
Also, I had to max out all of my trim pots, and I can still get the self-oscillation each of the drum channels, if I turn the decay pot up enough. I will have to be careful with that one.
Other than those issues, it sounds and works great.