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Building Look Mum No Computers 3340 Simple VCO
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Building Look Mum No Computers 3340 Simple VCO
zarar
Hi!

I find Sam very inspiring and thought I'd give the CEM3340 simple VCO build a try as I had most of the stuff laying around.

After a couple of hours of soldering where I checked all the resistors with a multimeter before placing and checking the power connectors to ensure there's no shorts i figured I'd give it a try using a powersupply from a Dreadbox Hades DIY i had around.

Plugged in some headphones in the Square output jack and plugged the power in... nothing, just a very very slight static hum.

As I'm quite new to the DIY scene, I've been trying to troubleshoot the circuit, checking connections from the IC pins to power etc, measuring up the total resistance here and there.

My question, how would you go about troubleshooting this circuit? Anyone else that has built it?

Also, what's the possibility of that i've fried the CEM3340 chip? No funny smells or smoke has been present. I had to push quite hard to fit it in the socket though.

Here's a link to the build instructions:

https://www.lookmumnocomputer.com/projects/#/cem-3340-diy-simple/

And a big shoutout to Sam for an inspiring channel!
CLee
I would have to check the data sheet but it’s very likely that you can’t drive headphones from the Square Wave output. The schematic buffers the Tri out so attach your phones there. Depending on their impeadance you may not get much of a signal there either, but you could at least confirm it’s alive. And you won’t fry an expensive chip.

I understand his goal is a minimal circuit, but you should put bypass caps on all the power pins (100n ceramic right near the power pin of the IC between the pin and ground) and maybe electrolytic (around 47ufd) at power entry to the module.
zarar
CLee wrote:
I would have to check the data sheet but it’s very likely that you can’t drive headphones from the Square Wave output. The schematic buffers the Tri out so attach your phones there. Depending on their impeadance you may not get much of a signal there either, but you could at least confirm it’s alive. And you won’t fry an expensive chip.

I understand his goal is a minimal circuit, but you should put bypass caps on all the power pins (100n ceramic right near the power pin of the IC between the pin and ground) and maybe electrolytic (around 47ufd) at power entry to the module.


Thanks for the input! Bypass caps would be there to reduce noise and spikes, right?

Also, you say the schematic buffers the triangle out, doesn't it also buffer the square output? From pin 2 on the TL072.

CLee
That's the Sawtooth out on Pin 8, is that what you were measuring?

In any case, if you're attaching headphones, attach them to a 20 cent op amp not a $15 chip :-)
zarar
CLee wrote:
That's the Sawtooth out on Pin 8, is that what you were measuring?

In any case, if you're attaching headphones, attach them to a 20 cent op amp not a $15 chip :-)


Sawtooth out, my bad. But am i wrong in thinking the sawtooth out is buffered through the TL072 as well?

LMNC Stripboard Layout
CLee
Yes, It's buffered
l3v3l6
I haven't built this particular circuit, but I've created a working one using the reference schematic in the datasheet (using the AS3340). This is very similar to Sam's circuit.

The best troubleshooting tool is an ocilloscope. I use that to determine if I'm getting any meaningful output before sending to an audio device.

In my circuit I used +15 and -5 voltages. For testing CV input I tapped a pot into the +15 supply and sent it to one of the CV inputs. To get the pulse wave you need to supply voltage to pin 5. For testing I used another pot to supply this pin. I sent output directly unbuffered to a small mixer with the output going to a audio monitor speaker. I will be adding buffering to my circuit this weekend.
l3v3l6
Quote:
Also, what's the possibility of that i've fried the CEM3340 chip? No funny smells or smoke has been present. I had to push quite hard to fit it in the socket though.



You're probably okay. I put my chip through some horrible abuse when working on my circuit. I had a +15 supply hooked up incorectly and the chip was getting too hot to touch. It survived.
Thinredline
Hi all, new to the site/new to diy synth. Idk if you ever worked it out, but i made the same circuit sam showed from his vid. I noticed that my wiring job was wrong the first time round. I was only following the drawn schematic image, but i was translating that to a breadboard. No sound, but When i went over it again i followed his stripboard layout and noticed i had a cpl of the resistors wired in the wrong spots, and the trimpot wired wrong...also when i hooked it up, i ran the output coming from the 072 to a lm386 amp i made, and connected old pc speakers to it...scared the bejeezus out of me at 1am when the sound came out but was glad it worked. Hope you got it working!! Now trying to figure out what else i can do w this chip, and the as3310 chip i got too. Im all self taught no backgrnd in EE or electronics, i just know working on this stuff and trying to figure it out puts my ADHD at ease lol.
stormkraft
Hi guys. Sorry for digging up this old post but I'm a little stuck. I built my CEM3340. It works very nicely except every now and then the output goes all distorted and noisy for a few seconds before coming back to normal. It's pretty random and inconsistent. Any idea what could be the cause? It usually happens when I'm sending CV but sometimes not.
nigel
stormkraft wrote:
Hi guys. Sorry for digging up this old post but I'm a little stuck. I built my CEM3340. It works very nicely except every now and then the output goes all distorted and noisy for a few seconds before coming back to normal. It's pretty random and inconsistent. Any idea what could be the cause? It usually happens when I'm sending CV but sometimes not.

Hard to tell from that little information. It sounds more like a physical problem than an electronic one. I'd look for a wire (or component) not soldered correctly, so that it's intermittently losing contact.
stormkraft
nigel wrote:
stormkraft wrote:
Hi guys. Sorry for digging up this old post but I'm a little stuck. I built my CEM3340. It works very nicely except every now and then the output goes all distorted and noisy for a few seconds before coming back to normal. It's pretty random and inconsistent. Any idea what could be the cause? It usually happens when I'm sending CV but sometimes not.

Hard to tell from that little information. It sounds more like a physical problem than an electronic one. I'd look for a wire (or component) not soldered correctly, so that it's intermittently losing contact.


Cool thanks, I will do. I did make a bit of a mess of where I soldered the IDC socket so ended up cutting that part off the stripboard and resoldering it on the opposite side - perhaps something went wrong in that process. Every time I think I get the hang of this soldering thing I rush and shake and muck something up lol.
paukl.at
Since the thread is already dug up, let me dust it off a little more smile

I also recently built this oscillator, then I also built Sam's 8-step sequencer (from here https://www.lookmumnocomputer.com/projects/ ).

I currently have a really frankenstein setup (a few modules in Sam's easy custom size (20cm height) - vco, sequencer, clock (all of those in the wild west wires everywhere style) and 3 eurorack modules (headpone output, mixer [diy kit], vcf [diy kit]). Maybe I find the courage one day to post a picture of it here, currently I'm a bit embarrassed by all the high quality diy gear pictures that I saw so far Mr. Green ).

Anyway my actual quetion/problem has to do with tuning the vco.
I realize that currently with just 1 vco it doen't really matter, but since I want to expand the setup soon, I wanted to give it a first try and ran into this strange issue:

Since I don't have any fancy midi / electronics gear, I thought I'd just use a few steps of the sequencer and set them to 0, 1, 2, 3, 4 volts.
I did that by simply hooking up the output to a cheap multimeter and turning the knob until I saw those values.
Then i plugged the cv back into the vco and placed some smartphone pitch tuning app in front of the speaker.

My problem is that I could never find any position on the trim pot which gave me a frequency factor of 2 between 2 volt levels.

That's how it should work, right? So if I get 400Hz for 1V, i should get around 200Hz for 0V and 800Hz for 2V, right?

I never got to a factor of 2 (best values i ever saw was something like 580Hz vs. 960Hz for the +1V step, so a factor of ~1.65).

What was also really strange is that at certain positions of the trim pot, the "function" of the main tune knob (how is that typically called on a vco?) got "reversed", meaning when i turned it from full left to full right, the pitch would first decrease then increase again or things like that (instead of just always increasing).

Sorry for the really long explanation, in short my 2 main questions:
* Am I just completely dumb and doing my tuning attempt all wrong?
* Does this problem sound like anything obvious? (My guess would be that I messed up some resistor value maybe?)
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