Building Look Mum No Computers 3340 Simple VCO

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zarar
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Building Look Mum No Computers 3340 Simple VCO

Post by zarar » Fri Aug 10, 2018 6:43 am

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/proje ... iy-simple/

And a big shoutout to Sam for an inspiring channel!

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CLee
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Post by CLee » Fri Aug 10, 2018 7:39 am

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.

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Post by zarar » Fri Aug 10, 2018 8:47 am

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.

Image

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Post by CLee » Fri Aug 10, 2018 8:54 am

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 :-)

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Post by zarar » Fri Aug 10, 2018 9:25 am

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

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Post by CLee » Fri Aug 10, 2018 9:48 am

Yes, It's buffered

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l3v3l6
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Post by l3v3l6 » Fri Aug 10, 2018 12:24 pm

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.

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Post by l3v3l6 » Fri Aug 10, 2018 12:55 pm

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.

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Post by Thinredline » Sun Oct 07, 2018 12:13 pm

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.

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Post by stormkraft » Tue Jan 15, 2019 12:53 am

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.

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Post by nigel » Tue Jan 15, 2019 7:13 am

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.

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Post by stormkraft » Tue Jan 15, 2019 7:17 am

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.

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Post by paukl.at » Tue Jan 22, 2019 5:50 pm

Since the thread is already dug up, let me dust it off a little more :)

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 :mrgreen: ).

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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Post by Bardan » Thu Jun 20, 2019 3:55 pm

Hey everyone!

I'm digging up this topic again, (sorry about that, i can just make another one if i need to..)

I've build sam's VCO along with the power supplies from Frequency Central..
Now, everything seems correctly wired and everything, but when i plug it in, the +12V LED on the PSU turns off, and no need to tell you that I hear absolutely nothing from the Module.. :bang:
i'm getting desperate here as no one (Frequency Central or even Sam) has responded to me..

(btw, i'm fairly new to the diy world of electronics.. but here it really makes no sense..)

I can send you guys detailled pictures of the layouts and everything if you want, but have you encountered such a problem?

Thanks in advance! :)

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Post by CLee » Thu Jun 20, 2019 5:08 pm

Bardan wrote:Now, everything seems correctly wired and everything, but when i plug it in, the +12V LED on the PSU turns off
If it does this you most likely have a short circuit somewhere. Check for solder bridges or a component or wire wrong.

Maybe you have the power reversed.

I'd suggest pulling out your expensive chip and see if the power shuts down with no chips in the sockets. Then check with a volt meter that the power is correct at the IC socket for +12, -12, and ground.

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Post by LeBoi » Fri Jun 21, 2019 1:59 pm

Hi everybody

Since this is a post about the CEM3340 I tought it would be the right place to ask about something that has been bugging me on the CEM3340's datasheet.

I am in the process of designing a schematic for a simple DIY CEM3340 oscillator but I don't understand what this specification in the absolute maximum ratings means: " Voltage between frequency control pin or reference current pin and ground pin + or - 6.0V "

Frequency control pin is pin 15 which is said to be a virtual ground (summing amplifier). Reference current is pin 13 is also a virtual ground...

If someone understands what it means it would really help because I don't want to fry the chip or something...

Thanks!

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Post by nigel » Fri Jun 21, 2019 7:43 pm

LeBoi wrote: " Voltage between frequency control pin or reference current pin and ground pin + or - 6.0V "

Frequency control pin is pin 15 which is said to be a virtual ground (summing amplifier). Reference current is pin 13 is also a virtual ground...
Just what it says - the voltage on pin 15 must be between -6V and +6V. (Which is measured relative to the ground pin.) Same for pin 13.

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Post by LeBoi » Fri Jun 21, 2019 10:34 pm

Thanks for the answer !

However I don't think it's that simple because pin 15 is a ground so there no voltage relative to the ground pin there ?

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Post by nigel » Sat Jun 22, 2019 12:29 am

It's not a ground pin, it's a virtual ground, which means that under normal conditions, the internal circuitry of the chip will try to keep that point at 0V. If (for example) you connect that pin directly to the 12V rail, the chip will promptly blow itself up trying to sink enough current to short out your power supply. As long as you use the normal schematic, and just combine control voltages at that pin through 100k (or higher) resistors, it will stay at 0V.

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Post by LeBoi » Sat Jun 22, 2019 2:17 pm

I did not know the virtual ground worked like that thanks alot for clarifying !

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Post by Bardan » Sun Jun 23, 2019 9:18 am

CLee wrote: I'd suggest pulling out your expensive chip and see if the power shuts down with no chips in the sockets. Then check with a volt meter that the power is correct at the IC socket for +12, -12, and ground.
Indeed when the ICs are removed the +12V keeps lighting!

Now, the multimeter shows me that i get 4V out of the FC diy power supply (starting from the power regulators.. maybe they should be the other way around?)

Anyway, i've checked on the IC socket but only found a few resistance here and there but no continuity.. soo idk)

Thanks for the help! :)

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Post by Bardan » Mon Jun 24, 2019 3:00 am


Indeed when the ICs are removed the +12V keeps lighting!

Now, the multimeter shows me that i get 4V out of the FC diy power supply (starting from the power regulators.. maybe they should be the other way around?)
As it turns out, the power regulators were indeed flipped (the 912 where the 812 should be etc.)
I couldn't remove them without damaging them since that i read 38V when i tried putting them back in.
So i've order some more and hope for the best! :mrgreen:

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Post by Bardan » Sun Jun 30, 2019 2:11 pm

Quick update:

I put the regulators back to where they belong, i now get about 12V on one supply, and a bit more than 14V on the other one.
SHould I worry about this difference or is it insignificant?

Now I need to figure out where the short is on my VCO, as it doesn't happen with the ICs off of the board.. :)

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Post by nigel » Sun Jun 30, 2019 7:18 pm

Bardan wrote:I put the regulators back to where they belong, i now get about 12V on one supply, and a bit more than 14V on the other one.
SHould I worry about this difference or is it insignificant?
78XX / 79XX regulators have a tolerance of about 5%, so a 12V (positive or negative) regulator should be somewhere between 11.4V and 12.6V.

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Post by Bardan » Tue Jul 02, 2019 2:46 pm

It works!!

Thanks for the help guys!

Now i didn't dare to plug it to the supply that fires out ~14V..

How insignificant is this 2V difference in a modular synth application?

Thanks! :mrgreen:

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