Advice for troubleshooting 3340 based VCO

FAQ
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This is a wiki post; please correct and improve!

This is written regarding the 1222 Performance Oscillator module, but similar things apply to the simple stripboard oscillator or other oscillators based on the 3340 chip. For the stripboard version also see

If you are not getting good tones from the oscillator, do the following. If with the 1222 you are also not getting good behavior from the tuner, don’t waste time on trying to fix that yet. Get good tones on the outputs first — the tuner won’t behave nicely until you do.

Start with the usual generic troubleshooting procedures:

After that, check if you have 4.0V from the TP1 test point to ground. If not, do this:

(By the way, R42 shown there is specified as 220 ohms but it really should be higher; 220R results in too much current through the voltage reference U4, potentially causing bad behavior and possibly damaging U4 permanently. If you used 220R this would be a good time to replace it with something between 1k and 4.7k. For more on this and other mods see 1222 Tuner VCO Module - #203 by antoine.pasde2 and 1222 Tuner VCO Module fixes/improvements - #6 by eric .)

Once you have 4.0V at TP1, check your control voltages. Do this:

The center note trimmer should have +12V on one end (measured between there and ground), 0V on the other, and something between 0V and +12V on the center terminal, depending on the trimmer position.

The fine tune pot should have +1V on one end, 0V on the other, and something between 0V and +1V on the center terminal, depending on the knob position.

The octave selector switch should have +4V on one end, 0V on the other, and 0V, 1V, 2V, 3V, or 4V on the center terminal, depending on the knob position.

With nothing plugged in, the 1 V/oct and CV input jacks should have 0V on the sleeve terminal and nothing on the tip terminal.

Also check the connections and component values going into pin 13. R4 should be 1.5M and should connect to +12 V. R2 should be 470R and connect to ground.

Adjust the center note trimmer; somewhere in the middle of its range you should get an audible tone.

If all voltages are correct and you still don’t have good outputs, go back to the start and double check everything. If it still isn’t working, you could have a bad chip; try replacing the ICs one at a time. If that doesn’t work, post a query to 1222 Tuner VCO Module . (Please don’t post queries to this topic; solutions, yes, queries, no.)

Tuning

If all those connections and voltages are good then it’s a matter of getting the scaling right. R3 (connected to pin 1) should be 24k, R5 (pin 2) should be 5.6k, RV1 (TRK trimmer, between R3 and R5) should be 10k.

For tuning, it’s best to have a scope, frequency counter, or an instrument tuner that tells you frequency. Lacking those you can do your best with any tuner. Set both the TRK and HFT trimmers to the middles of their ranges. Put 0 V and 2 V alternately on the V/Oct CV. These should be accurate voltages or at least have an accurate difference of 2.000 V. The resulting pitches should differ by 2 octaves; in frequency terms, the ratio of the two frequencies should be 2.000. If not, change the TRK trimmer and check both voltages again. (Both frequencies will change, but it’s just the ratio you need to pay attention to.) If the frequency ratio is going in the wrong direction, change TRK back the other way. Keep repeating until the frequency ratio is 2.000 (or the tuner says it’s as close to 2 octaves as you can get).

Then put 3 V and 5 V in and do the same thing, but this time use the HFT trimmer to get the frequency ratio to 2.000. After that go back to 0 V and 2 V and do it all again from there, with the TRK trimmer, then 3 V and 5 V with the HFT. It might take two or three times through the whole routine before everything’s set. For details on this procedure see

If you hit the limit on either trimmer and can’t get to 2.000 ratio then there’s probably a bad connection or incorrect component value. Check those again.

The tuning procedure’s much less tedious using a MI Module Tester, but first you have to build and troubleshoot that, so maybe do it the old fashioned way until you’re making a bunch of VCOs or have other need for module testing and troubleshooting equipment…

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Hello guys, have someone met with sympthoms like this? No led glowing, and I’ve got a big 0 on the screen, short noise signals from the outputs

IMG_20201229_004050
IMG_20201229_0040502736×3648 1.35 MB

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tiny

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Did you go through the calibration procedure? The oscillator may be tuned so low that it’s acting as an LFO.

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Wow, maybe that could be the problem, thanks!

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I remembered this was happening to me for my stripboard 3340s, hope thats what this is. You would expect to hear some pattern of thump thump thump.

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Hello friends. I have built a few modules without many chips other than op amps and they have worked (avalanche, all about circuits, shapes vcos, kassutronics adsr, henry thomas vca). The great thing is I can follow the signal path and see where I messed up and fix it.

But I got two AS3340s from ebay and after spending literally 20 hours debugging it, switching chips, etc. I still get no signal out of either chip. I’ve checked all the solder points, continuity, that the ‘outside’ cv is flowing correctly and mixing correctly and hitting pin 15 correctly. I checked VCC adn ground correctly. I tried a range of resistance between -12V and pin3. I even tried +12V, -5V supplies.
I pulled them out and built the simplest LMNC stripboard schematic and no signal. Actually not quite true, I get a single high signal on pin 8 and sometimes something like that on pin 10 or sometimes 0. I get something like +0.5V on pin 3 (shouldn’t that be -7.4V with zener inside!).

So my thinking is a) either I messed up the chips early in my build and both are broke b) the chips were fakes or didn’t work in the first place, c) I still am making some simple mistake in wiring this up, d) I am an idiot. All four could be true. My question for you is have you seen anything like this before and which of these four causes do you think it is (and yes I know you will flame me and call me an idiot, but I got there first so no glory for you in doing so please also choose another answer!).

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Welcome to the forum, which you will find to be (so far!) pretty much 100% flame-free and name calling free. It’s weird, I know, but there it is.

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Thank you! by the way, I’ve watched many of your videos (at least I think that’s you from behind the mask) and learned a lot.

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did you get them from cabintech or a private seller if it was private seller its a good chance you got burned. but then again you may be missing some dumb little thing , I hate that shit it is so frustrating .

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Hey devicex,

I got them from syntaxis-pl. The odd thing is they have printing on them that doesn’t look like anything I have seen otherwise. But I do understand syntaxis is typically a real supplier in poland of actual instruments. Perhaps I fried them myself in the beginning!

A

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well hopefully not and its just some thing you are missing somewhere else .

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The Alpha chips tend to look used, also when new:

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Can you read this? Doesn’t this look different to you? image

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Nah, i have 10 of them and thats just what they look like.

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I realised a while back that everybody who posts here seems to be spookily chill. I don’t think I’ve ever been on a forum that was quite as friendly and full of productive, supportive and cooperative people.

Maybe it’s got something to do with [forum creator and eponymous computer-free person] Sam Battle’s perpetually sunny disposition setting a good example. I know I myself can be fairly prickly, so something about the atmosphere brings out the best in me. I suppose that’s true of everybody else, even if some of you are genuinely saintly in every forum.

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Hey thanks for looking. Ok if I have the right chips and they don’t work, i probably fried them at some point! Oh well. I guess I have to use regulaaar circuits instead of a fancy all-in-one chip for a bit!

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So a quick update - I certainly did fry a few 3340 chips somewhere. I got a new set and when being more careful got them to work nicely and as expected. Though I find tuning them to be difficult over more than a few octaves. I have started down the Hordijk tuning the as3340 with the four pots and I’ll see how that goes. Has anyone else tried that and it worked well for them?

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Yet another 3340 question :joy:

I’m tinkering with oscillators for the first time starting with essentially the 3340 datasheet, except using -5v for power, a la kassutronics. Also have a simple output buffer for each waveform.

I don’t have any detuning problems (at least not bad enough for me to complain about) but adjusting the pulse width introduces a DC offset. This is not good? Isn’t it best to stay centered around 0v?

Here is quick 10 second look at what I’m seeing on my scope:

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The 3340 pulse wave output is 0 to about 10.5 volts. Centering that on zero is the output stage’s job.

I see other problems here, the pulses aren’t square (not flat) and apparently only 4 Vpp (or at least 10 Vpp is a more common design choice).

So what is your output stage?

If you’re using a decouping capacitor then it’s doing about what I’d expect: if the pulse width were 99.99% it would want to put the top of the waveform close to 0 V and if it were 0.01% it would put the bottom close to 0 V. The capacitor will also distort the square wave in the way seen here. Better to not use a capacitor to center the output but (as in the Kassutronics design) to subtract a fixed offset.

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