Masa921 Micro VCO

23isgood

Hi folks. I recently received the parts I needed to build this VCO so I built it on my breadboard. When I power it up nothing comes out. When I touch some of the resistors and capacitors it starts oscillating. In particular R3 (10k) and the 47k resistor at the CV input, and it starts oscillating when I touch C1 the 470pf cap. Whats going on here? Why is it not oscillating on its own?

Also I noticed from the pics on Masa'a site that his VCO has a different value resistor than stated on his schematic. It looks like a 332 ohm resistor.

Can you guys share some tips on what I should do? When I touch a resistor is it creating more of a resistance? Does this mean that a higher value resistance is required?

thanks
pete

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Masa Micro VCO

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23isgood · Posted: Sun Mar 04, 2007 2:16 pm Post subject:

Nobody has any advice Sad

ian-s · Posted: Sun Mar 04, 2007 3:02 pm Post subject:

Touching a resister will not increase it's resistance. It might fix a bad connection on the breadboard, or introduce your body capacitance into that part of the circuit.

332 ohms does not seem like a probable value for a resister.

Sorry cant be of much help.

23isgood · Posted: Sun Mar 04, 2007 3:39 pm Post subject:

Well it clarifies the issue of touching a resistor. So our bodies act like a capacitor when touching things. I will try rebuilding the circuit. This will be the third time. Though its not to bad since this is such a simple circuit.

thanks

blue hell · Posted: Sun Mar 04, 2007 4:33 pm Post subject:

I'm not sure how exactly this oscillator is supposed to work.

I see a current source built arounf Q4,5 sending current into C1. To make the circuit oscillate C1 will have to be dischared again. Apparently this is done by the voltage rising over C1 making the part around Q2,3 go into conductance until the voltage drops enough to make it restart (and that's the part I don't quite understand).

When however this is true the emitter of Q1 should be at some positive voltage to make it work I'd think, which means that one of the alternatives from the (R, Vr, Vr) box should be connected - preferably the R opion to keep things simple.

Would this give any clues for investigating things further ?

23isgood · Posted: Sun Mar 04, 2007 7:49 pm Post subject:

Well I built it again with the same results. Has anyone else tried to build this? I know there where a few people interested. I think I will try one more time with different transistors.

Uncle Krunkus · Posted: Sun Mar 04, 2007 8:14 pm Post subject:

I agree with Jan. Try connecting one of the options in the top left box. If that connection is left floating it would be very susceptable to body capacitance effects. Well, that's what I think! Very Happy

23isgood · Posted: Sun Mar 04, 2007 8:38 pm Post subject:

Well I did try it. I connected R2 to a 100k resistor then to +12v and that didn't work then I connected R2 straight to +12v with no difference. I did not add the hard synch circuitry, i'm not sure if that would make a difference? Though I had added it before.

I am using a 7812 and 7806 for the +12v and +6v. I'm feeding +15v into the 7812 which is feeding the 7806. I checked the voltages coming out of those and they are correct.

When I touch the resistors and it starts oscillating I noticed that the tracking is backwards. On the left of the keyboard the octaves are high and to the right of the keyboard the octaves are low.

ian-s · Posted: Sun Mar 04, 2007 10:06 pm Post subject:

blue hell · Posted: Mon Mar 05, 2007 6:53 am Post subject:

If it doesn't oscillate what is the voltage over C1 ?

23isgood · Posted: Mon Mar 05, 2007 1:53 pm Post subject:

I finally got it to oscillate but it wont track pitch. The CV input doesn't sound like its working. If I increase the resistance going to R2 from R the pitch goes down. At the low freq's it sounds very meaty as Andrew had said. Lots of bass. This could be a nice VCO if I could get it to work properly.

CJ Miller · Joined: Jan 07, 2007 Posts: 368 Location: 127.0.0.1

Some interesting circuits from this person! Could be perfect for some kind of portable EMS-like noisemaker. Don't know if I'll try this one though. Reminds me of the old ARP VCOs. I read online that there is another VCO which uses 2SC1583s for the expo pair, anybody have a diagram for that one?

Adam-V · Posted: Mon Mar 05, 2007 7:01 pm Post subject:

Is this the one you were thinking of?

René Schmitz VCO3 http://www.uni-bonn.de/~uzs159/vco3.html

EDIT:
Oh, and greetings from down under. I just realised I hadn't introduced myself here yet!

Cheers,
Adam-V.

andrewF · Posted: Tue Mar 06, 2007 3:38 pm Post subject:

v-un-v · Posted: Tue Mar 06, 2007 4:40 pm Post subject:

23isgood · Posted: Tue Mar 06, 2007 5:47 pm Post subject:

Ok I finally got it working. I followed Andrews advice and I just removed the Oct/V CV circuitry and placed it in the LIN CV and now it works! Its a falling saw tooth wave shape. It sounds great, though it does not track all that well but it could be used for all sorts of things. Check out the sample I just recorded. I'm using a CGS Sequential Switch to trigger the VCO and I got the VCO going through a CEM3320 filter and thats it. Now I going to make some PCB's. I love it when things work after spending time troubleshooting. I still have to test the hard synch.

thanks for the help folks
pete

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micro VCO sequence

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andrewF · Posted: Tue Mar 06, 2007 8:21 pm Post subject:

well done Pete
sounds good

you'll probably never be able to play Bach with it, but should be useful for many patches.

If you make a PCB, consider getting the transistors close enough to touch each other, or at least gloop the whole thing once its finished and working. I'm sure Masa921 originally made this cct as a little blob in an attempt to maintain some thermal stability and reduce drift.

just curious, did you leave the V/oct input floating or is there still a pot there?

23isgood · Posted: Tue Mar 06, 2007 10:23 pm Post subject:

I left out all the V/oct circuitry including the trimmer. I have been designing the PCB with all the transistors close toghether. I'm fairly new to designing PCB's so I hope it works ok. I will let ya know how it comes out. I noticed that when I blow on the transistors the pitch changes slightly so as you mentioned I will cover it using some kind of goop. I'm going to try designing the PCB to hold a few VCO's on it.

Fernando · Posted: Tue Mar 13, 2007 12:16 pm Post subject:

maybe Magnus Danielson V/oct to V/Hz converter will work fine here...

andrewF · Posted: Fri Mar 30, 2007 9:15 am Post subject:

hi
i was hunting thru my Takeda files for a schematic and found this forgotten gem
a dual discrete VCO based on masa921's miniVCO
plus a PCB layout

http://www.aleph.co.jp/~takeda/radio/harvest/img/schem/DualTBVCO3a-schem.gif

http://www.aleph.co.jp/~takeda/radio/harvest/pdf/DualTB-pattern.pdf

and while you are there check out the dual 555 VCOs

http://www.aleph.co.jp/~takeda/radio/harvest/ExpSchems.html

Fernando · Posted: Fri Mar 30, 2007 9:52 am Post subject:

hey, the dual discrete VCO looks great, so simple!
thanks!

Scott Stites · Posted: Fri Mar 30, 2007 10:19 am Post subject:

Yeah! Forgot about that - Motohiko sent me some samples he'd done with it at the time (it's my firm belief if there was a God, Motohiko would be the reigning rock and roll king in all of Japan and beyond).

His electronics projects certainly are outstanding, as well.

Dang, I've still got a Steiner VCF board around here somewhere that he did for me.

Cheers,
Scott

23isgood · Posted: Fri Mar 30, 2007 12:39 pm Post subject:

Hey cool a PCB! I guess im going to build this one.

thanks for posting it.

pete

janvanvolt · Joined: Nov 24, 2005 Posts: 285 Location: Mainz, Germany

andrewF · Posted: Wed Apr 18, 2007 4:46 pm Post subject:

I see on the schematic one FET is a p-channel - 2sj103, the other fets are ordinary n-channel, i used the sk30 on most VCOs i built from CGS and Takeda San.
The npn and pnp transistors - Often Takeda San used C1815 and A1015, which are general purpose audio, I don't know if he used any special types in this design, presumably he would have labelled them if he did.

Probably safe to use your favorite npn and pnp transistors but the FETs will be fussier.