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 Forum index » DIY Hardware and Software » YuSynth
VCO - how to debug
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akorbeti



Joined: Oct 30, 2008
Posts: 22
Location: Greece

PostPosted: Sun Feb 08, 2009 1:08 pm    Post subject: VCO - how to debug Reply with quote  Mark this post and the followings unread

Hi there,

I recently made two VCOs but when I test them, they do nothing. Well at some times there is a low freq noise coming out of all the outputs. The power is ok. I get 27-28V between rails. As I didn't have a 1% mica , I got many 5% ones (silver micas) and tested them so I found two with exactly 220pF capacitance and put them.

My question is, how could an analog newbie debug the circuit? Any pointers would be most welcome.

Also, there is some soldering flux remaining on the board. Could this be conductive enough so as to produce problems?

As I havent yet finished my Midibox CV converter, I use a 9v battery with a potentiometer to simulate a voltage of a common note that would be around 3-7 volts in the inputs. Is this a really dumb thing to do?

Sorry for my infantile questions but I am a bit of a newbie and moron combined Smile

Thank you all in advance,

Aris
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yusynth



Joined: Nov 24, 2005
Posts: 1219
Location: France

PostPosted: Mon Feb 09, 2009 12:44 am    Post subject: Re: VCO - how to debug Reply with quote  Mark this post and the followings unread

akorbeti wrote:
Hi there,

I recently made two VCOs but when I test them, they do nothing. Well at some times there is a low freq noise coming out of all the outputs. The power is ok. I get 27-28V between rails. As I didn't have a 1% mica , I got many 5% ones (silver micas) and tested them so I found two with exactly 220pF capacitance and put them.

This is not a problem as long as they are thermally stable.


akorbeti wrote:
Also, there is some soldering flux remaining on the board. Could this be conductive enough so as to produce problems?
It is good practice to remove solder flux as it may create issues in part of the circuit with high impedance.


akorbeti wrote:
As I havent yet finished my Midibox CV converter, I use a 9v battery with a potentiometer to simulate a voltage of a common note that would be around 3-7 volts in the inputs. Is this a really dumb thing to do?

Well you need no external CV source to check the VCO. The coarse tuning potentiometer (FREQUENCY) let you set the frequency of the VCO from 16Hz to 30kHz...

OK, did you follow the trimming instruction on my web page ? Have you an oscilloscope to check the signals ? Did you etch your PCB yourself ? Have thorougly checked the quality of solder joints, checked also that there is neither solder bridge nor broken track (checked with a continuity tester). Have you checked that the components are installed correctly (no inversion) ?

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Yves
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akorbeti



Joined: Oct 30, 2008
Posts: 22
Location: Greece

PostPosted: Thu Feb 12, 2009 2:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi there again,

I followed your advice and doublechecked each component. There was one busted 10Ω resistor that gave wrong voltage to the TL074s.

Now, it sounds like something resembling a VCO but I think there is more stuff to fix.

The oscillation is very unstable and can't get it to go lower than 400Hz without losing the signal. I followed the tuning procedure but didn't help.

I use TL072s instead of OPAs. Could this be a hinder?

Anyway, thank you very much for your designs. I hope I can get them to work properly sometime to enjoy them too Smile

Cheers,

Aris
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akorbeti



Joined: Oct 30, 2008
Posts: 22
Location: Greece

PostPosted: Tue Mar 24, 2009 12:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi there again,

after long sessions of debugging I finally made it to work. The only issue I have now is that it drifts a LOT!. When I blow hot air with my mouth on the LM394 + resistor (not tempco), it gets a semitone off!! Then I blow from further away, thus cooling it, it gets a semitone off in the opposite direction.

Is this the normal driift I get without a tempco or am I having another mistake on the circuit that I should work on finding out ?

Can get a sample if you want to hear it!
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yusynth



Joined: Nov 24, 2005
Posts: 1219
Location: France

PostPosted: Tue Mar 24, 2009 12:45 am    Post subject: Reply with quote  Mark this post and the followings unread

akorbeti wrote:
Hi there again,

I followed your advice and doublechecked each component. There was one busted 10Ω resistor that gave wrong voltage to the TL074s.

Now, it sounds like something resembling a VCO but I think there is more stuff to fix.

The oscillation is very unstable and can't get it to go lower than 400Hz without losing the signal. I followed the tuning procedure but didn't help.

I use TL072s instead of OPAs. Could this be a hinder?

Anyway, thank you very much for your designs. I hope I can get them to work properly sometime to enjoy them too Smile

Cheers,

Aris


The fact that you had a busted 10 ohm resistor mean that there was some kind of short-cut. This means that some sensitive components may have been damaged too by the shortcut.

I would suggest that you replace all the active components if you have not done it already.

Check that the capacitor is well soldered, check also that the tempco resistor has both legs soldered to the right pads or to the right pins of the LM394, what tempco resistor are you using ?

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Yves

Last edited by yusynth on Tue Mar 24, 2009 2:23 am; edited 1 time in total
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akorbeti



Joined: Oct 30, 2008
Posts: 22
Location: Greece

PostPosted: Tue Mar 24, 2009 12:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Ummmm it is not a tempco Sad

It is a simple 1K 1% resistor.

Embarassed

With such a resistor is so much drift normal ?

Anyway, Thank you very much for such swift answer!!!!!

PS. Where can I find tempcos? (I'm in EU)

Thanks again,

Aris
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yusynth



Joined: Nov 24, 2005
Posts: 1219
Location: France

PostPosted: Tue Mar 24, 2009 2:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh yes that's completely normal that you have such a drift if you are using a mere 1% resistor. You must definitely use a tempco resistor 1K 3000/3300ppm (Positive temperature compensating resistor), the tempco is here to compensate for temperature dependent frequency drifts.

You can find these tempcos in EU at farnell :
manufacturer reference TYCO ELECTRONICS - 1624329-2
Farnell product reference : 1174306

otherwise at www.vintageplanet.nl :
C-2AQ1001F 1K 1% 3500ppm Tempco Resistor (Q81) . . . . . N 3.99 4.7
:

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Yves
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akorbeti



Joined: Oct 30, 2008
Posts: 22
Location: Greece

PostPosted: Tue Mar 24, 2009 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much for your answer!

this means that the rest of the VCO is probably right!

Thanks again for your wonderful designs. Ive also made a moog filter clone from your designs and it sounds absolutely billiant!!!

Cheers!
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yusynth



Joined: Nov 24, 2005
Posts: 1219
Location: France

PostPosted: Tue Mar 24, 2009 2:33 am    Post subject: Reply with quote  Mark this post and the followings unread

akorbeti wrote:
Thanks again for your wonderful designs. Ive also made a moog filter clone from your designs and it sounds absolutely billiant!!!

You're welcome Wink Have fun with these modules

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Yves
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