VCO - how to debug

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

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

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

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

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

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

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!

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

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

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

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

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
:

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

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!

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France