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 Forum index » DIY Hardware and Software » YuSynth
VCO 1V/Oct Tracking Problem
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Marco Piatti



Joined: Aug 08, 2018
Posts: 6
Location: Argentina

PostPosted: Wed Aug 08, 2018 6:37 pm    Post subject: VCO 1V/Oct Tracking Problem
Subject description: Can't track more than one octave properly
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Hi.

I made a VCO with a LM394 (metal can package) and it's respective 3300ppm 1K tempco resistor.
I'm calibrating the 1V/Oct Tracking with my Arturia Keystep's CV output and i can only manage to get just one properly tuned octave before it starts drifting off. By the point i get the second octave right the first octave i trimmed before goes out of tune.

This left me astonished since I expected to get it to track at least 3 or 4 octaves and I'm really clueless at this point. Maybe my LM394 isn't working properly?

Are there any reference voltages i should check around the expoconverter?

Thanks in Advance.
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JovianPyx



Joined: Nov 20, 2007
Posts: 1693
Location: West Red Spot, Jupiter
Audio files: 194

PostPosted: Thu Aug 09, 2018 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Link to schematic?
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Marco Piatti



Joined: Aug 08, 2018
Posts: 6
Location: Argentina

PostPosted: Thu Aug 09, 2018 11:47 am    Post subject: Reply with quote  Mark this post and the followings unread

http://yusynth.net/Modular/Commun/VCO/new/VCO-V2.gif
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JovianPyx



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Location: West Red Spot, Jupiter
Audio files: 194

PostPosted: Thu Aug 09, 2018 1:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

I see a note on U7A output that should be 5 volts +/- 2%, so I'd check that. I don't see any other reference voltages. I'd measure it both at the source (U7A pin1) as well as the points where it's used.

I'll guess there are good instructions for adjusting pots T1 and T2 and that you've followed them.

There are notes regarding resistor changes if +/- 12v PSU is used. These are important if you didn't use +/-15volts. Any other PSU arrangement such as +/- 9v won't work without appropriate recalculation of those resistors.

Anywhere that a resistor is marked with a precision such as 1% - that should be strictly followed.

Yusynth is respectable stuff, so I'd trust that the schematic is accurate. But you don't mention the construction technique - if this is a purchased board you populated, have you checked that all the component values are correct? Soldering quality can be an issue. I've fixed more than one thing by reflowing every solder connection. Bad solder connections can add resistance and some cases, if there's dirt, oil or corrosion (even if you can't see it) involved resistance or even a crappy diode can be formed.

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Marco Piatti



Joined: Aug 08, 2018
Posts: 6
Location: Argentina

PostPosted: Thu Aug 09, 2018 3:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, I'm 100% sure I've caused the problem somewhere, I've strictly followed every notation and step but there's always a way to jinx it.

It came to my mind just now that my on-board voltage regulators are not exactly outputting the calculated 14.2V reference voltage marked on the site but a bit less (around 13.7V) that made the 5V voltage drop to 4.5, so i changed resistor values to make it 5 again.

Could this also be altering something at R20? Since it's the pull-up resistor LM311 Comparator.


This happens in all 3 VCOs I made, so I don't really think it's a "Situational" build problem like a bad solder point (I wouldn't discard it though).

I'll check if all my resistor values are correct just in case
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JovianPyx



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Posts: 1693
Location: West Red Spot, Jupiter
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PostPosted: Thu Aug 09, 2018 4:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't see how R20 or it's function would be affected.

I agree that it's likely not a situational problem if the problem is copied identically into 3 units.

14.7 volts isn't out of spec for a 15 volt regulator which is probably what the +/- 2% note on the 5 volt reference.

If it won't track several octaves (as it should do) then something else is wrong.

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