VCO Square Problem

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15khz
 
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Joined: Mon Apr 03, 2017 7:35 am

VCO Square Problem

Post by 15khz »

Hello, I bought a willzyx kit a couple years back and have had it tucked away for a while, only just now getting around to it.

I'm having an issue where my square wave seems to morph shape as I change the CV to increase the pitch. It has a strange attack to it. The pulse width is also wrong but that could be due to me using a sound card scope. The saw wave works perfect, the problem is only with the square.

I found a couple similar threads where users noted their square had a similar shape until they installed R36. My R36 is installed and checked to be the right value and tested good. I've swapped out every component in the circuit, double-checking the components with my meter and cross-referencing with both the online build guide and the printed instructions that came with the kit.

I'm going to compare the voltages to the modified eagle layout tomorrow and also desolder the components in the square circuit to check for shorts. Hopefully that will fix it, but in general I would be very appreciative of any help regarding what components I should be looking at here.

I've attached some pictures to show the waveform at increasing pitches.

Thanks!
squarehi.PNG
squarehi.PNG (25.96 KiB) Viewed 1076 times
sqauremid.PNG
sqauremid.PNG (21.53 KiB) Viewed 1076 times
squarelo.PNG
squarelo.PNG (16.4 KiB) Viewed 1076 times

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antto
 
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Joined: Thu Apr 15, 2010 3:21 pm

Re: VCO Square Problem

Post by antto »

first, you have to know a few things when measuring with a soundcard
most soundcards are AC-coupled, which means you can't measure a DC signal (the 303 oscillator is biased)
unlike a scope, the impedance of the soundcard is probably lower, so when you attach it to a "weak" part of the circuit - the low impedance of your soundcard will probably affect the circuit, and your whole measurement may be inaccurate

when i probe the VCO with my soundcard, i put at least a 100k resistor in series on my "probe"

i suggest you try attaching your soundcard thru a resistor - 100k or more (you will have to push the gain in your audio recording) and see if that changes the shape

the 303 square indeed varies in shape depending on the osc frequency, but things don't look exactly sane in your pics

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15khz
 
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Joined: Mon Apr 03, 2017 7:35 am

Re: VCO Square Problem

Post by 15khz »

Wow, didn't even think of that but it makes total sense. I went ahead and put an 100k resistor in series with my probe and the square looks perfect now!

I've gone ahead and borrowed my friend's actual scope for the rest of the project so this doesn't happen again.

Thanks!

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