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 Forum index » DIY Hardware and Software » Thomas Henry designs
XR VCO (almost) worked first time on the breadboard :-D
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elmegil



Joined: Mar 20, 2012
Posts: 1448
Location: Chicago
Audio files: 14

PostPosted: Thu Apr 26, 2012 12:01 pm    Post subject: XR VCO (almost) worked first time on the breadboard :-D Reply with quote  Mark this post and the followings unread

I get a pulse no problems. My sine/triangle is weird, and I wouldn't have even found this if I'd used the specified switch... For some reason if I have the switch in sine or triangle mode, I get no output. However, if I have it in the middle (I'm using an on/off/on switch because it was handy) I get oscillation at the same frequency as the pulse.

If I adjust the skew, about half way it switches from a sine to a triangle (I think, just identifying by sound), but I hear no difference other than that--no gradual change, just a sudden switch in shape.

I'll hook a scope up to it later tonight when I can do real troubleshooting, but I couldn't wait any longer to hear it Smile
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elmegil



Joined: Mar 20, 2012
Posts: 1448
Location: Chicago
Audio files: 14

PostPosted: Thu Apr 26, 2012 8:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

So now comes the time for the questions.

First, I figured out my big mistake; had incorrectly wired up the sine/triangle shape selection circuit, both 5.6k's were going to +15 and the cap went to +15 instead of ground as well.

So now I was getting sine/triangle output, and moved on to adjustments.

The closer I get to correct sine shape, the lower the output voltage; I was down to almost 2Vpp by the time I was reasonably happy with it. The triangle was at 6Vpp, and I had already increased R21 as mentioned to 22k.

Interestingly, my square wave output is nearly 26Vpp at least according to my scope.

The adjustment also seems to have dragged the wave down DC wise to a level where the offset trim isn't able to help. I probably ought to double check that though.

So I'm not sure how/why tweaking this 500R trimmer drops me so hard. I need to go back and look at the datasheet more thoroughly.

Another thing I'm wondering about is resistor precision. I've just used standard 5% resistors everywhere. I could probably do with replacing the input stage resistors with 1%s where I can (I have plenty of 100k 1%ers), but that's more a matter of making sure that all the various inputs are on the same scale. Is there anywhere else in the circuit where 1% is going to make a big difference?

Speaking of precision, I have a mylar cap as noted for C7 & C8 (though not using C8 just yet), but everything else is either standard ceramic or aluminum electrolytics. Are there any of those that are likely to cause me grief if they're not matched or precise?

I'm still hearing/seeing the initial skew basically take me from no change, no change, wham now it's an octave higher and skewed. That's listed as a linear pot, should I actually be using an audio pot? And I'm wondering if whatever that is about could be related to my low levels on the sine.


Of course I'm half thinking out loud here. There's plenty for me to go back over and double/triple check for myself. I found a couple good suggestions for tests I haven't done here: http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=synth_vco And I will be digging further into here as well: http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=synth_xrvco_inside

(Thank you warrenmaximus)


But if anyone sees something here that strikes them as 'oh yeah, that would be THAT problem/mistake', I'm all ears....
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