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 Forum index » DIY Hardware and Software » Thomas Henry designs
Thomas Henry VCO 1- Fixed Frequency Issue
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riprickwright



Joined: Dec 03, 2016
Posts: 2
Location: USA

PostPosted: Sat Dec 03, 2016 6:31 pm    Post subject: Thomas Henry VCO 1- Fixed Frequency Issue Reply with quote  Mark this post and the followings unread

Hello All:

I've been a lurker of these forums for a year or so now, mainly building VCOs that don't require an OTA thanks to posts from the members of this community and the skills that I've gained from my EE internships. Now that I have graduated, started my job, and settled in my new place I have finally have the time, money, and energy to begin building more complex circuits and hopefully give something back along the way.

In order to keep debugging simple I decided to only bread-boarded what was on one page 1. So my only expected output is the triangle wave. The image attached shows the only output that I receive. Instead of a 10VPP triangle wave with variable frequency, I get a distorted ramp wave that's range is -15 to ~+10V and has a fixed frequency of ~30.48kHz. I think the waveshaping issue is due to the fact that I'm not using using the exact resistances that are specified (no more than 200 ohms different) but I'm a little lost on why the frequency isn't changing.

My initial thought was that the bias current into the CA3080 was flawed, however, using my multimeter along with adjusting the pitch CV and the tuning pots I've noted that the max current that is sinking into the bias pin is a little less than 640uA. Because of this I'm beginning to suspect that I received a bad batch of CA3080s from ebay as I've tried 4/5 of them and the result is the same.

So my question is, what is the expected output current range from the CA3080 to the TL072? Also, has anybody come across anything similar with these CA3080s and if so should I switch to an OTA that's in production such as the LM13700?

Any insight will be much appreciated.


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riprickwright



Joined: Dec 03, 2016
Posts: 2
Location: USA

PostPosted: Thu Dec 08, 2016 4:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

It looks like the CA3080 was the culprit. I lucked out and was refunded my money Smile I replaced the CA3080 with a LM13700 and it works like a charm! Here's the output with the course tuned all of the way up.


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Cfish



Joined: Feb 24, 2016
Posts: 473
Location: Indiana

PostPosted: Thu Dec 08, 2016 7:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Wow, I have purchased a lot of 3080s on eBay and not had any trouble.

Do you mind saying who the seller was, as I would like to avoid them.

TH VCO1 was my first real synth project I built. Used all TL071s spread out on a 5x7 perfboard so I could get in and test easily. It's a great VCO.
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