LMNC VCO Potentiometer sweep.

[Marcuz]

I have just got this VCO working thanks to the kind people on here and was buzzing when I nearly blew my eardrums out when I plugged in the square wave and discovered the thing worked this morning. I was just wondering if there was a way to make the course tune potentiometer sweep in a larger radius? I appear to be getting a full pitch sweep but ot doeant start till about the ten o'clock position on the pot?

[pjbulls]

If I understand correctly your tuning pot does change the frequency of the VCO, but only reaches audible frequencies from about 1/3rd rotation onwards.

There's quite a few ways to skin this cat, and we'd have to know whether you built the 'basic' or 'performance' VCO. Both are based on the 3340 IC but they implement tuning a bit differently. Assuming you have the performance one, the coarse tune pot looks like this:



You can have the pot 'start' at a higher voltage (frequency) by adding a resistor between the first leg and ground. The larger the resistor the higher the voltage will be when the pot is fully CCW, I'd start with something in the 33K region. This will not change the top end, if you want that to go higher as well you could reduce the value of R23.

In the basic VCO the wiring is a bit different but the same principles apply.

[Marcuz]

Thanks for your help I will give this a try.

[Marcuz]

Tried this and worked a treat. Thanks. Instead of the fixed resistor could I add another pot so I could adjust to my liking?

[pjbulls]

Yes, definitely. You could also put a little trimpot on the PCB instead of one on the panel (for set-and-forget adjustments) but that's up to personal taste.

I noticed that Ray Wilson has an article and even a calculator on this specific topic on this site, might be useful.