XRVCO init skew

[Pehr]

Hi!

I've built the XRVCO, but I'm having a little problem with the initial skew.

Nothing really happens when I turn the knob until around 12 o'clock and it seems that it doesn't skew enough at fully cw.

I'm running on dual 12V supply, do I need to make any major changes for that?

My first thought is to change R44 somehow... lower?
http://www.birthofasynth.com/Thomas_Henry/pdf/XR-VCO/xr_schem1.pdf

Any ideas?

[Tim Servo]

Hey Pehr,

My first thought is maybe R44 is too large. This controls a DC offset voltage going into the Skew CV summing amp (IC3a), and the offset might be wrong if you're running on +/-12V. I'd try putting another 2.2M in parallel just because that would be quick and easy to do, and see if that has the desired effect. If so, then experiment with different values. My guess would be something around 1.5M to 2M. Once that is set, you should hear the effect of turning the manual Skew control earlier in the pot's rotation. Once that is set, if the manual Skew control doesn't have enough range (but the Skew CV is okay) then make R40 smaller. If the manual Skew AND the Skew CV both suffer from not enough range, then maybe make R41 larger. I don't know if you're using a Magic Smoke TH-102 PCB or if you've made your own, but these same tips should apply in either case. Hope this helps!

Tim (a little skewed) Servo

[Pehr]

Thanks for your answer Tim

no, I'm using a home etched bugbrand PCB

I'll investigate R44 first, I think that would be enough.
Since at fully cw it is almost at full skew.

I haven't tested the CV response yet, i'll do that too.

to be continued...

[Pehr]

adding positive constant voltage centers the range of skew

however, the skew doesn't skew enough
even when I keep adding positive voltage

I know that it will not be perfectly symmetrical.
but it does NOT look as good as imgae below.

every other minima differs ~2V

should I examine R11 and R17?

[Pehr]

here's my xr.
as you can see, it doesn't skew enough...