Increasing Oakley VCO "Course" knob range

[analogsplitter]

I would really like to have the "Course" knob on my Oakley VCO be able to sweep the VCO's entire frequency range. Right now, the knob only spans a couple octaves (and I need to use CV control to adjust it further).
How would I go about doing this?
Thanks much!

[terjewinther]

I believe the range of the tune knob is set by the resistor leading from the knob to the CV summing op-amp, so changing the resistor should change the range.

[Synthbuilder]

How would I go about doing this?

Like Terje says you need to change the value of the resistor that connects to the wiper of the coarse tune pot. That would be R8 on issue 7 VCO boards. You need to lower the value to make the pot more sensitive. Halving the value will double the range of the pot.

But be warned increasing the range of the pot may make the VCO very difficult to tune. Pots are not accurate devices as the wiper rarely moves smoothly over the resistance track, so even if you are careful, trying to tune it to a certain frequency can be problematic.

[analogsplitter]

Great info, and thanks for the word of caution regarding tuning.

Would it be ok to put a potentiometer in place of that R8 resistor? I like the idea of being able to have a variable range for the course knob (controlled by a "R8 pot"), which would allow huge freq sweeps and also moderate fine tuning.

[Synthbuilder]

Would it be ok to put a potentiometer in place of that R8 resistor?

It'll not be a good idea to use a pot on its own as you don't want to reduce R8 to zero ohms. But a 220K pot wired as a variable resistor in series with something like 140K might work.

Alternatively, you could just use a simple switch. The switch would be wired in series with something like an 140K resistor and then wired in parallel with R8. Turning the switch on would put the unit into 'wide tune' mode.

[analogsplitter]

Thanks, I never would have known about keeping it from going to zero ohms. Nice idea on the switch, it could also be a performance tool (jumping between frequencies with a switch).

If I go with the potentiometer, I'll get a 220k pot and the 140k resistor. What would you recommend if I get a few pots and resistors of different sizes, in case I need to experiment a little with the values? I'm not too familiar, I should be looking for "linear" pot right? Any other specifics (feel free to pass on a link to parts if you like).

Thanks again for all the help.

[KSS]

Alternatively, you could just use a simple switch. The switch would be wired in series with something like an 140K resistor and then wired in parallel with R8. Turning the switch on would put the unit into 'wide tune' mode.

Yes. It's a really good choice for a VCO!

@analogsplitter
Here's how it looks on a vintage Polyfusion VCO*.

Pull pot or push-push pot can allow the mod -depending on PCB and pot mounting- without needing panel changes.

*There are a few other mods on that densely paneled VCO and one -the toggle switch next to one of the Voct input jacks- might be a better way to 'jump between two notes'