VCO sine wave adjustment

[darkomen]

I'm in the process of adjusting the waves on the VCO. I scoped the saw and triangle waves and got them looking good and well adjusted according to Ray's VCO page. The sine on the other hand is not looking too well, it just looks like bumps (or hills without the valleys). I fiddled around with the adjustment trimmers but didn't really get anywhere. Has anyone encountered this before, and if so any advice on how to adjust it correctly?

Thanks!

[jordroid]

If you have a nice looking triangle, there shouldn't be too much between you and a good sine. The trimmers are a bit fiddly and interactive, but if you suspect that something is wrong at least there are not too many components to check.

Check everything around U5-A, make sure the trimmers R60 and R64 are correct and soldered well, and that R61, 48, 44, 45, 51 and 53 are the correct values/no cold joints.

Bumps without valleys sounds like it is rounding the triangle (distorting) nicely but is improperly biased, so i would suspect something around R60/R61 isn't right, but that's just a guess.

If you aren't getting anywhere you could try popping in a new 13700 just to be sure.

I have built a few of these and the sine shaper does work nicely, so you'll get there

[darkomen]

So I took another look at my ramp and triangle on the scope. I had the ramp centered around ground, but when I adjusted R5 and R7 to get the triangle it is no longer around ground (that is ground is the center of the scope right?), and the triangle, although it looks good, is not centered either. Could this be causing my problem? Is there an initial set up with frequency that I should be using? Could the 1v/oct adjust or high freq adjust have any effect on this?
Also, what does "cold joint" mean, I've never heard that term before.

[jordroid]

R5 should be the only trimmer you need to adjust to get a good looking triangle, and when the triangle looks right the ramp should be centered around 0 volts.

After the triangle looks right only then should you worry about R7, in order to center the triangle around 0 volts.

After the triangle both looks good AND is centered then you can start messing with the sine adjustments.

With my MFOS VCOs the triangle, and thus the sine would change a bit at the extremes of the frequency range, a glitch would appear on one side of the triangle at the low end of the range, it would look great through most of the middle, and then a glitch would appear on the other side of the triangle at the high end of the range.

This is caused by slight changes in the amplitude and bias of the ramp core through the frequency range. It is not a problem in practice, because the glitches are at the extremes of the range where i am unlikely to be playing anyways, and the effect is just a few extra harmonics added as it gets higher in frequency(and i mean like dog whistle territory).

So my suggestion is to trim the waveshapers with the frequency set to the middle of where you will be using it most, and when things are looking nice sweep it up and down a bit to see if things change and readjust as necessary.

The 1v/oct and high frequency trimmers should not have an effect on the waveshape trimming, unless something is wrong.

A "cold joint" or "cold solder" is a joint that has been soldered, but is not electrically sound. Lots of google hits on what it is and how to avoid and correct it. A visual inspection is often good for finding funky joints, if a joint looks grainy, dull, pitted, cracked or dirty it should be suspect, though sometimes you will not be able to tell visually.

If you cannot get the triangle to look good AND have the ramp centered at the same time, then something is probably wrong around U4, most likely a wrong resistor value, i think, or possibly something is strange with the ramp core, but that seems less likely.

Also, ground is only at the center of the scope if the scope is set up properly, I don't know what your equipment or your experience with it is, so i'm just throwing things out there.

Cheers

[darkomen]

So I readjusted the triangle, and have it centered around ground. The sine wave seemed easier to adjust this time, although it doesn't look perfect. I still have hills, but the valleys of the sine are slightly better.
Well, I have another mfos vco that needs wave adjustment, so I'll compare how it comes out and maybe I will see my problem.

[jordroid]

Cool, they are a bit fiddly to tweak, sounds like you are pretty close. It is often recommended to trim the sine by ear after it is fairly close, if it sounds good, it's good!

jordan