Current Lover 2015 Bias Procedures O-Scope

[Gristlepig]

Hey folks,

Sitting in my test rig I've got here a Current Lover 2015. Biasing it by ear has been a bit of a struggle as I can't seem to get a deep flanging effect without distortion.

I believe it was mentioned in the build notes that modifications to the bias may be necessary when running at 15v. Does anyone have any suggestions as to what might need modification? Perhaps raising the value of the bias pot?

I've seen it said that this circuit is best biased using an oscilloscope. I do own a nice, old school scope and sig generator, but it has been years since I've used either. Is there a basic procedure for biasing this baby up using the scope? Ideally one that might take into consideration my incredibly rusty skills on the scope?

Thanks so much folks.
Y'alls rock.

[alanp]

Most obvious to me would be to stick the o'scope probe on one of the BBD's output pins, and twiddle the bias until you get the biggest clean signal out possible.

[pdhonte]

Hi,
There's something on the Mistress Mystery page http://www.metzgerralf.de/elekt/stomp/mistress/align.shtml.

[Gristlepig]

Thank you, pdhonte!

That looks like exactly what I was looking for. Now to figure out which EM version corresponds to the Current Lover, but that shouldn't be too tough.

Thanks again!

[Gristlepig]

Got it running all nicey nice.
Sounds great.

And it was a necessary endeavor to get reacquainted with the O-scope.

Thanks again y'alls!

[pdhonte]

Hi,
That's great. I've never tried the scope calibration, was it easy in the end?
Cheers.

[Gristlepig]

Yes and no...
I'm really rusty with the scope, so I think I pretty much just biasing by ear while looking at some wacky squiggly lines on the scope screen. My ears just got burnt out listening to that 200hz sine wave for hours...
What WAS super helpful were the old EHX instructions to bias the 4013 clock at 35hz.

Since I'm running this jammy at 15v, I had to make some adjustments to the clock and bias trimmers. The 100k bias trimmer seemed inadequate since I was getting flange at the absolute end of the 100k trimmer's range and I assumed the "sweet spot" was just beyond it's limit...
Since the next greater size trim I had was 500k, it seemed fruitless to increase the pot value. So, after socketing and experimenting a bunch, I swapped r10–the 82k resistor between VB and the bias trimmer for a 47k. This put the sweet spot at just about the center of the bias pot's rotation.
But I was still getting a rather abrupt sweep of the flange effect—almost like there's a big hump in the middle of the sweep. The lead up and trail off are fine. It's much more noticable imputting a sine wave vs a guitar tone. I read on a couple forums that the 15k r30 resistor was too low and should be raised to something like 39k for 9v operation. So I figured at 15v, 22k would be a good place to start. No noticeable difference in sound, though raising that resistor value did put 35khz outside the range of the clock trimmer. So back to 15k it is.
Got it sounding pretty good, save the hump in the middle of the sweep.
Anyone have any ideas as how to smooth that out? I tried a different 3007, but no change. Maybe I should swap out the clock chip and see what happens?
Go easy on me. I'm a fuzzhead, these modulation circuits give up their secrets very slowly with me...

[Scruffie]

You need to bias the clock for 70khZ, the original used a 512 stage chip, as we're using a 1024 stage, to get the same delay time range, the clock frequency has to be doubled, that's probably why your sweep sounds off.

[Gristlepig]

Thank you so much Scruffie!

And 70khz was right around where my clock trimmer jumped to when I switched the 15k resistor to 22k.

Will report back results of next round of tweakage.

[Gristlepig]

Well, I biased the clock at 70khz, and while this definately smoothed out the sweep, it did introduce some very noticable, high pitched background noise which followed the sweep of the vfo. Stumped... and starting to feel like I was in over my head, I decided to see what would happen if I raised r30 from 15k to 22k. Much better, but still a bit noisy, I just went by ear from there and got it sounding perfect. Thanks for your input there Scruffie. It sounded pretty bad, comparitively at 35khz.

Things then got really interesting when I tried to box it up...

I Drilled my 125BB using the appropriate drill template and opted for top mounted jacks, eschewing the fx loop. Well, the jacks wont fit as my caps are too tall... same goes for the larger footprint of the 3prr footswitch pcb, which overlaps the main board and cannot fit for that big 220uf cap...

I finally shoehorned the whole hot mess in the case by moving several electros to the bottom of the board. The Footswitch had to be raised considerably, as the 3PRR overlaps the board by about 5mm. And the there's the cap I had to lay on it's side over a bunch of resistors to make way for the input jack. The pcb is a bit less elegant looking now, but holy shit it's finally boxed!

Thanks y'alls for your help.