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Music Easel Pitch/Frequency CV input question.
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge  
Author Music Easel Pitch/Frequency CV input question.
Lento_Zoom
I'm looking for a solid answer on this and can't find one anywhere.

I can't get either oscillator to track when I run pitch CV from the 218 to the MO frequency or CO pitch CV inputs. I can get like a C-F with the CV slider all the way up.

Everything tracks fine when I use the normal Keyboard Pitch input.

Is this expected behavior?

If so is there any way to change it?
ArguZ
Depends...
The CV inputs are resistor "locked"
Dave Brown describes on his page that the values used in the BOM are giving you exactly that and need to be changed.
https://modularsynthesis.com/roman/buchla208v2/208spss.htm
two thirds down , among the Modifications list

So if you have a clone 208p or 208r chances are that is it.
On the BEMI the values are fine and lowering the slider should give a pretty good result.
Lento_Zoom
Thanks! I'll check that out. I do have a clone.
Lento_Zoom
ArguZ wrote:
Depends...
The CV inputs are resistor "locked"
Dave Brown describes on his page that the values used in the BOM are giving you exactly that and need to be changed.
https://modularsynthesis.com/roman/buchla208v2/208spss.htm
two thirds down , among the Modifications list

So if you have a clone 208p or 208r chances are that is it.
On the BEMI the values are fine and lowering the slider should give a pretty good result.



Is this the section that you were referencing?

"On a recent repair the CO coarse slider useful range was only about the first half travel. I cut the trace on the motherboard and added a series resistor in series with R45 to reduce the gain of the CO coarse slider. You can cut a trace on the rear of the motherboard at the X and jumper a resistor between the pad and feed through (marked by arrows). I determined the resistance to add empirically."
davebr
Lento_Zoom wrote:
ArguZ wrote:
Depends...
The CV inputs are resistor "locked"
Dave Brown describes on his page that the values used in the BOM are giving you exactly that and need to be changed.
https://modularsynthesis.com/roman/buchla208v2/208spss.htm
two thirds down , among the Modifications list

So if you have a clone 208p or 208r chances are that is it.
On the BEMI the values are fine and lowering the slider should give a pretty good result.



Is this the section that you were referencing?

"On a recent repair the CO coarse slider useful range was only about the first half travel. I cut the trace on the motherboard and added a series resistor in series with R45 to reduce the gain of the CO coarse slider. You can cut a trace on the rear of the motherboard at the X and jumper a resistor between the pad and feed through (marked by arrows). I determined the resistance to add empirically."

No, that is to make the Coarse frequency slider have full range without a dead zone at the top.

The CV inputs with the sliders at the top have a 120K series resistor to the CV summer. R14 for the CO which sets the scale is 120K with a 20K trimmer in series with it, so say 130K typical for 2V/Oct. You should be able to tune the CO to 2V/Oct with the CV attenuation slider. However, if your 208 is tuned to 1.2V or 1V/Oct there is too much attenuation in the CV input to scale. You would also have to change resistors on the motherboard.

Dave
Lento_Zoom
Thanks Dave! I think I understand. I did change card 7 r14 to 44k to get 1.2v/oct tracking. So then I could probably get away with adding a 68k resistor in parallel with r22 on the mobo?

Would I want to do the same thing for r42 on the mobo for the MO frequency cv slider? I changed card 6 r5 to 91k and r58 to 390k.

davebr
Lento_Zoom wrote:
Thanks Dave! I think I understand. I did change card 7 r14 to 44k to get 1.2v/oct tracking. So then I could probably get away with adding a 68k resistor in parallel with r22 on the mobo?

Would I want to do the same thing for r42 on the mobo for the MO frequency cv slider? I changed card 6 r5 to 91k and r58 to 390k.

The CO CV input resistor is R42, the MO is R44. Those are the resistors you want to lower for 1.2V tracking on the CV input. Candidly I would just put a trimmer on it and tweak it until close, then measure it and use that value. Or, you could either replace the resistor or add a trimmer in parallel and actually scale it when the slider is at maximum. I don't know if there is room on the back side of the PCB to do that so you would have to check.

Dave
Lento_Zoom
Ahhhh. Thanks! I’ll let you know how it goes. I won’t be able to work on it until Monday.

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davebr
Lento_Zoom wrote:
Ahhhh. Thanks! I’ll let you know how it goes. I won’t be able to work on it until Monday.

Note you don't want to take those trimmers all the way to 0. The "proper" way would be to like use a 25K series resistor with a 50K trimmer to give you 25K - 75K range and to drop the original resistor out of the board. Without a series resistor just set the trimmers to mid-point to start.

Dave
Lento_Zoom
Thank you again Dave! I got it all set today. I didn't have any 50K trimmers and ended up doing 390K resistors and 10k trimmers after soldering wires to the board an using alligator leads to select values. Tracking well enough now with the CV sliders at max. If we ever cross paths around town, I owe you beer/beverage of your choice!
davebr
Lento_Zoom wrote:
Thank you again Dave! I got it all set today. I didn't have any 50K trimmers and ended up doing 390K resistors and 10k trimmers after soldering wires to the board an using alligator leads to select values. Tracking well enough now with the CV sliders at max. If we ever cross paths around town, I owe you beer/beverage of your choice!

Glad it worked out for you. If you want to cross paths, just visit the vintageTEK museum on a Friday and before 3pm. I am generally there every Friday. It's a pretty special place.

Dave

governor blacksnake
from my build notes:

Card 6:
Change R5 to 82K (scales keyboard CV in)
Change R9 to 191K (offset)

Card 7:

Change R14 to 60K (scaling)
Add a 2M resistor between molex pin 3 and -15V (more offset)

When calibrated, this should give good 1.2v/oct scaling, and with the mod freq slider all the way UP and principal freq slider all the way DOWN, you can quickly fine tune them to unison and use keyboard voltage for a nice tuned response.
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