Scott Stites/Buchla 291 adaptation

[Scott Stites]

If you mean the schematic on my original site, I had a VC Res circuit in there at one point. I've lost the ability to edit that site, so it's more or less a trainwreck of things I'd like to change (or delete).

I agree with Peter, BTW, about the voltage control - there are much easier ways to control the frequency.

Cheers,
Scott

[numbernone]

First question for the sucessful builders of the 291... what the hell did I do wrong?? I made a PCB based from the Verbos design, included Scott's changes, and altered any values to gel with the revised schematic. I have been over the layout and parts placement with a the fine tooth several times, and everything looks A-ok.

Having now completed the build...NADA!

Not even a peep. I traced the signal thru the inputs to the TL082. Its fine until it enters the IC. Should I be able to pick up the signal from the C output? I have nothing. I should note that I am using the simple signal tracer from the MFOS site.

I got the idea to skip the input mixer section, and jump my input directly to the other end of "C". Now it gets weird, the signal passes thru all three outputs with no filtering whatsoever...the same straight square wave I am feeding it comes out the LP HP and BP outs. This seems incredibly strange to me, as if there were no circuitry at all between. none of the controls have any effect. Am I going crazy??

Thanks for suffering this fool

[Scott Stites]

Hmmm....The TL082 is acting as mixer. You shouldn't be able to hear the signal at pin 2, because this is virtual ground, but you should hear it loud and clear at pin 1, which is the output of the first stage of the mixer. And, of course, you should be able to hear it loud and clear at pin 7 (point C) as well.

Bypassing point C should work fine - in fact, that's the original input of the filter - the mixer was put in there just to easily deal with multiple input signals.

The fact that you can hear the signal on all three outputs isn't crazy, but the fact there is no filtering going on is obviously not right. It's possible the control section isn't drawing any current through the LEDs for one reason or another, or something else is going on (gee, that's helpful, isn't it?).

There could be some basic misconnection that's making it happen all wonky. You might try solving the problem concerning the mixer first - it may lead you to what the problem is, and, if so, will be generally easier to tackle.

Of course, check your rail voltages and whether those voltages are making it to the supply pins of the op amps. Make sure ground is ground. Check the output of the TL072 with a DMM - is it railed high or low? If you put in a sine wave instead of a pulse wave at point C, does it sound clear on the output or is it all harsh and distorted?

Make sure D1 and D2 are each pointed in the right direction as well.

Cheers,
Scott

[numbernone]

OK, rail voltages are +/- 14.75 VDC, +/-15 before the 10 ohms. I checked that the IC supply pins are getting their respective voltages. The output pin from the mixer IC is showing like +13.5 V. D1 and D2 are properly oriented.

I have tested several 82s in the mixer, no change from one to the next. I pulled all the IC's except the mixer, same deal. Is that Ok to do?? I would assume that there should be no net change to that small circuit if the others are pulled.

I have found that I used a 68k resistor before D1 rather than 68 ohm. I will swap it out today if I have or can cobble something close. Will this have a big effect?

One thing I did find was a tiny bit of cat hair stuck in a solder point on the positive voltage, it did appear to bridge the negative supply!!!! YUCK!!! It did not seem to affect my meter readings but... I have always wondered, if the + and - rails are bridged what will be destroyed??? I understand the ICs will be shot, does this also extend to polarized cap? Vactrols?

Thanks for all the help.

[Randaleem]

[numbernone]

Update with some progress.

The mixer section still aint happenin'. Its just weird. I am hopping straight to the old input. Some filtering is starting to happen. for a square wave, the BP out has a good sweep with the initial center pot. FM input is also functioning tho not a giant range. Resonance is adding a bit of peak. At this point the BW pot must be fully CCW to pass a tone, the slightest CW turn pulls the signal out instantly, its CV is acting like a gate, cutting in and out. HP out acts just like the BP.

LP is passing no signal. Now the Center freq CV is functioning and in observing its behavior, it appears to function almost bipolar, with zero input at its center position. However the sweep it is producing sounds exactly the same to my ears, CW or CWW. But damn it does sound nice! Sine waves are in the same boat, although the resonance seems much more pronouced on the square wave. Probably forgetting a bit but I'm hungry!

So thats where I stand before dinner tonight. Good bit of progress , finish line hopefully in sight. Incomplete yet BADASS filter.

[numbernone]

Thanks for the info Randal, the NYC allegory really hits home! Trying to ride my bike as much as possible these days. As my last post says it is starting to get its legs but is hardly up and running yet. I will change that resistor post haste.

Looking forward to those QFG boards, as well as your model of the 291 should it come to pass.

[Randaleem]

[numbernone]

Swapped out the 68k to 68 Ohm. Sounding much more rubust but my silly mixer problem is still present, as well as the Bandwidth cutting it all out. LP out is now in action as well so its coming together in fits and starts!! WOOWOO

[Scott Stites]

How's the filtering coming along Numbernone?

Oh, yes, I somehow missed this:

[Randaleem]

Scott,

Makes sense. BTW, I think I've seen those quotes somewhere before

Thank you,

Randal

[numbernone]

Well I have been in Montana for the last 5 days...visiting my girlfriends family. Only to be chased away by the most raging forest fire in years!!! Serious yikes...Needless to say i will be working on the 291 again starting today, and certainly pestering the smarties with ????s.

Thanks for asking!!!

[Scott Stites]

Oh, that's a shame. I heard just yesterday on the radio those trees have really been taking a beating from some beetle or whatever. I was in CO earlier this summer, and saw the same thing going on the with the trees in Rocky Mountain National Park. Made the north part of the park look like one giant tinderbox.

[numbernone]

Ok, I have found that a mega slight adjustment to the BW manual pot, say 5 degrees seems to encompass the full range of its function, any further CW and the pass band is closed completely (I think). Is there a single resistor perhaps that is controlling the range? I had to swap out one on my wave multiplier that had bogged down the PWM range, hoping something similar is happening here, and a wizened one may point it out before I start plucking out parts. Thanks.

[keithwin]

Wow I am glad I found this topic.
I started a 291 clone years ago from Marks design and was never to happy with my result - never had the time to troubleshoot either.
Its just been on a shelf looking good.


I hopefully will be changing some 68K resistors this weekend,
Thanks Scott!!

[3vcos]

Where did you find the skirts for the blue buchla knobs?

[keithwin]

Group buy years ago, they are one piece moulded,
I got the small and med knobs - The large ones as on the
oscs we too expensive.

[3vcos]

Oh well, I have lots of the skirtless buchla style knobs. Anyone know if you can just buy the skirts anywhere?

[Noiseconformist]

[funkyfarm]

Hi,

I've just discovered mark's blog today, and it reminded me that only one 291 is not enough for the greedy man I am.

My only complain about the 291 that is already here : I have to use (waste) a complete three channel mixer before feeding CF CV input...
(keyboard tracking, plus a bit of envelope, and just a touch something else...).

So, I need more CV inputs on the next BPF.

[funkyfarm]

[ericcoleridge]

I've just etched my first Verbos 291 PCB. I was hoping someone might be able to help me with a few details that I'm unsure of:

I've read here that the (2) 2N3566 gen purpose NPNs (per filter) can be substituted with 3904s, and that the pin-outs are compatible. Is there anyone here who has successfully completed the 291 using this substitution?

Also, the (2) diodes in this circuit are both 3.9v Zeners, yes?

Last, which of these caps call for critical high quality polystyrene/silver mica? I'm guessing the 910pf; any others?

I appreciate any suggestions anyone could make!

[guitarfool]

[ericcoleridge]

On the Buchla schematic, and the Marjan Urekar re-draw, there is a 5k pot for 'Q'. However, on the Verbos PCB, the pads for this pot are labeled 'Res'. Is this the panel mount 'Q' from the Buchla module?

Then, there are pads labeled 'Q' for a 100k pot. Looking at the schematic, this looks more like an input attentuator to me. What I'm wondering is, was this a trim pot on the Buchla 291? As far as I know, there is no panel mount input attentuator on the 291, so, I'm assuming it was a trim. But why does Verbos label it 'Q'? This is very confusing.

[Luka]

i have a new adaptation of the verbos layout with mods if anyone is interested. input mixer (with a error fixed from scott's mod schematic), cv processor, hp/lp tap/buffers - single filter per board.

here is a jpg
ill upload the pdf soon



Q has always been resonance in my book, and its on the buchla panel