Redrawn Buchla 158A VCO schematic

Fernando · Last edited by Fernando on Thu Dec 08, 2022 5:39 am; edited 7 times in total

Hi all! hope you are all fine

I made a redrawn of the Buchla 158A schematic, the original is hard to read.
It includes the extra transistor and resistor Mike Peake (ebolatone) indentified in actual 158 boards and some notes.

I don't have a web site, so I'm posting it here for the moment, so it's accesible to anyone.

I'm finishing a pcb for it too, with a couple additions, but that's a different affaire.

Greetings and hugs for all!

Buchla_158A_redrawn.pdf

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Fernando · Posted: Wed Apr 13, 2022 2:51 pm Post subject:

A couple of key links with previous work:

http://ebolatone.blogspot.com/2012/12/buchla-158a-pcb-build.html

https://modularsynthesis.com/buchla/158d/glm_158d.htm

Grumble · Posted: Wed Apr 13, 2022 2:58 pm Post subject:

One remark:
R4 with the label Fmax, if that is a potentiometer, it is good practice to connect the wiper to the pin that is not connected right now (pin 1 in your drawing).
The reason for this is that when your pots get a little older and starts to wear you always have at least the max impedance of the potentiometer (1k in this case) between the connections and not infinit impedance.

Fernando · Posted: Wed Apr 13, 2022 3:05 pm Post subject:

Thank you! they are all trimmers (pots are in wiring page)
The one with one pin unconnected is as in original schematic

AlanP · Posted: Thu Apr 14, 2022 2:29 pm Post subject:

Interesting. How dependant is this circuit on specific transistors? (I've built the 258 VCO before, and it is really, really dependant on selection of the sine waveshaping FET.)

Fernando · Posted: Thu Apr 14, 2022 6:26 pm Post subject:

Peake · Posted: Wed Apr 27, 2022 2:52 am Post subject:

2N5460, definitely.

vladosh · Posted: Mon Nov 14, 2022 6:14 am Post subject:

Thanks for sharing this ,i've built over 10 x 258 VCO's from a layout that is available here on forum and the last 2 times i couldn't get it to work ,and gave up ,and now it's me without a proper Buchla VCO ,this should do very nice ,i will share a PCB layout for this if it's ok ,thanks a lot
cheers

Fernando · Posted: Mon Nov 14, 2022 4:01 pm Post subject:

Well this layout is already done
I posted it in case people want some pcbs (I posted my pcb design on another thread. It is the original circuit intact plus a CV mixer and an output amp)
But of course, do what you wish
In any case it's nice to have a clear schematic now : )

Fernando · Posted: Mon Nov 14, 2022 4:23 pm Post subject:

Not the last version I have, but it's illustrative (10x10cm pcb) :

PM if interested

158A_top.png

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vladosh · Posted: Tue Nov 15, 2022 2:21 am Post subject:

Looks great ,i was gonna try single sided but then i couldn't resist ,here's mine .. still need to recheck it few more times before printing ,thanks again for sharing this redrawn version ,i assume all is correct in the schematic ,buchla stuff is so hard to find ,i was just looking at that 257 processor and someone has told me that the schematic is correct but it's still not all that clear ..synth is a never ending

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Fernando · Posted: Tue Nov 15, 2022 6:40 am Post subject:

Excellent!

Just to clarify: my pcb lets you choose between the original power scheme of +15V direct to 3 transistors and the rest of the circuit thru R53 and C9 (as I understand you did) and a modern power scheme (power in to ferrite beads and 10uF)

Additionally, my pcb let you choose between original freq control INT/EXT and a modern CV mixer that lets you have both the freq pot and as many inputs you want, all with bipolar attenuators.

Also my pcb have an output amp to adapt the original 1V RMS (2.83Vpp) to 5Vpp, 10Vpp or what you like.

I did all film caps with multi-size footprints and FETs (Q7, Q15) with a footprint that let you use many different devices.

I did a 100x100mm pcb so it can be used in small systems (Eurorack) or larger systems (Serge, Buchla, MOTM, etc). Thru plated holes, etc., very good quality

vladosh · Posted: Tue Nov 15, 2022 7:12 am Post subject:

i was just excited to see a well documented buchla module ,to be honest i am not so certain that i will build it ,tho i want to ,i hope i will ,about those add ons and mods ..you are right it's prolly better way to fit into modular ,i should maybe make some high gain amps to boost the signal if it's quiet ..in the end i will be so happy if it works ,maybe to add that 156 processor ,anyway ,i believe there might be some negative voltage here and there in my modular ,i wonder how this VCO behaves on negative voltage ,it's descrete i guess it won't do any harm ..even so ..it is still worth building ,thanks

Fernando · Posted: Tue Nov 15, 2022 8:20 am Post subject:

of course it worths! it is a lovely discrete VCO!!! :D

vladosh · Posted: Fri Nov 18, 2022 1:39 am Post subject:

i fixed it up a bit and shared on my blog ,i hope it's ok ,also did a layout for Buchla 106 https://synthacks.blogspot.com/2022/11/buchla-158.html , i wonder if i ever get to building this stuff .. more then 10yrs of building synths so often it is hard to sit down and do some soldering ,i wish i could get some window size Modcan and clean up my room

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Fernando · Posted: Fri Nov 18, 2022 3:14 am Post subject:

nice!

vladosh · Posted: Fri Nov 18, 2022 3:49 am Post subject:

i so want to make some other buchla layout but not knowing what schematic is correct ..is hard .

vladosh · Posted: Tue Nov 22, 2022 9:15 am Post subject:

made a new version layout for this oscillator as the first one was bit all over the place

BUCHLA 158 CORRECT V2.JPG

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Fernando · Posted: Tue Nov 22, 2022 12:37 pm Post subject:

very cool

vladosh · Posted: Mon Nov 28, 2022 11:29 am Post subject:

this is how i plan to fit 2 x 158 on a panel ,there is also 194 ,106 and LPG ,wonder if i want to make 265 or souce from unce instead of that bezier digital thing

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Fernando · Posted: Mon Nov 28, 2022 1:27 pm Post subject:

nice! Sauce of Uncle is a good option IMHO

vladosh · Posted: Wed Dec 07, 2022 6:59 am Post subject:

What transitor testers are you using folks ? J-Fets have been an issue since i got some fakes yrs ago and i can't always order from trusted sellers . I just received a tester from Ali , but when i plug in a P Channel Fet it says it's an NPN ,now there are some other measurements there but it does not identify it's pins ,the J112 or 201 is measured like resistor ,some 50 ohms or so ,not sure what it means ,anyone ?
Thanks

Fernando · Posted: Wed Dec 07, 2022 7:44 am Post subject:

I use the usual China-made clone of the originally open source ATMEGA project. It costs around $12 and it's fantastic.

I also have a Meterman 37XR, which can measure capacitance and inductance in addition to the usual readings.

There is always a problem with JFETs with high Idss (let's say >20mA). Transistor tester says it is an UJT or whatever. For those few you need to test them in a real circuit. I think I have a couple of test circuits diagrams, I'll post them if I find them.

I have some 2N4339 in TO-18, Idss specs are 0.5-1.5mA, readings are 10 to 30mA. In this case I suspect they may be fakes, for instance. But you can't never be sure with FETs! So it's better to test them on a real circuit.
But 2N4856 Idss specs are 50mA minimum. I accept the tester gets confused with this one.
Maybe this happens because this testers supply only 6mA for most readings.

----> Anyway this topic deserves it's own thread. Open a new thread?

Fernando · Posted: Wed Dec 07, 2022 8:01 am Post subject:

right now I only find this one (see attachment)

But this simpler one may be more practical:
http://www.hawestv.com/amp_projects/design_page/idss_tester_and_tutorial.htm

I also found this:
https://stompville.co.uk/?p=665
https://stompville.co.uk/?p=112
https://stompville.co.uk/?p=601

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vladosh · Posted: Wed Dec 07, 2022 8:50 am Post subject:

i don't know maybe there is obvious way to understand , maybe when i plug in a J112 and it says pin1 and pin2 are resistor of 50ohms ,maybe pin1 and pin2 are source and gate ,and there is some formula to calculate everything from say 56.71 ohms but i never really cared about math to be honest,i was expecting to plug in a jfet (and i tried good transistors) and it shows me what is source ,gate and drain ,and something to understand what rating is the transistor ..guess it's not that simple for 12$ ..it's not about the money ,it's just that it's sort of useless to me now ,i'll just be careful where i buy the transistors and maybe i dunno build my own tester