292C secrets

I cracked open my first 292c repair last night. I found some interesting things during this repair. First, there are some parts in the actual 292c that are not on the schematic. I probably found the most confusing way to draw this, but I just added it on top of the schematic in red. the cap around IC7 is going from inverting input to output, even though my artwork may look misleading. I'm not sure what the point of that resistor/cap kludge in the middle is. My guess is to roll off the highs (around 24k) a little bit in gate mode only. This makes me wonder why nobody has complained about oscillations in all this cloning that has been going on. Check out the hacked solution to solve the lower gain on the second channel. I don't think that's factory.



Second, the channels don't match! Well, the first one on the left has an error, so that doesn't count, but the second one doesn't have a 100p around IC7 at all!?

I love Buchla repairs, they are like an archeological dig.

Just for fun

I have been way sick for the last week. My tonsils got infected and I haven't been doing any work. I took this shot just for fun. These modules will never live together, but it's a fun hypothetical 12 spacer. Now I just need to make them all work...

Unearthing Greatness

In the last trade I did, I ended up with 2 modules that seem to be very early examples. The first is a 292B labeled "Prototype" on tha back of the panel. It's written in pencil, funny enough. This one worked well and matches the 292B I already had here.


The front panel was dirty, I had to take all the parts off to clean it up.




The other, a beat up old 207, has "Model" on an orange sticker, stuck to the back of the panel. I believe this means that Don built this up for the assemblers to use a reference while they were hammering them out. This thing had a few broken sliders, so I decided to replace all of them, since I was ordering a set for the 207 I was building from scratch. Synth Restore in the UK has re-manufactured sliders for ARP synths, and these were CTS too, so they work for Buchla stuff. To my horror, the inside of this thing was filthy! Balls of hair at the top and bottom of each slider and crumbs were everywhere.


I took out the sliders and blew out the crap to get started.


The sliders went in really nice and even though the shafts are shorter than Buchla used, they look great. Now, I just have to finish his buddy...

ADSR and the Buchla

The Buchla is not usually associated with keyboard type ADSR envelopes.

I patched up a 281, plugged into the 107 voltage controlled mixer to make some percussion sounds. The good thing about a 100 series gate like this or the 110 is that they are all transistor and don't have the characteristic Vactrol mushy attack. Everything has it's place after all.

I have a loop going, so the envelope keeps triggering, while I work on it. I started with function generator 1 in looping mode and it's trigger out plugged into the trigger in on function generator 2. In this patch, both are triggering at the same time, and I'm taking the voltage out of the combiner section at the bottom. It is a diode combiner and not a mixer, so it puts out whichever is the higher of the 2 voltages at any given time. If the second function generator is set with a slow attack, you can create a cool, compressed drum type sound, where there's an attack followed by a swell in the decay portion. If you put the switch into AR mode on function generator 2 you can play this from a keyboard with sustain.


Then I put the 281 into quadrature mode. The pulse output from function generator 2 plugged back to the pulse input of function generator 1. This gives you a AHDSR of sorts. The Attack control of function generator 1 controls the attack time. The attack control of function generator 2 controls the peak hold time. The decay control in function generator 1 and the pot in the combiner section control the decay and sustain (unfortunately decay is the time of decay AND sustain so you have to massage them to get what you want). The decay control on function generator 2 controls the release. This setup is good for making hard punching sounds, because of the peak hold. Again, put the switch into AR mode on function generator 2 for sustain.



The final option I can come up with right now, is to invert the second function generator. The first function generator's pulse input is where the envelope is trigger from. The quad mode is on. The cv out of function generator one is plugged into a voltage processor and turned up to 100%. The CV out of function generator 2 is sent into the voltage processor and inverted, it's inverted gain will then be the sustain control. It should be in AR mode. The attack control on function generator 1 is the attack, the attack control on function generator 2 is the decay. The decay control on function generator 1 is the release and the decay control on function generator 2 should be all the way down (it's sort of a delay until the next envelope fires if this is all looped). What I like about this is how it always completes the full attack and decay faze before it starts the release, unlike a "normal" ADSR. That makes it great for sending a pulse from a sequencer and getting percussion envelopes with full control.

Oh, and you can tweak the shape of the segments with feedback too...

Music Easel Oscillator

The oscillator section in the 208 module can be seen as a rest stop on the way between the 258 and the 259. The "timbre" section is introduced as well as a switch to select waveforms. It's the first time Don implemented a dedicated modulation oscillator and trimmed keyboard CV inputs.

Interestingly, the original version of the 208 used a totally different design than the 258 or the 259, with Vactrols in the oscillator core. I don't know how many of these were made, but it was fazed out in favor of the 259-like design you can find schematics for on the web.

The "Timbre" control introduces wave folding on the sine wave, but does not effect the other waveforms at all. There is a separate, unlabeled knob above the waveshape switch that controls the mix between sine and the other shapes. The wave folder's effect disappears at the full CW position of this control.

I built a clone of this oscillator about 6 years ago, using 2 single sided photo etched boards. I modded a few things to make more sense in a dedicated module. There are a few lessons I learned, that I would do differently, if i did it again. It is a nice design and I think it has validity, even alongside other Buchla oscillators.



Going through my box of abandoned old PCBs, I got the idea that maybe this would be a cool module to throw together, considering the hype surrounding the Music Easel. Maybe it's time to revisit this one....

Musings on Voltage Processors

Some time in 2007, the idea of "building a better voltage processor" came up between Reed and me. The problem goes like this....



256
pros: good for mixing, inverting and scaling. Huge output voltage swing possible, for black knob era systems where you really need 15 volts to open some parameters.
cons: All it can do is mix and invert. Only 2 sections. No indicators. All black banana jacks. (just kidding Rick)


257
pros: able to mix, scale, invert, crossfade, manually offset, VCA
cons: no gain, can only mix 2 CVs. Because it uses PWM for the crossfade, it doesn't like audio rate CVs. Only 2 sections. Confusing nomenclature.

256e
pros: able to crossfade. invert, scale, mix (kinda). Breakpoints!
cons: only will work with up to 10 volt CVs. No real mixing.



The first idea was to make a quad voltage processor module that had 2 sections of 256 clone and 2 sections of 257 clone. That seems like a cool enough idea, but then I thought, "what about making a design that can do what both do and including 4 of those?"

The idea is to have the ability to mix 3 CVs, crossfade, scale (from a knob or another CV), offset. It should have an LED to show it's state. I had a design made up and a front panel worked out, then my laptop was stolen by a Gypsy in Barcelona. These print outs are all that remain. Anyway, I am ready to dive back in and I will get something together.



It will be like a 257, but with 2 reversable CV inputs. The PWM crossfader will be made from newer, faster parts. So the cutoff frequency will not be so low. The 257's triangle oscillator, for the PWM, is 21kHz. I think I'll try to take it up to 100kHz or so. This shouldn't be a problem for modern analog switch chips. I think I'll use center detented pots for the reversing CV inputs. Here's the 257 schematic, BTW.



With this configuration, you can mix 3 signals, one with voltage controlled gain. It's not the 4 signals of a 256, but it's close. It can do everything a 257 can do, with a bit more speed. It's 4 processors in one module space. It'll have LEDs on the outputs. the output will not swing a full 15 volts, because I don't want a 24 volt supply line (200e and many 200 systems don't have one). It won't even try to do the breakpoints, we'll leave that to the 256e.

Desoldering Station

Last week, I became the proud owner of a Hakko 472D desoldering station. This amazing device heats up a solder joint and vacumes out the molten solder. I had been using the Soldapult large size hand pump sucker thing up until I got this thing.

This allows me to reclaim chips and other parts from a circuit board, rather than resorting to cutting off the legs. It is a work of genius. Thank You Reed!