Buchla 208 V1 Stored Program
Sound Source Module

I built the Buchla 208 Stored Program Sound Source module for someone else. This is the original two-board set now referred to as V1.  They sent me a complete kit of parts and I assembled and tested the module.  Many of the components are sourced through Mouser but specialized parts, panel, knobs, and PCBs have specific sourcing and I do not know the details.

Music Easel Manual 1974   

A scan of the manual

Music Easel Manual 1974   

A better color scan of the manual contributed by Luther R.

Building the PCBs took time since the boards are dense and the reference numbers take time to locate. ICs orientation is not consistent which makes it a bit more error-prone to build but pin 1 is a square pad which makes it easy to verify the correct IC orientation.

The build thread is very long so you have to go through it all to pick up the modifications and build tips.  I spent a lot of time studying the PCB assembly to make sure I understood completely how it all went together.  15 mm standoffs are used between the panel and PCB1 and 12 mm standoffs are used between the two PCBs.  2x10 headers connect the two PCBs but the pins are just a bit short for the 12 mm spacing so I soldered the male pins elevated off the PCB (the plastic headers are not seated against the PCB but the pins extend through).

I test fit the 805-44P connector to determine the spacing of the edge fingers but the connector easily splits if it is not soldered into the PCB.  DO NOT test fit the edge fingers in this connector without it being soldered in place or you will end up with this!  Replacements are available on Amazon and eBay.

 

The banana, the three 3.5 mm connectors, and the 1/4" connector install on the panel.  There are also eight standoffs that screw to the panel.  I soldered the program board connector to the edge finger PCB after the module was assembled.

 

Here is the process I found best for assembling this module.

Assembly Process

  1. Install all the 15 mm standoffs to the front panel.  Use a nut driver to tighten them from the rear to avoid damage to the front.  Install 15 mm standoffs in all additional PCB1 hole positions to provide a height reference for the switches and potentiometers so as to not warp the PCB.

  2. Start PCB2 first by installing and changing any SMT parts.  It is easier to solder these with no other components on PCB2.

  3. Install and solder the 2x10 female headers on the back of PCB2.  Then insert the male pins into the headers and seat them fully.  Sandwich the two PCBs together and screw them together.  Solder the male pins on PCB1 at the correct height.  The plastic header will not be seated on PCB1.  Separating the boards afterwards is difficult because of the 160 pins.  Work each connector slowly to not stress either PCB.

  4. Install standoffs on each PCB around the edge on the component side to provide support when you soldering.  There are many capacitors that get bent if you lay the PCB down. 

  5. I used water wash solder and installed all parts except for the polystyrene capacitors, switches, and potentiometers.  After washing the board I used no-clean solder for the remaining parts.

  6. Pin 1 of all IC pads is square so you can easily double check the orientation of the ICs by making sure that the pad near pin 1 is square.

  7. The DPDT switches I used mount further from the PCB than the SPDT switches.  Mount all of the switches and solder only one pin.  Install the backing nuts, sandwich the PCB and panel and screw them together.  Adjust the backing nuts near the standoffs to the correct height.  Remove PCB2 and set the backing nuts on the other switches to the same depth (this eliminates PCB warpage).  Re-sandwich the PCB and panel and reflow the pin and press all the SPST switches flush with the panel.

  8. I installed the backing nuts on the 5 large potentiometers and soldered only one pin.  I set them flush to the panel similar to the SPST switches.

  9. The bushing on the three small potentiometers will not extend through the panel.  I soldered the tabs and pins flush with the rear of the PCB to extend them as much as possible.

  10. Do NOT test fit the edge connector into the 805-44P connector.  Just solder the 805-44P connector to PCB1.

  11. I installed the LEDs loose and bent one pin over to retain them.  The silk screen has a flatted side but my LEDs did not so I had to verify anode and cathode.  When PCB1 is finally mounted to the panel you can extend the LEDs into the panel and solder them at the correct height.

  12. Carefully inspect every solder joint on PCB1 before installing PCB2.  Be sure to check the switches, potentiometers, and LEDs for any missed pins.

  13. Use extra standoffs on the rear corners of PCB2 to protect the PCB components when the module is set on the rear (see side view photo below).

 

PCB1 is complete except for the LEDs.  Standoffs are installed front and rear to help protect the parts as I work on the PCB.

Hi-Res image of PCB1 control side

 

PCB2 is complete except for the reverb tank which I will install last.  Standoffs are installed front and rear to help protect the parts as I work on the PCB.

Hi-Res image of PCB2 component side

Hi-Res image of PCB1 solder side

The banana, three 3.5 mm jacks, and the 1/4" jacks are now wired to the PCB.  I used 1" lengths of colored wire to match the color of the banana jacks.  All the correct standoffs are in place for mating PCB2.

Hi-Res image of PCB1 component side and panel wiring

 

I left a 3.65 mm gap between the top of the edge fingers PCB and the bottom of the Program Board connector.  Basically the top of the PCB is even with the bottom of the holes in the terminals.  That gives me a reasonable amount of terminal to solder to the finger and a good insertion depth into the 805-44P connector.  I cleaned the edge fingers when done with a pink pearl eraser.

 

This image shows the standoffs in the corners to protect the PCB components and the plastic strips on the male pins above the PCB to allow the pins to fully seat.

Hi-Res image of module side view

 

Here is the finished module.  It is quite nice looking.

Hi-Res image of 208 Stored Program Sound Source module front

Hi-Res image of 208 Stored Program Sound Source module rear

 

 

Verification

Verification is pretty straightforward.  I did find that the thresholds for different functions required a fairly precise 10 volt pulse.  I had to create a Buchla keyboard pulse to verify the envelope generator transient and sustain mode.  The keyboard pulse is a combination trigger and gate with two different voltage levels.  Once I generated the correct waveform the two modes were easy to verify.

Many of the reference numbers are covered by the parts so it is difficult to locate them after the PCB is built.  I needed the reference numbers for PCB2 and Roman sent me the file.

PCB2 Reference Designators

 

 

In this scope image you can see a two level keyboard pulse I generated with my ComputerVoltageSource.  In transient mode the envelope delay is based on the delay control setting.  It actually is delayed by the trigger portion of the keyboard pulse and then by the delay control setting but the trigger is short so it is not noticeable.

 

In the sustain position the gain is increased such that the pulse from IC19-10 is the entire width of the keyboard pulse which is longer than the delay control setting and dominates.

 

The four random outputs are unique.  Output 1 (top is just an attenuated version of output 4 (bottom).

 

 

Calibration

Calibration was pretty straightforward except for the 1.2V scaling.  Setting the complex oscillator range from 55 Hz to 5 KHz would not allow the front panel trimmer to reach 1.2V/octave.  The front panel scale and the range control are two separate gains in the CV circuitry.  I adjusted the front panel trimmer to about 3/4 so I would have some margin and then adjusted the range control for a final 1.2V/octave setting.  The resulting complex oscillator range was a bit broader which is nice.  With the fine tune to full CW the complex oscillator range is quite high although the amplitude drops off past 6.5 KHz.

Complex Oscillator Range with front panel control
Min Frequency Max Frequency

Fine Tune at full CCW:

44 Hz 6.5 KHz

Fine Tune at full CW:

100 Hz 18 KHz

 

I likewise had similar issues with the modulation oscillator and followed the same proceedure for setting the tracking to 1.2V/octave.  It likewise had a broad range mostly at the very low frequencies.

Modulation Oscillator Range with front panel control
Min Frequency Max Frequency
Fine Tune at full CCW: 0.05 Hz 24 Hz

Fine Tune at full CW:

0.9 Hz 41 KHz

I found that I could shift this range slightly up by adding a small amount of control voltage at the IC30A summer.  A 1M resistor from pin 2 to 7 would shift the range up nicely although I left it unmodified.

Modified Modulation Oscillator Range with front panel control
Min Frequency Max Frequency
Fine Tune at full CCW: 0.1 Hz 53 KHz

Fine Tune at full CW:

0.2 Hz 91 Hz

 

Operation

I have just a short video of the 208 Stored Program Sound Source running a short patch.  This is an interesting module.

Video Demo

 

 

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