Buchla 259 Complex Wave Generator Module

I built a Buchla 259 Complex Wave Generator module for someone else.  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, the panel, knobs, and PCBs have specific sourcing and I do not know the details.  This module is in the Buchla format with V1.0 PCBs.

Buchla 259 Complex Wave Generator description from the Buchla Synthesizer User Guide, November 16, 1981.  This document doesn't describe the autotune switches which disable the FM input for tuning.  I would be interested in what autotune is supposed to do.  The clocks of IC11 and IC15 are connected to ground so these parts do nothing.

 

 

I made reference designators from the PCB images.  The components cover the silk screen legends once populated which makes it hard to debug if anything is wrong.

         

PCB 1 Front Reference Designators

PCB 1 Rear Reference Designators

PCB 2 Reference Designators

 

 

Construction

There is a single build thread and no modifications listed for the V1.0 PCB.

259 V1.0 Complex Wave Generator build tips

  1. The 3 pin header and shunt are not listed on the PCB2 BOM.  I used Mouser 517-929647-01-03-EU pins and 571-8815452 shunts.

  2. The Mouser 667-EXC-ELSA35V ferrite beads listed in the BOM are obsolete.  I used Mouser 623-2743001112LF.

  3. The BOM lists a Mouser 23PW247 for C7 and C22 which is a radial capacitor and they are very prone to bending.  A Mouser 23PS247 axial capacitor might be a better choice.  The PCB lead spacing is 10mm and the axial length is 12mm so the leads would wrap around the capacitor slightly.

My Modifications and tips

  1. The tempco resistors are mounted on PCB2 close to the transistor arrays.  I epoxied them on top of Q1 and Q5 and wired them to the PCB.

  2. I used single pin SIP socket posts for R*.  I tried different values for R* but it worked best with no resistor installed.

  3. I improved the sine wave shape by changing R110 from 3K3 to 787R.  Scope images of before and after are  in the Modifications section near the bottom of this page.

  4. I improved the symmetry of the modulation oscillator low range greatly by using IC4C to switch in a 0.47 F timing capacitor in parallel with C7.  Lift pins 10 and 11 of IC4.  Wire pin 10 to a 0.47 F capacitor.  Wire the other lead of the capacitor to the right side of C7.  Wire the left side of C7 to IC4 pin 11.  Scope images of before and after are  in the Modifications section near the bottom of this page.

Front Panel build process
Here is my process for building the front panel.  There are a lot of components to align and this process worked well.

  1. Install all the banana jacks.

  2. Install the 3.5mm jacks so the terminals face the edge.

  3. Install each switch and solder one terminal.  Make sure that the momentary for S1 is down.  Install a backing nut and internal tooth washer on each switch.  Install the front panel.  Use a long rod and screw the backing nuts flush to the panel. Reflow each single pin soldered so the switch will self-align to the panel.

  4. Install each 9mm potentiometer and solder just the wiper pin.  Install the front panel and align the shaft of each potentiometer in the hole and solder one support pin.

  5. Push all the LEDs through nearly flush to the PCB.   The LEDs do not have a flatted side so the long lead fits on the round side of the silk screen.  The shoulder of the long pin is also angled. Push a small piece of static foam on each LED pins to hold it in place.  Use static foam of the same thickness to provides a flat surface to set the PCB on.

  6. The 16mm potentiometers cannot mount flush to the panel as there is insufficient lead length to extend through the PCB.  I wanted to maximize shaft length so I used a 5/16" internal tooth washer and increased the inside diameter slightly with a round file to fit the 7.5mm potentiometer threads.  Mount the washer and break off the mounting tab on each potentiometer and press the leads into the PCB but do not solder  (it is easier to install the front panel if you are not trying to align the bushing to the hole).

  7. Solder wires to J9 and fold the wires over.

  8. Install the standoff with a screw in the upper right corner of the PCB.

  9. Install the front panel and screw the three standoffs on to hold it.  Install the screw in the middle standoff.

  10. Use a long rod and make sure the switch backing nuts are flush against the panel.  Install and tighten the nuts (use heat shrink over the socket and cut out the opening to not scratch the panel).  Reflow the single soldered pin on each switch to relieve any stress.  Solder all pins.

  11. Make sure the alignment on each 9mm potentiometer is correct and solder all pins.

  12. Install the washer and nut on each 15mm potentiometer and tighten.  The pins should be flush with the back of the PCB.  Solder all pins.

  13. Remove the static foam from each LED, make sure the pins are straight, press it down until it seats in the panel, and solder the pins.

  14. Use a small hook and fish the wires from J9 through the PCB and solder.

  15. Install and solder the remaining jack wires.

 

Build time for PCB1 and the panel was 3.75 hours.  This is PCB1 ready for the final front panel installation (the image is a composite of two separate photos).

 

I use color coordinated wire for the banana jacks although there is only the single blue jack.

 

The tabs on the 9mm Alpha potentiometers contact the PCB so I cut the top and bottom tabs off of Pot4, Pot11 and Pot13 since there are runs directly beneath the tabs.  I do not like relying on solder mask for insulation.

 

The square pins do not fully seat into the headers with the height of the standoffs.  I insert the square pins fully into the female header and assemble the PCBs together.  The pins are flush with the PCB and fully inserted into the header so the alignment is correct in all directions and then soldered in place.

 

Build time for PCB2 was 7.5 hours.  The resistors take a long time to install since there are so many of them and they are spread all over the PCB.  I use Kester .020" diameter #24-6337-6401 331 water soluble core solder so take time every hour to wash the PCB.  The trimmers, vactrols, polystyrene capacitors, and power cable are added after washing.  I epoxied the vactrols onto their respective transistor array and used SIP socket posts for R*.

 

There are two 2x15 headers on the rear of the PCB.

 

 

Operation

Calibration and full checkout of all inputs and outputs took about 2.5 hours.  I calibrated the modulation oscillator at the center of the frequency control to 440 Hz (high) and 4 Hz (low).  In high range the minimum frequency was 32 Hz and the maximum frequency was 8 KHz.  The waveforms are well formed.  This image is a composite of three scope images showing all three waveforms at frequency control mid setting.

 

In low range the waveforms are a bit more distorted but generally pretty good.  This image is a composite of three scope images showing all three waveforms at the frequency control mid setting.

 

As you go to the very low frequencies the waveforms distort quite badly.  At the low end of the frequency control the oscillation stops.  This image is a composite of three scope images showing all three waveforms at the frequency control min setting.  I made modifications to significantly improve the waveforms.  Scope images of the improvement are in the Modifications section near the bottom of this page.

 

This image shows the difference in levels and offset between the modulation output and the CV output.  The modulation output is a +/-2V output while the CV output is a 0 to 10V output.

 

I calibrated the principle oscillator at the center of the frequency control to 440 Hz.  The minimum frequency was 29 Hz and the maximum frequency was 9.8 KHz.  The sine wave has a bit of flattening.  I made modifications to improve the sine shape.  Scope images of the improvement are in the Modifications section near the bottom of this page.

 

At the maximum frequency the square wave shows some leading edge rounding.

 

This image shows the amplitude modulation.

 

This image shows the frequency modulation.

 

This image shows the timbre modulation.

 

This image shows the principal oscillator with all modulation modes on (phase, amplitude, frequency, and timbre).  It produces quite a complex waveform.

 

This video shows the principle oscillator phase lock capability to an external oscillator.

Video Demo

 

 

Modifications

I did some modifications to improve the flatted top of the sine wave (left image).  I changed R110 to 787R (1K paralleled with R110)which rounded the top nicely (right image).  The sine trim TR1 adjusts the gain a bit more with this mod so I could adjust the sine amplitude to match the square amplitude.  This is a much better sine wave.

    

 

This image shows the principal oscillator sine and square outputs.

 

The modulation oscillator waveform distortion on the low range was quite significant.  IC4C switches in a different bias point for the expo converter on low.  I made some modifications to improve the match of the charge and discharge current sources but they negatively affected the waveforms on the high range.  I decided instead to use IC4C to switch in a higher value integration capacitor which works very well.  This image is a composite photo of the modulation oscillator lowest frequency before and after triangle wave (scope horizontal and vertical settings are different between the two and the top waveform is AC coupled).  The resulting symmetry is very good..

 

Here are three scope images of the modulation oscillator output and the signal output at the lowest frequency of 1/3 Hz.  The signal signal output is capacitively coupled so there is droop in the waveforms.  However, all are much improved with a near perfect duty cycle.

 

The forums mention a "bleed" of the modulation oscillator in the principle oscillator with all the modulation switches off.  This module had a slight "bleed" that I could hear it as a timbre change in the square wave output.  I could not hear it in the sine wave or final output.  I investigated the principle oscillator circuit and found that the square output would vary in width depending on whether the modulation oscillator output was high or low.  I took a scope image of the principle oscillator output with the modulation output high and inverted and added it to an image with the modulation oscillator low so similar colors become gray and unique colors stand out.  You can see the difference in width circled in green and is about a 100 S delta.

 

I decoupling some of the logic ICs (IC6, IC8, IC9, IC10 and IC11) in the modulation oscillator with a 0.1 F capacitor across the IC power pins.  This helped minimize the "bleed" so it was barely perceptible but I could still see some pulse modulation on the scope.

 

I investigated further and found the +15V shifting about 20mV depending on the polarity of the modulation oscillator output.  This shift is independent whether the +5V is internal or external.  This module appears to be very sensitive to +15V variations so I changed to a dedicated  power supply with short cables and could no longer discern any "bleed".  My recommendation is to make sure you have a well regulated power supply with short cabling for this module.  I didn't analyze the power distribution on the PCB but suspect some modifications in the distribution and isolation of power routing would certainly improve performance.

 

 

259 Second Build

I built a second 259 for another customer.  This customer supplied SMT tempcos which I also epoxied to Q1 and Q5 and wired them to the PCB.

 

I took some time to further investigate the bleed between the oscillators.  The architecture is such that the AM modulation is always enabled but the wet path is broken with an analog switch when turned off.  I surmised that perhaps there was still AM modulation that might be impacting the power supplies that was causing the bleed.  I looked at moving the analog switch from the wet signal path to the modulation index path but on this build I could not see or hear any bleed between the oscillators with all the modulation switches turned off.  This build used LF412 op-amps which are not my favorite, no additional decoupling capacitors, and a Power One HDBB-105W-A with a 12" power cable and the +5 jumper set to external.  The HDBB-105W-A is a large supply with 1.5A of +/-15V and 12A of +5V so it is well regulated and this seems to confirm my earlier experience that well regulated supplies are the key to minimizing bleed.

 

 

259 Third Build

The low frequency modulation oscillator waveforms looked very symmetrical on this module so I did not do the modifications.  However, I could not tune it to 2V/Octave with the stock resistors.

 

The Principle Oscillator square output showed some oscillations on the rising edge.  You can barely see them in this scope image outlined in red.

 

Increasing the sweep speed to look at the rising edge shows the oscillations.

 

U25 is a comparator to generate the square wave output from the triangle.  It has no hysteresis which is simply bad design practice for a comparator.  I lifted the triangle input, pin 2, and added a 10K series resistor.  The other end of the 10K can connect to the anode of D9.  Then I added a 1M feedback resistor from pin 7 to pin 2.  The resulting square wave output is now clean.

 

 

 

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