I built a Buchla 296 Programmable Spectral Processor module for someone else. The customer 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. Although all the resistors and some ICs are pre-soldered there is still considerable time required to build this module.
Buchla 296 Programmable Spectral Processor
The PCBs come with most of the SMT parts pre-soldered. The two large electrolytic capacitors below the power connector have to be added.
The cards have been scribed and can be built as a larger PCB. Then they can be snapped apart and then the Molex connector and trimmers installed.
Card 7 requires two SMT resistors be replaced.
I made reference designators from the PCB images. The components generally cover the silk screen legends once populated which makes it hard to debug if anything is wrong. However, in the case of the motherboard all the reference designators are next to the components. Reference designators for the cards are provided.
Motherboard Controls Reference Designators
Motherboard Component Reference Designators
There is a single build thread and three modifications listed for the V1.0 PCB.
296 V1.0 Spectral Processor modifications
Note: These modifications are included on the V2 blue PCBs.
Note: These modifications are included on the V2 blue PCBs.
296 build tips
Front panel build
As I checked the linear potentiometers I noticed that some PCB vias are immediately below the metal tabs and could touch. The metal case of the potentiometer is floating but I didn't want the possibility of contact. The metal tabs are longer than the plastic tabs on each end so the metal tabs directly contact the PCB.
I decided to add some tape insulation beneath the metal tabs as I don't like relying on solder mask to provide insulation and I solder the vias so they can be taller than the solder mask. Long term the tape adhesive will loosen but the tape is sandwiched between the potentiometer and PCB and also increases the spacing to the PCB slightly.
Likewise the tabs on the 9mm Alpha potentiometers contact the PCB so I cut the top and bottom tabs off of Pot20 and Pot21 since there are runs directly beneath the tabs.
Build time for the motherboard was 2.5 hours. You can see the tape insulation beneath the sliders. I installed the LEDs when I installed the front panel.
The panel took 2.75 hours to assemble and solder all the wires to the motherboard. I used an insulating washer on the left screw since it is close to some vias.
The cards took 5 hours to assemble to the point of breaking them apart. After breaking the boards apart I used a file lightly to clean the edges up. Then I added the Molex connectors, did a final wash, and installed the ICs and trimmers.
I added 0.1 µF decoupling capacitors on each of the cards at the edge connector to pins 13 (gnd), 14 (+15V) and 15 (-15V).
Total build time was 12 hours: 3.25 hours for the motherboard, 2 hours for the panel and wiring, and 6.75 hours for the cards. Here are the finished cards.
The 296 took a total of 12 hours to build.
Calibration was very straightforward. I first went through first with a DMM and adjusted all the envelope followers to <2mV. The only exception was outputs 8 and 9 on card 5. The trimmers didn't function nearly the same as the others but I was able to get the offset down to ~20mV.
The filter topology is designed around three cascaded single op-amp multiple feedback bandpass filters in a stagger tuned configuration. Each of the filters is a small frequency offset from the next to get a wide and flat passband with steep slopes. You tune each bandpass for maximum output at its specific frequency so there are three trimmers per channel except for the <100 Hz channel. To adjust the frequency trimmers I setup my MSO4054 4 channel scope with channel 1 with frequency measurement on the input, channel 2 on the comb odd output, channel 3 on the comb even output, and channel 4 on the individual envelope CV outputs. I simply then dialed in the frequency and adjusted the appropriate trimmer. There is a nice calculator for this single op-amp multiple feedback bandpass filter at sim.okawa-denshi.jp/en/OPttool.php.
There is a regular pattern to follow for calibration. First, monitor the EF output with a DC voltmeter and adjust the EF trim to as close to 0V as you can get. Then monitor the channel output with a scope, set the input frequency to what is shown for Trim #1 and adjust, then set the frequency for Trim #2 and adjust, and then set the frequency for Trim #3 and adjust. Then go to the second channel on that card. You soon learn the pattern of adjusting the lowest trimmer, then moving up two, then moving up two, then going to the next to the lowest trimmer, then moving up two, then moving up two. I kept a marker on the card I was adjusting so I didn't venture to either side. Calibration is pretty quick.
Trimmer Location Chart
The following table is my measured outputs for the envelope follower CV output, the non-attenuated channel output, and the attenuated all output with the sliders set to 0 dB. If you compare the non-attenuator output levels and ignore the <100 Hz and >10 KHz since those are not the center frequencies, the levels are all within +/-0.7 dB. The attenuator outputs have some inherent variability with setting all the slider exactly at 0 dB but those outputs are all within +/- 0.8 dB.
|Source||Input||EF Max||Non-Attn||Atten Out|
|Frequency||Level||Level||Out Level||0 dB Level|
I swept VCO (yellow trace) with a ramp (cyan trace) for the all signal input to the 296. The magenta trace is the output with all the sliders in the 0dB position and the green trace is the 1 KHz EF output.
This image is the even comb outputs with the odd frequency notches.
This image is the odd comb outputs with the even frequency notches.
This image inverts and adds the even to the odd outputs so similar colors show as gray. You can see how the two spectrums interleave nicely.
This images shows a notch filter with the 1 KHz slider set to minimum.
This image shows a bandpass filter with the <100 Hz and >10 KHz sliders set to minimum.
This image shows the Programmed All output with the frequency and width controls set to the 12 o'clock position.
This image shows the Programmed All output with the frequency control set to 9 o'clock and the width control set to 3 o'clock.
This image shows the Programmed All output bandpass filter with the frequency and width controls set to minimum and a 10 volt CV in the 1 KHz CV in.
This image shows the Programmed All output notch filter with the frequency control set to 12 o'clock and the width control set to 9 o'clock and a negative CV in the 1 KHz CV in. Note that the channel CV adds to the frequency and width controls so in this case a negative CV will decrease the amplitude.
This image shows the Programmed All output bandpass filter with the frequency control set to 2KHz and the width control set to 9 o'clock. I am using white noise as the input and have my scope in FFT mode. The frequency control setting at 2KHz is fairly accurate.
These three images show the Envelope Decay time settings (short, medium, and long settings).
This is a video of the 296 used as a vocorder. The programmed spectrum transfer switches are opposite of what is in the Buchla Synthesizer User Guide, November 16, 1981. The microphone is plugged into the even input, the carrier is plugged into the odd input, and the output is from the programmed odd output. You will note in the video that the right switch, and not the left switch, is up which is opposite the documentation and the original panel legend.
You can see the difference in the Buchla Programmed Spectrum Transfer legends (left) and these legends (right).
The programmed spectrum transfer switches S2 and S3 center traces are easily accessible on the back of the motherboard and the modification is to cut and swap the middle switch pins with wires.