Buchla 257 Control Voltage
Processor Module

I built a Buchla 257 Control Voltage Processor 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, panel, and knobs have specific sourcing and I do not know the details.  Here is a photo of an original module.


There is only a single PCB in the module.



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 Front Reference Designators

PCB Rear Reference Designators


I've mounted the potentiometers and switches by soldering a single lug.  After assembly I reflow the pin so the part will relax into the proper location.  The LEDs are unsoldered and I push them flush to the panel before soldering.


Everything is finished except for the power cable.




Initially my 21 KHz triangle wave did not oscillate.  I discovered IC4 pin 14 was at -13V which reversed biased the input of IC2 pin 6.  In looking at the circuit I discovered that the 0.2V clamp was not implemented as per the original schematic.  This clamp would have kept IC2 pin 6 from becoming reversed biased.  Below outlined in red is the original clamp circuit.


IC4 has one unused op-amp that is unused and not terminated so I decided to add the clamp modification.


The oscillator now runs at 21.7 KHz.


For some reason, I never documented the modification but was asked in an email. I believe this is correct.

Everything on the original schematic in red needs to be added. The op amp in lower left is the spare op amp. Pin 4 is 15V and you can see connected to it are two 33K resistors (it is a TL074 so the pin numbers are different from the original schematic). You can see on resistor going to pin 5 and the other going to pin 6. The pin 5 also connects to the diode that goes to ground soldered at the resistor leg on the PCB. The 680R feedback resistor is connected between pins 6 and 7. Note that the vertical leads across the top actually descend and solder to pin 6. The horizontal leads descend and solder to pin 5.

 Then we wire over to the other op amp where we add 3 diodes. The cathodes of the diodes connect to the IC pins, and all the anodes are connected together.

Here are two side photographs to help understand the modifications.


The trimmers set the center off position of the Va control.  I simply put +5V into Va, set the Voffset to 0, and adjusted the trimmer so the output was 0V with the Va control centered.

This is an interesting circuit with three voltage inputs, a constant multiplier, an index, and an offset.  This image shows the voltage processing of two ramps.  I have the M control set at center and have an external -5V into the M input.  Vb as directd by M to the output.  I am not using Va as I don't have enough scope channels to display five signals.


This scope image shows the same setup with Voffset applied to shift the output positive.


Removing the Voffset and changing M to -5V directs Vc to the output.


An M index of -2.5V directs Vb and Vc to the output.