Buchla 245 Sequential Voltage
Source Module

I built a Buchla 245 Sequential Voltage Source 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.  PCB1 has SPDT switches for Start and Stop with LEDs above them.  The panel had holes for illuminated SPST switches so I had to use panel mounted switches and left the LEDs off.  I couldn't find small SPDT switches so had to modify the Stop circuit for a SPST switch by grounding IC5 pin 12 and adding a 1 uF capacitor across the Stop switch to debounce it, and grounding IC5 pin 12 (SW1 bottom pad).

 

PCB1 has a few discrete parts but is mostly the potentiometers.  You can see in this image SW1, SW2, LED1 and LED2 are not installed.

 

The discrete components are all mounted on the rear.  You can see the wires from the panel mounted switches connecting to SW1 and SW2 and the Stop debounce capacitor in the lower right corner.

 

The front of PCB2 is just the board-to-board interconnects.

 

The components are on the rear of PCB2.

 

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.

PCB1 Front Reference Designators

PCB1 Rear Reference Designators

PCB2 Front Reference Designators

PCB2 Rear Reference Designators

 

 

Operation

There is very little documentation on the245.  It is comprised of two separate sections: Pulser and Sequencer.  The Pulse output must be patched to the Advanced jack in order to operate.  Start and Stop are self-evident but releasing stop advances to the next stage.  This allows you to manually advance the sequencer with the stop button.

The outputs across the top are pairs of gate outputs indicating when that stage is selected.  The inputs across the bottom select the stage.  If you have the stage switches set for a Stage1-Stage2 sequence and pulse the Stage 3 input then the sequencer will begin a Stage3-Stage-4-Stage5 sequence so you can "flip" between sequences.  The Analog input is a VC control to select a stage. 

This scope image shows the pulse which is a combined trigger and gate at the minimum pulse length.

 

The Pulse Length control sets the width of the gate portion as seen in this scope image.

 

The Reference is a Saw output of the Pulser.  The internal time modulation will pickup noise and should be set to center when the external input is not used.

 

This photo shows the Pulse, Reference, and two channels of output.

 

This scope image shows modulation of the pulse length (e.g. gate) time.

 

This scope image shows modulation of the intenal time.

 

 

Calibration

There are just two trimmers.  TR1 sets the clock frequency so I calibrated for 2 Hz at the 0.5 knob setting.

TR2 sets the tracking of the Analog input.  I calibrated it to change stages every 2V (Stage2=2V, Stage3=4V, Stage4=6V Stage5=8V).

 

 

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