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




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.




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).



Additional Observations

The original 245 used a 24V supply and the logic was a mix of DTL and early TTL that ran from +5V.  The output buffers seem overly complicated with an op-amp and transistor inverter.  This redesign uses CMOS with a 15V supply and the 24V circuitry in the output buffers appears to be driven from 15V with no other changes.

I was contacted by an individual who had a 245 that didn't function properly when isolating a single stage and then selecting the stages around it.  I helped him by email and we found that the first stage had some oscillations.  This module is basically digital but is designed with all analog circuitry.  Two transistor latches form each stage with a third transistor to set the latch.  The latches all have a common reset which is driven by the advance input.  This input pulse is conditioned to be fairly short and the latches have a second set input that is driven as selected with the switches through a capacitor.  When a latch is set, it elevates the reset level to prevent other latches from being set.  When reset, the falling output is inverted to drive the appropriate set.  Thus the propagation time of the latch being reset has to be longer than the reset pulse so the proper latch can be set.  I've worked on a similar design with the 123 and 146 Sequential Voltage Source and found that the layout was critical to proper operation.  Stray coupling would prevent the stages from proper functioning and I ended up hand tuning each stage.

He determined that a number of resistors had been changed from the original schematics and set some of the voltage thresholds quite high.  He reverted back to original values which improved, but did not 100% fix the issues.  These are his notes; I have not verified his results.

I made the changes to the resistors that provide the latch voltage and sure enough the functionality is much improved. It is still not 100% perfect: I’ve noticed that if for instance I try to “trap” the sequencer at stage 3 using the switches to isolate it, an advance pulse will still escape back to stage 1 (and then go 1,2,1,2 etc). Other than that I think everything works as expected: combinations of switch settings and stage select work as they should, and so on. The analog select overrides everything else which I think it what it should do, at least going by the schematic. I am not able to induce oscillations.

To lower the advance pulse voltage: R174=15k;   R163=5k6

To lower the latch voltage: R60, R78, R93, R108, R123=68k;   R66, R84, R99, R114, R130=15k

To lower the voltage at the inverted gate transistor (may not be needed): R68, R86, R101, R116, R132=47k 

I have had a bit of a look at the layout and can see a few things that might cause some strange coupling issues. Long parallel traces, especially for stage 1 which was more troublesome than the others, and some baffling routing from Q12 to R66 with traces on both sides of the board, forming a loop around the base of Q14.