Buchla 281 Quad Function
Generator Module

I built a V2 Buchla 281 Quad Function Generator module for someone else. They sent me a mostly 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.


Assembly of PCB1 is straight forward.


The power cable wires into discrete pads on the rear and I wire tied it to one of the connector holes.


I used sockets for the CA3080 OTAs as they are both rare and fragile.


This photo shows the completed three PCB module.


I routed the flat cables a bit different than other photos I have seen.  I routed the cable that goes around the middle PCB on the side that had additional clearance.  This cable needs to be split to fit around the center standoff.  I made the other cable long enough that I could extend the PCB out for service.  I used 20mm spacers between the panel PCB and the center PCB and 15mm spacers for the rear PCB.




This image shows A triggering B.  You can just barely see the magenta trigger pulse on this scope image.



The red banana jack output is a trigger at the end of a cycle.  Zooming in you can see it is a +15V pulse about 150 S wide at mid-voltage.


This scope image shows Out A - D in quadrature mode.


This scope image shows the same Out A and B as above and the Sum outputs.


I calibrated Section A to 10 seconds and then adjusted the slope of Sections B - D to be parallel.




The V3 PCBs have a few issues that are easily corrected.  The BOM for the 10V regulation resistors are swapped.  R169 should be 270R and R270 should be 1K8.  Pin 1 of IC7 and IC14 are driven from a saturated op-amp output through a diode.  IC7 and IC14 are powered from 10V so you need a 47K resistor in series with pin 1 to limit the current.  You can lift the pin and add the resistor  but since the parts were in sockets, I chose to cut traces and add the resistor on the rear.  All cuts are on the front of the PCB.  Cut the trace to cathode of D2, to cathode of D7, right side (towards center) of R43, right side (towards center) of R109.  On the rear of PCB2 add wires to connect the cathode of D2 to the left side (towards center) of R43, and the cathode of D7 to the left side (towards center) of R109 (the left side because you are working on the rear of the PCB)..  Then add 47K resistor from the left side (towards center) of R43 to pin 1 of IC7, and the left side (towards center) of R109 to pin 1 of IC14.


There are four op-amp inputs that are driven by diodes so they float when the diode is cutoff and noise will affect the outputs.  Simply add four pulldown resistors to ground.  Add 100K resistors to ground to IC6 pin 3, IC6 pin 5, IC12 pin 3, and IC12 pin 5.  Nearby decoupling capacitors provide ground pads.


On this version I could not get the Transient vs. Sustain to work at all regardless of the trigger levels.  While the input is true the RS flip-flop is held in the Attack state so it sustains until the input goes false.  I assumed the circuit was designed for the "stepped" Buchla trigger-gate but the comparator level is set at 0.7V.  When the switch bypasses the 100K, the level does change, but it is always greater than 0.7V.  I tried changing the threshold but could not get the circuit to operate properly at all.  I simply changed the input 100K resistors to capacitors to provide a narrow trigger in transient mode and the full width in sustain mode.  Change R146, R148, R163 and R165 to 0.01 F capacitors.


Transient mode is shown in this scope image.


This scope image shows the sustain mode where the attack is held at 10V until the input goes false.