Buchla 258J VCO

Quantity 2

This is my build of the J3RK Buchla 258J VCO board, as discussed on Muff’s message board:

Board thread

Build and mod thread

I put it into my famous 1U 16-position MOTM grid panel.  Here’s the Front Panel Express file.

Buchla 258J VCO FPE panel

This is a music VCO with 1V/octave tracking over several octaves and a temperature compensated exponential voltage converter.  I would say its most notable features are a voltage-controlled wave shape, and a very nice linear FM.  The schematic diagram provided by Mark Verbos matches pretty closely to the PC board.

My build and modifications

  • The builder decides between mixing a square or sawtooth wave with the sine wave in the wave shaper.  The choice can be made by a selection of resistors at build time or by adding a rather complicated switch.  For simplicity I chose the sawtooth version.
  • The PC board offers three outputs:  the main wave shape output, a triangle and a square wave output.  These outputs are roughly 5 volts peak-to-peak.  I added a quad op amp to the board to buffer these outputs and also the sine wave output.  The buffers have gain to bring the levels to a standard 10 volt p-p level.  (I’ll add the buffer schematic later.)
  • The PC board has frequency control voltage summing for a coarse tune pot, two 1V/octave inputs, and one reversible CV input (which can easily be doubled to two inputs).  My exponential CV inputs include a 1V/octave input jack, one attenuated FM input, one reversible CV input, and also a fine tune pot for which I had to add a 4M7 resistor to the board.
  • For the linear FM section I selected a 1uf polyester film capacitor with a 39K series resistor and a 50K audio taper attenuator pot.  These values work well for me, giving a good depth of FM, and control over the lower levels of modulation.  Low frequencies pass quite well, allowing for slow linear tremolo.
  • I added some 100nf bypass caps to the underside of the board.
  • As to the much debated choice for switching transistor (a rare 2N3638 is called out), I substituted a BC560C, which works fine.  I always print out a data sheet when installing transistors to insure correct lead placement.  The PC board marks E, B, C, for the transistor leads on the top of the board, which helps.
  • I added a 100K linear pot to attenuate the wave shape CV in and left off the 150K resistor from the board.

First Impression

The initial frequency trim pot lets you set a bias point for the coarse range.  At first I set the lowest frequency to 20 Hz.  As this produced a high end of 35 KHz, I decided to set the high end to about 25 KHz and let the low end fall where it might, which turned out to be under 1 Hz!  A very nice range.

I was able to set the 1V/octave tracking for at least three octaves of accuracy and perhaps more.

Adjusting the wave shape trim pots, I had a surprise.  The VC wave shape affects both the sine and the wave outputs!  I was very glad that I brought out the sine wave (which is tapped from the drain of the J201 FET and buffered to bring up the level).  Nicely, the sine output maintains a constant amplitude as it changes shape and becomes edgy.  The wave shape output that mixes in a sawtooth increases in amplitude as it shifts from ‘sine-like’ to ‘saw-like’ in shape.  The ‘sine-like’ shape yields low harmonic content, but is not as smooth as the sine output.  (I’ll post some demo recordings.)

Pictures

Board

Behind the panel

Buffer wiring on the underside of the board

Here’s an image of an output buffer schematic.  My buffers are essentially the same as the shaded portion in the upper right.

output-buffers

The bracket that I made from thin aluminum stock from Lowes, shown before bending.

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9 Responses to Buchla 258J VCO

  1. Neutron7 says:

    Very nice job (again)
    I really like this oscillator.

  2. Matt says:

    Hi there,

    Would you be able to share the schematic for the buffers?

    Thanks.

  3. Richard says:

    See image in the updated post, above.

  4. Fer says:

    Hi Richard, did you selected the J201 for Idss .7-1.2?

  5. Richard says:

    Fer, no. I did not have very many J201, and I’ve never selected them.

  6. Fer says:

    Thank you

  7. Fer says:

    Hi Richard. I’d also like to tap the sine and bring it to the panel (through a gain buffer)
    Can you give some more detail about how you did it please?
    Did you take the sine from the point between J201 leg and 680 ohm resistor and then to the input resistor of the buffer? (I mean the upper 680R that is connected to a 6k8, not the 680R which is connected to ground)
    Thank you in advance

  8. Richard says:

    As mentioned above, “tapped from the drain of the J201 FET”. Drain is clearly marked on my PC board. I think that’s the junction of R57 and R58 on that schematic.

  9. Fer says:

    I just found an annotated schematic and see the correct junction (I was wrong above), as you say Q10-R57-R58.
    I avoided using the pcb as a reference and used the schematic, since I may use a different FET and pinout could differ. Thank you very much Richard.

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