This MEMS 258C is a faithful recreation of the 258C Dual Oscillator with a panel PCB for easier DIY construction.

To make construction easier, MEMS has designed a front panel PCB for wiring of all the panel components. This much simplifies the DIY construction vs. hand wiring all of the components as on my earlier 258C build. This PCB can be configured for both the 258A or 258C and has additional trimmers for better calibration.

The 258C features two CV inputs with +/- CV controls. CV1 also has a fine adjust +/- CV control. As designed, scaling is done for the CV2 input with full CW rotation of the input control. CV1 is not scaled and requires operation of the fine adjust CV control along with full CW rotation of the input control for correct scaling. 1.2V/Oct scaling requires four resistor changes which are included in the build instructions. Located on the PCB are also two 20T trimmers which allow for precise CV1 and CV2 scaling. An additional trimmer sets the maximum frequency of the coarse control. 

This panel PCB is also setup for a switching Tinijax so that with no plug inserted, the outputs are cross coupled to the FM inputs. You can ignore the switched connection and just use a non-switching Tinijax if this feature is not desired.

The main PCB is an authentic reproduction of the 258C which uses an µA726 heated exponential converter. MEMS supplies a SMT adapter PCB which replicates the µA726. The adapter mounts using wires to the µA726 footprint. This photo is the main PCB before the µA726 adapter and ICs have been installed.

This photo shows the assembled panel PCB.

This photo shows the assembled main PCB.

This photo shows the rear of the assembled module. The panel PCB trimmers can be accessed from the sides and bottom. The main PCB trimmers can be accessed from the rear.

Calibration

Calibration consists of setting the µA726 Replacer operating temperature, scaling the oscillators to 1.2V/Oct, setting the low frequency offset, setting the maximum coarse control frequency, and adjusting the sine shape and waveshape controls.

The calibration for the µA726 Replacer is on a this page.

These are the various trimmers on the panel PCB for calibration.

Note the panel PCB logo reflecting their new arrangement with Buchla.

These are the standard trimmers for the main PCB calibration.

Scaling Adjustment

The original 258C has no scale trimmers. The panel PCB has scale trimmers for CV1 and CV2.

The BOM is setup for 1.2V/Oct scaling with the four 100K resistors on the panel PCB. These need to be changed if a different scaling is desired.

Set the CV1 attenuverter control to full CW. You have to decide what to do with the CV1 fine control. I centered it so I could slightly detune CV1 +/-. Scale CV1 precisely with the panel trimmer. The design is quite accurate for 4 octaves and goes a bit flat on the 5th octave. Adjust the high frequency compensation and then recheck all octaves. The CV2 scales the same way. I use my VCO Scaler for easy calibration (I donated this design to MEMS who made me a PCB).

Offset Adjustment

I calibrate the offset with the fine frequency control centered and both CV attenuverters centered. Turn the frequency control full CCW and adjust the offset trimmer for 5 Hz. On this particular module I could not get above 3.8 Hz. I added a 100K resistor in series with both R11 to adjust to 5 Hz.

Coarse Control Range

The maximum frequency is typically beyond 20 KHz (this module was almost 40 KHz). Turn the frequency control full CW and adjust the panel trimmer to set the maximum frequency at 20 KHz.

Sine Adjustment

The 258C has no symmetry trimmer and symmetry may not be ideal at the minimum and maximum frequencies. I adjust my sine shape at ~ 440 Hz. Adjust the sine trimmer for best shape. Depending on your JFET a resistor tweak of  R60 may be required.

Waveshape Adjustment

Adjust the waveshape trimmer for best ramp and square. Depending on your JFET a resistor tweak of R43 and/or R56 may be required. The waveshapes are reasonable up to about 10 KHz.

I used J201s for all the JFETs. There is a wide variation in JFET specifications. I had to increase R46 to 820K for the ramp and 2M for the square to have a reasonable range for the waveshape control. The wave starts to morph in the first 5 - 10% of rotation and reaches full amplitude at CW.

The waveshapes looked pretty reasonable all the way to 20 KHz.

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