I've repaired and calibrated a number of 258 V2 Dual Oscillator modules. Some track, some do not and I've never understood why. I decided to build one for myself and dive deeper into the circuitry. I wanted to experiment with the difference between the discrete current source and an op-amp current source.
I used tape and reel 2N3904s for the differential pair. I epoxied them together and soldered them in place of the uA726. I was going to use a 150R SMT tempco for R33 and R89 mounted on top of the pair but decided to change resistor values so I could use a 1K axial tempco instead because of the better specifications.
With the high frequency trim (Linearity) removed I was able to get 6 octaves of range at 1V/Oct CV. I was surprised I didn't need a high frequency trim. The discrete current source didn't seem to be an issue either. I experimented with an op-amp current source but it didn't change things appreciably.
258 V2 Modifications
I built a Buchla 258 V3 Dual Oscillator 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.
The V3 includes a CA3046 transistor array which is used for the exponential pair along with a heater to operate at 55°C junction temperature. Each oscillator is separate on the PCB including the power supply connections. I chose to use a single Buchla edge connector with two power cables to the module.
The sine waves are very clean on this oscillator.
The waveshapping is different between oscillators, one having a waveshape to a saw and the other to a square.
Calibration is pretty straightforward on this module although the expo pairs are heated so you need to calibrate that first. Here is the calibration for the heater that I chose.
I had to increase R6 and R68 from 91K to 100K to be able to calibrate to 1.2V/Oct. The 1.2V calibration is through the second CV input and attenuator, and potentiometers will have some variation as to their minimum resistance. I set linearity to center, then calibrated at 1.2V for a full octave, then set the CV to 3.6V and adjusted the linearity for a full 3 octaves. Checking at 1.2V and 2.4V the frequencies were correct. Then I set the Frequency to CW, Fine to center, and adjusted the initial trimmer for 5 Hz.