I built a V1 PCB set after the V2 and V3. V1 is designed for LM741 op-amps, uA726 expo pairs, and a number of vintage transistors. I chose to build this with TL071 op-amps, a matched pair of 2N3904 with a 1K tempco, and all 2N3904/6 transistors. I used cut tape for the transistors so did not match the expo pair. I used a 1K tempco so had to modify the CV gain appropriately and I left out the high frequency compensation parts.
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
This is a 258 I built for a customer that uses two of the Portabellabz 208 pA726 adapter boards. I had to drill three holes and jumper one trace that was in the way to mount them so no trimmers were covered. The two yellow wires on the adapters eliminate the need to run two additional wires between the PCBs.
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.