This is the first version of the 258, so sometimes called the no letter (as in A, B, C). This MEMS version was built with the panel PCB as used on the 258A and 258C so includes the two fine controls. The panel PCB provides more precise scaling and adjustment of the frequency control maximum.

The PCB was designed with only three trimmers - Offset, Scale, and Sine. There is no trimmer for the Waveshape although the schematic recommends which resistor to tweak, so the second trimmer from the bottom on the left replaces it. This board was built with matched dual pair transistors with no temperature compensation for the µA726.

Operation

The 258 performed very well, comparable to the other 258 variants. I had to add a 56K resistor in series with R15 to be able to bring the minimum frequency up to 5 Hz. On the bottom oscillator I had to increase the series resistor to the Coarse Maximum trimmer to trim it down to 20 KHz.

These three waveforms demonstrate well formed sine, ramp, and square waveforms.

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