You need Technote 01 for the Minibrute,
It says something like this:
let the synth warm up for at least 10 min
Connect a digital tuner to the output,
Slide up the sawtooth waveform fader to maximum level.
Press the Octave buttons until reaching the center position: green LED is lit.
Make sure the Fine Tune knob is at center position.
Find a plastic (!!) 2mm flat screwdriver,
Press the lowest key on the left of the keyboard, and hold it down.
The frequency should be a perfect C at 130.80 Hz (0 cent).
If it is not the case, turn the Slope trimmer to reach this value.
The Slope trimmer is nearest the MIDI in/outs, the other one is the Offset trimmer
1) Press the upper most key on the right of the keyboard and hold it down.
Turn the Offset trimmer to reach C at 523.30 Hz (+/- 5 cents).
2) Press the lowest key on the left of the keyboard, and hold it down.
Turn the Slope trimmer to reach C at 130.80 Hz (+/- 5 cents).
Repeat steps 1) and 2) until both notes are in tune (+/- 5 cent)
Full “0-cent accuracy” can not be achieved: +/- 5 cents tolerance generally is OK.
Expect some drift at the extremes (up to +/- 20 cents): C-3 will *not* be accurate, neither will be C5.