I reckon there might be a few abandoned Zero4 mixers out there if not re-purposed or just sold on because of this fixable issue. You might have ran into this issue whilst trying out many various DJ or production antics that you can do with what is  a pretty versatile digital/analogue mixer.

These issues only occurs in full MIDI control mode and not analogue/digital or analogue/digital/Midi hybrid mode.

symptoms: If you found that the MIDI implementation is all messed up whilst in full MIDI mode when using it with various pro audio apps when it had previously worked without any issue, you might need to find some way of clearing the MIDI mapping so it can return to the default values.

I'm not sure if there are many other scenario's where the MIDI mapping might get a little stuffed up, but from experience it might have happened whilst using the Traktor Scratch pro software MIDI interface adaptor to make the Zero4 work with the Traktors layout and then returning to something like Cubase or Ableton Live to find that it no longer works when trying to assign the MIDI controls in learn mode. I only mentioned the applications in that order because I started out on Cubase first.

The typical symptoms are that various midi control signals are doubled up to not allow you to individually assign them to individual control elements in your chosen production/DJ software. You might also find that control properties that you need to assign also won't be correct although still assignable on those physical midi controls that actually still function.

Trying to change the MIDI signal bit mode within your chosen software doesn't work either. (i.e.. all variants of Absolute and Relative)

If you check the MIDI implementation from within the Zero Edit software that came supplied with the mixer you'll also find many of the MIDI key signals are missing for many control elements on each of the physical MIDI layouts on each of 4 channel control sets.

If any combination or most likely all of the above apply you will need to reset the mixer.

- Hold down the "GATE PLAY" and "CUE SPLIT" button at the same time whilst you Zero4 mixer is powered off.

- Power on your Zeor4 mixer whilst still holding down the a fore mentioned buttons.

- You should now see the letter "in it" within the BPM LED reads out display.

- Keep holding the buttons down till the mixer fully boots up to show the BPM count at 120BPM again.

You should now find that normal assignable MIDI control and "MIDI learn" features have now been fully restored.

You will need to re-install the official Korg Traktor adaptor software again if you wish to use Traktor pro again.