Behringer xenyx QX1222 USB microphone not working

[Aheld]

Hello Forum,

I have a Behringer sound mixer QX1222USB for repair, where the USB microphone is not working.
The USB connector seems to be very special. It has the normal 4 pins (I think for power supply) and in addition 5pins (I think for sound).
I found out which pins are used for sound, but for what are the rest of the pins used for?

The sound mixer it self is working fine, but it seems that the mixer is not realizing that the USB adapter is connected and is not switching the path of the microphones.
I tried internet search, but neither USB connector pinning is available, nor a schematic for the QX1222USB (the normal QX1222 is easy to find :-) ).

Any help is highly appreciated.

So long.
Aheld