OK, all is in the tittle.
I leave in France, but nowadays it can be easier to send it far away to the right person.
I bought it at a very low price on Ebay with a set of filters (100x100). But it rate 2800 K° in front of a new redbeam, and 4800 for a cloudy sky. So it's clear that it's all lower than what it would be.
Don't know if it is just a calibration issue or perhaps the head is dead.
So now I look for the right doctor, hoping it will not be too expensive...
Many thanks for your answers !