Thanks Dave !
Franck

Jimmy, Dave,
I've found 2x 20K resistors in an electronic store in Paris. I'll try this week end to sum the 2 signal from XLR to pin 4 of the jack i.e.:
- orange signal to pin 2 XLR + 20k resistor from pin 2 XLR to pin 4 of the 1/4jack.
- violet signal to pin 3 of the XLR + 20k resistor from pin 3XLR to pin 4 of the 1/4jack.  
I'll let you informed if any progress.
Regards
Franck

I've done the wiring yesterday night.(I could not wait the week-end to do it !)
Now both jack and xlr are working correctly.
I would like to thank you all for your help and support to fix this issue. I think I wouldn't have being able to do the appropriate corrections without your support and knowledge.
Once again thank you !
Franck

Franck,
Great job, congratulations!  And you've learned something about electronics too.
Enjoy this bass, you are lucky to have it in your collection!
Jimmy J

Congrats!!