ok. I followed all the step recommended on Apple site : Resetting an AirPort base station FAQ - Apple Support
I tried several time the 3 differents resets on airport
- Soft reset
- Hard reset
- Factory default reset
And in the last weeks, here is the sequence :
1. iMac & Airport TimeCapsule went fine
2. iMac TimeCapsule on Airport failed : dont remember if something special occurs ? (may be on 2015-october, new MacOs release ??)
3. with iMac : tried all 3 resets ... still fail
3. iMac TimeCapsule transfered to a USB external drive
4. new Macbook, christmas gift 🙂, and work on the first time with Airport : Wifi & Timecapsule
5. re-tried all 3 resets, including unplugged ...
6. iMac : still does not work
7. MacBook : still work fine
8. I'me writing on this site to resolve
I turned ON and OFF my iMac several times.
I unplugged and plugged Airport.
I plugged the iMac on Ethernet wire (just to try). but presently unplugged, only WiFi
BOB