Why would you doubt that the ML socket doesn't work?

It is a standard feature/connector and doesn't have to be activated, like some tv/boards.

The only thing you can check are the contacts inside the ML connector. If they all are present without strange bends, the ML will most probably work fine. It isn't a connector which you use for other reasons then ML. And so a faulty connection isn't made easily. (only if the former owner did some strange things with the ML wiring.

A suggestion:

1. Examine the Masterlink cables first, especially the connector ends.  You will either have a B&O store purchased cable ( fully intact, with set lengths ), or a ML custom length cable put together by someone. I have made and pulled custom cables for three properties ( 13 rooms ), and I used a fly fishing set with a magnifying glass to complete them correctly.

As I recall there are 21 different wires inside each ML cable: If a few are cut, or broken, there will be scratchy or no sound at all. If on the other hand, your sound is good, then you are done.

2. If you are going to possibly add more speakers along the way, look into buying an M-L junction box. We kept adding more Beolab 2000 speakers here and there, and the junction box is well worth the nominal cost.

Thank you for your answers.

The master link socket is in very good condition like for the rest of the Ouverture, so I guess it will be ok.

I've got the PDF of the service manual and I can see we can entered in dignose mode but did not catch almost anything as this guide is dedicated to service maintenance specialist. So I will keep simple and suppose if the aspect is good the functionality is good as well.

I'm sure it will work.