Venue 11 Pro 7130 HDMI problem

As subject says : Venue 11 Pro 7130, problem with HDMI output. More details : As of September 3rd, BIOS and video drivers are current. When using dock's HDMI output, everything works fine no matter which device is used to display. When using the tablet itself, the built-in mini HDMI output works on a 22" LG monitor, so I know the mini HDMI connector works.The user wants to use the tablet without the dock for occasional presentations in the conference room's 55 inch TV using a mini HDMI to HDMI adapter. It's been used for other laptops with success. The Venue is the only device which refuses to cooperate. The tablet doesn't even seem to detect that the TV is connected. I tried it in another room which has a different TV brand but same result : ok with dock, nothing without. I found a thread about something similar, but it turned out to be a HDMI to DVI problem which is not my case. Can anyone help me with this ?