T440 Ultra Dock ext. screen

[hhmcsv]

I would like my screen on the T440 (running Win10pro) visible on an external screen (Philips TV) using the HDMI connection from the dock. I have read the other contributions containing the upgrade to firmware 2.30, but the solution is not stable.
Occasionally, using this sequence would give a result, but seldom:
1. Apply power to TV set, but do not turn it on
2. Apply power to the Ultra dock
3. Place the T440 in the dock and turn it on (usually from sleep)
4. Turn on the TV
But on most occasions, both screens turn black, and it is not possible to get any picture on either PC or TV (which just states "no signal")
When it works, both picture and sound comes over fine (in control panel, the TV sound function has to be set active)
Connecting the TV directly from the T440 using a dvi-hdmi converter cable works flawlessly.

What do I do wrong?