Ha ! Found it myself ....
I don't know how I got there anymore, but at
http://www.linuxhq.com/ldp/howto/min.../powersav.html I found that I should have set option ¨DPMS in the monitor section of the /etc/X11/XF86Config file.
That worked ! My monitor first blanks, then shuts off (power-led orange ). When I activate mouse / keybord you hear the monitor highvoltage crisp and it's on again.
The highvoltage crisp is the sign that the monitor was fully powered down.
Note, that there are more interesting stuff on the site mentioned ( vsync blanking / hd spin down / ..etc... )
Thanks all for helping, hope this helps some else too
Rolf