HP XE3 notebook

hi!
the background light of my HP XE3 notebook is not working any more.
i can not measure any voltage on the 2 pins for the neon light tube.
there has to be always something like 500 V, hasn't it? (with and
without the tube?)
on the 5 wires wich come from the pc to the high voltage converter there
is 5 V on 2 of them, around 1.5 V on 2 othes. always measured against
the black one which i believe to be GND.
is it normally the converter or the lamp which breaks down?
thanks and greetings,
peter