NeXT Cube - powers on briefly then off again

[Toby Thain]

Hi,

After a few years I wanted to try my Cube with the soundbox/VGA splitter 
configuration for the first time. (I have the fading phosphor N4000A so 
have been keen to have an alternate video solution.)

However, all is not well. Using the keyboard power button, the machine 
powers on for a couple of seconds, just long enough to see a NeXT logo 
and grey desktop on the VGA (yay!) but then powers itself off again.

Any clues?

Thanks in advance

--Toby
(not having a good classic computing week)