I done the power modem from PC PSU mod using Granty's SB5100 tut, I just adapted the tut slightly and used a 250 instaed of the SB5100, you can also use a 100, 120 or 200 as they all use the same PSU.
With a little Modifcation You Can Also Fit The Other Ambit Modems..
just to had to cableguys69`s method .
instead of soldering to the board of the modem you could buy from maplins a dc plug check the size and a spare y power lead ..cut the black wire and yellow from one end of the lead and solder to inside of plug check the supply iam sure that the outer casein is earth (-black) and the inner is live (+yellow) Use a Multimeter
this way you can remove the modem and connect any ambit to this plug. also it is a constant 12v supply..
Dc Power Plug (Check Size)
Remove the yellow and black wires from one end of the male plug and strip back and solder to the correct dc plug.(look at the dc plug that powers your modemf or size)
Black -should be soldered to the outer casing and the yellow + soldered to the inner as in picture