Modem power adaptor
On the bottom of the Modem, it usually lists the type of voltage/current ratio, for example, My modem is a 12VDC in, output is .75A Through the modem. Each modem has its own power requirements. Therefore if they dont match, you have the chance of either underpower (which means no use) or overpowering (and killing your modem permanently)
Best suggestion si the same as above by cnavarro, Buy the Right power supply for it, dont just try anything. get the matching one. Otherwise it will be Incident A or Incident B. Neither would work well, and you would end up buying a new modem all together.