Any reputable manufacturer (I'm assuming USR, too), will have a symbol by the power plugin (I think its a UL thing) that is basically a dot and a half circle with a stem. Near there should be a power rating. One should be volts, one should be amps. match them The dot/half-circle will tell you the polarity. If you have an electronics store near you, take the modem in to them, and they'll be able to match it easily, for no charge. If you're in the US, it should be around $10 or less for such a plug. This works for just about everything electrical, btw, so have them explain it to you to save you time and effort in the future.