Power problem with a Studio 1558

When plugged into the wall, the LED on the ac adapter goes on (when not plugged into computer) as normal, no flicker or anything. But when plugged into the computer, it immediately goes out.  If plugged into the wall when already plugged into the computer, it blinks on for a sec and then quickly goes out.  My computer functioned as normally while this happened (just didn't charge) but is now almost completely out of battery power.  I also tried plugging the laptop in with the battery removed, but the same thing happened.

I've got a studio 1558 and the warranty ran out a few months ago.

My question is do I need to replace the adaptor or is it a more costly repair to the laptop?