I've just done the same thing with my wireless doorbell (replaced the 3xAA batteries in the chime unit with a 4.5v DC transformer.
I'm glad I checked the voltage of the cheapy cheap adjustable Maplins device first though because when set to 4.5v it was actually delivering 11.8v under no load and 10.9v under load!
The replacement delivers 4.9v (no load) and 4.2v (load) and works fine.