I'm not too familiar with how a wired door bell chimer works. But Just opened the cover of mine which was fitted to my new-build about 9 years ago. Below is a pic. From looking at it, what I can see is that the transformer has the mains input and the output seems to be from the 3 screw terminals that seem to be numbered 0 1 2 which have cat5 like cable stuck to it. Question is how is the transformer linked to the top 3 terminals? and what are the top 3 terminals exactly? I noticed that my transformer is rather hot to touch and has a slight swelling to the body in the middle. I'm concerned that this transformer might be on it's way out. Is it normal for these transformers to get hot? Shall I replace the unit?