My techie mate that fixes my PC when I have the occasional problem once told me that as an excersise in problem solving, the tutor on the course that he took, messed up all telephone wall sockets in the room. A good hour or so of swaping components and scratching heads went by with no success, all the time the tutor was sitting silently grinning at the front of the class.
Eventualy someone thought to check the quality of the phone signal. His point to the class of future computer fixers.........when problem solving......."always start at the wall"!!! I suppose its like with other electronic faults.....always check the fuse first before throwing the appliance away. Well spotted SRS and TW!!