Yesterday I was looking at one of those store-bought multi voltage AC/DC converter bricks. One of the big heavy ones that let you select anywhere from 1.5 to 12V output.
When I checked this one with a multimeter, the voltage was messed up. It was set to 1.5V, output was 5V. Setting it to 4.5V gave an output of almost 9V. And so on.
Inside, a typical chinese-made rectifier. A multi-tap transformer, 4 diodes, a capacitor, a selector switch and a power LED. Resistor from one of the diodes to ground, and resistor in series with the LED. That's it.
Checked the 4 diodes for shorts or opens, they're fine. Checked the 2 resistors, they both measure up OK. LED lights up ok. And, no sign of problems in the cap.
So that leaves only one culprit: the transformer. Might as well get a new brick I suppose.
Getting into a more detailed discussion, out of curiosity, what's wrong with the transformer inside? Most bricks I've seen that fail just go dead because the windings open. But this one seems to be putting out more voltage than necessary. What causes that?