Hi from a 68 yr old newbie who has apparently designed a Puff of Smoke add-on for his Uno.
All working fine from the USB, but not when powered from the barrel jack (correct term??).
I've had a good poke about but can't find exactly what the offending little #### is (that's exactly where the smoke came from, and there's a fine crack running across the top) - arrowed in pic. If I can identify and source a replacement I'll try to make less smoke soldering it in
Same answer as with the 101, it is a diode.
If the other components do not get hot when you are powering via the USB connector or the VIN pin, and if the Uno otherwise works, you can replace the diode if you want to use the barrel jack. Let the schematic be your guide. It looks like the diode is an M7 and if you google "M7 diode" you will find sources online. You can probably just use any general purpose rectifier diode or schottky diode. If you use a non-SMD diode like a 1N4001 you can bend the legs inward and cut them short, and hold it down and tack it into place with solder. The purpose of the diode is to protect from reverse polarity on the barrel jack.