a mega 2650 power supply mystery
You obviously have some form of DMM or volt meter at your disposal, since you know the voltages around the regulator.

Some places you should, therefore, probe for voltages, and what voltages you should expect.  With the barrel jack connected and powered up, and assuming a 12V supply, you should expect:


At the rear of the socket: 12V
At the socket side of the M7 diode: 12V
At the header side of the M7 diode: 11.4V
At the right-hand (VIN) pin of the regulator: 11.4V
On the tab of the regulator: 5V


If you find any of those are wrong then the component that comes before it may well be faulty.