Mega regulated 5vdc problem

Hello guys Wanted to know if anyone has seen this problem and knows what to test to resolve the issue. I have not been able to find a discussion related to this exact issue; only for blown 5vdc regulators. I have 4 Mega's laying around with exact same problem. They were all used at some point on a 3D Printer and I've shorted the 5vdc circuit on all the boards in various different ways, and of course damaged them. They all still power on and function fine from the USB 5vdc power but will not power up from an external power source like the Printer or a power supply to the on board Vin pin. This makes the SD card and wireless features of the printer useless and as a project board pretty much useless too as a stand alone.

Here's what Ive observed when Powering the Mega 2560 with regulated power supply @ 12.0 vdc on Vin pin and gnd to gnd. I see the following at the 5vdc regulator on the Mega's board.

Pin 1..... 0-vdc "gnd pin Pin 2..... 2.13-vdc "output pin" Pin 3..... 12.0-vdc "input pin" heat sink (large tab above regulator) 2.13-vdc

The output is approximately half the voltage I should be getting from pin 2. Because I was not exactly sure how to check the regulator by a meter, I purchased 10 replacement regulators a few months ago (planned for future burn outs) and tried replacing just one. The problem still exist so I'm assuming it might have something to do with the conditioning capacitor on the out put side of the regulator or further down stream. I'm just not educated enough to see what it might be or how to check and isolate the issue. Any help would be great! Thanks in advance.

Either the regulator is damaged and is putting out 2.13V or component has failed and causing a high drain that is dragging the +5V line down to 2.13V. Since you replaced the regulator and the "problem still exists" (does that mean 2.13V output?) then the latter is more likely. Does the regulator get very hot when powered with +12V?