USB issues with Arduino 2560Mega powered by external supply

Development Tools IDE 1.x
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Hello everyone,

I'm working on a revision of a project where I run a PID control loop on a modified Mega (USB socket desoldered and

replaced with a 4pin header with only the D-, D+, and GND pins connected) with a custom shield. This shield connects a 12V

DC source to the Ardunio's Vin pin. Currently with this setup, I can't detect the Arduino on my computer.

In the previous revision, I had the Arduino powered through the 5V pin, and it worked fine.

I have both R2 and R3 of the Megas and both don't work. I've also checked the pinouts for all the USB connectors/wires and they're correct.

I've read this thread: http://arduino.cc/forum/index.php/topic,82046.0.html and I couldn't find the T2 "transistor" (its a MOSFET right?) to desolder, but I've tried just desoldering the T1 mosfet and it didn't solve the issue.

Any help would be greatly appreciated!

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Update:
I've got it working after cutting the 12V trace on my shield to the Vin pin on the Arduino and connecting a 5V supply to the 5V pin instead.
I suppose this means there is something wrong with the Arduino's voltage regulation circuit, or there is something that I'm not aware of.

On a sidenote, I've also noticed that the Vreg on the R2 runs cooler than the one on the R3, probably due to the package size.