+5V on USBVCC when powering from external source?

[raschemmel]

The Mega has the same circuit (T1,T2)
Mega SCHEMATIC

[Garreth]

Pulling doesnt help.
But I may have figured it out. At my Mega board, the P-mosfet is labeled as A1SHB and googling this out returns me to this note http://pdf.datasheetcatalog.com/datasheet/vishay/70627.pdf . It has different Vds, and lots of other parametes, so maybe I got the mega built on cheaper parts?

[Wawa]

I took out everything from the board - no shields, no sensors, nothing. Then I connected ONLY 9V battery to 2.1mm jack power input and I started measuring.

Are you sure the battery remained more than 7.3volt (6.6volt Vin + 0.7volt reverse protection diode) during measurements. 9volt batteries droop very quickly.
Leo..