Arduino Ethernet Board - Problem with external Power Supply

Hello!

I have a problem with my new Arduino Ethernet Board R3.

If I connect the Arduino via a FTDI Interface I could upload sketches and everything runs perfect. But if I don't use the FTDI Interface but a external Power Supply (which I also use with my other arduinos) I can't use the ethernet. The Board seems to power up and the power lamp is on but the arduino doesn't send any data and pinging doesn't make any response ...

I'm using an 1000mA adapter with a 2.1mm center-positive plug. I tried 6V, 7.5V, 9V and 12V.

If I use the FTDI Interface the power AND the L9 lamp is on. If I use the power adaptor only the power lamp is on.

Any Ideas?

Best regards,

Muetze303

Do you have a multimeter?

When external supply is set to 7.5V or 9V, what voltage do you measure on the 5V pin?

I measure 4.56V on the 5V Pin, regardless if I choose 7.5V or 9V.
I measure the same when I use the FTDI Interface.