thanks for the picture..ok so far
meanwhile i have adjausted a step-down-converter to 8V, which is now connected with Vin-Pin.
I guessed, that at the 5V-Pin will appear 5V .. but i am surprised 
voltmeter shows 8V here too !?????
and the other pins (reset,3V3) show 8V too !???
guess, the A.mega is damaged..