Hey lads!
Thanks for the answers! Here is the result of the past few hours: I've probably fried the sid, I don't know how. With the connection test I've measured only around 1,7-2,2V on the D6-D7 pins of the sid when inserted, 5V when pulled out. Then I checked the resistance between this two pins (out from the socket) and there was about 320 ohms resistance. Strange thing, the testtone was worked.... I had another (the last....) 8580, which has the proper voltages of all pins.... Now the other one is back in the C64, I'll test it tomorrow. I have no idea what was wrong...
Ps: the actual sid is just warm:)