iSight seems to start after opening but shuts down again

I bought a second hand and defective external iSight. The tube-shaped one. You start it by rotating the lens. It makes a focussing sound and the green LED turns on but then it shuts down. As if there is some short circuit or defective component. Anyone's got a clue to what could be wrong or what to check? I opened it but didn't find any visual problems. The firewire cable is ok. But Macs I hook it up to don't see the webcam.