I haven't tried an external monitor on the iMac DV 400 (these units are not in the same place btw). I have tried a homemade 20-pin to 15-pin VGA adapter for the iMac Indigo but nothing happened, I'm not sure if my homemade adapter works or not, though. Should the external monitor on the DV doesn't work, what could be the likely culprit?

(any ideas on the culprit for the other earlier iMacs?). Thanks in advance, again.