imac A1145 , sound only and no diplay

Hi Friends, I have imac A1145 , once turned on I can hear the sound but no display. What may be the problem? Light turned to white once turned on then goes to amber.
If I plug any external monitor on mini dvi port, will I get the output once turned on? or Only will work if the computer fully functional?

Is your iMac one of the iSight G5's? If so a lot of them died due to bad capacitors. Mine lasted 7 years 24 hours a day then one day bam.

Yes it is.
G5 20"/2.1/512MB/250GB

So , it got faulty motherboard?
then how the sound comes out?