Computer wont start, chime wont get complete

Hello, i recently got a Imac 24 inch 2007, i replace the super drive, but after that i put it together, and power on, i just hear the chime (part of it) and then goes off? it was working juts fine before.

so when i unplug the hd, it turns on, but eventually if i put it all together eventually wont turn on, it will just do part of the chime and then goes off, and if i leave connect it wont turn on, at all unless i disconnect the power cord. its weird.

when it doesnt do the full chime sound, the diagnositc LED turn on exept for the #4, it just shuts down

hope anyone can give some advice, i think there is a problem with a power suply, but not sure yet.

thanks