Imac 27" startup problem

Hello everyone,I have a problem with my Imac and before to do anything I want to show you it.I've posted a video on youtube at this address: https://www.youtube.com/watch?v=bOxXPTxavqM
Thank you all for any suggestion.

This may be a start for you.
Mac computers: About startup tones - Apple Support

Have you changed out the RAM in it?

I've changed the RAM for first thing but nothing... the startup tones indicate the RAM in fact.
I would to check the voltages of power supply but I don't have a pinout.
At last trying with the reflow of CPU or GPU.

(I've sent the mac to the technical service but they told me I must change all the logic board)