So, it all begun with a hdd failure.
The model of imac is A1419, i believe is 2015 model.
In the past i installed yosemite, from a hackintosh stick made with clover, an imac with no problem ( it was an older model of imac , the model that has the glass kept in place with magnets).
Now the imac was in good order except from the hdd problem. when you start it, it opened with a folder with a question mark.
First i started with cmd+r and open disk utility and the only partition was osx base disk 1,3 gb, and the whole disk was seen as 1.3 gb
then i inserted a stick with high sierra, with clover. it booted directly from the stick and the clover boot selection appeared and the only option was : boot from usb. i selected to boot from usb then an apple logo appeared ( but was much smaller in size, about 1 cm) and hang like this for about 5 minutes. i restarted holding the button and tried again but it was the same.
Then i tried to boot from a sierra usb with clover, and it happened the same. it hang after i choose to boot from stick ( with the small apple logo)
Then i sad, maybe will work with the yosemite stick
)
I inserted the stick and when i start the imac, after the first sound, the screen was dark but with some light underneath, and the starting sound repeated over and over again at 2 seconds interval.
And everything i tried after was in vain.
all you could hear was bang bang bang...
So i take it to a friend who took off the display, change the faulty hdd with a new ssd. he was the one who discovered that it will start with no screen at all.
The board has 4 diagnostic leds.
with the display attached 3 leds lights up, but the last did not , and the imac entered in the chime loop.
with the display detached from the board, all the for leds will light up, only one chime sound, and the vents will go to full speed.
we concluded that is a problem is with the video card. we looked up for a spare logic board on the internet.
Then i backtracked what i did and i found this forum.
AAAhhh
i had no idea of what can happen if you use a clover usb on a mac.
Ah ah ah.
Then i thought that maybe clover messed up something that has to do with the videocard and the retina display high resolution and if i can start it without the videocard, i can install the osx and update it and maybe it will update the efi by itself (i read somewere that wen you upgrade from sierra to high sierra it will also update the efi eprom)
But without retina display and with external monitor - same chime loop
Then i opened it with no display at all and did the pram reset, and it worked.
With the display attached it did not have time to do the pram reset because it will restart itself too often (after 2 seconds)
So maybe this will help some of you in the future.
unplug the display ( the screen) from the main board and see if it starts, and if it does, do a pram reset.