820-3481 imac 27" late 2013 simple upgrade question


Hi, i wanted to uügrade this imac but there faces me some problems.

1. upgrade from i5 CPU to i7 4.0Ghz (4790K)

insert the i7 there i have only 1. and 2nd led after a wilhle second turns off and first is only on then second turns on again and so on, no dong, nothing
insertet the i5 all is running like a charme

has anybody already doing an upgrade with a 4790K ?
or is it only compatible with the 3.5Ghz i7 ?

2. replace 3TB HDD with a 4TB NVMe plus adapter

inserted i5 all is ok
insert the NVMe
first led is on
after perssing power button only 1. led turns off and on again
after removing the nvme all is working good

inserting the NVME and the adapter in a A1502 2015 all is working good

what i do wrong here? sorry for that simple stuff but i tryed that already some houres now and i dont know what is the problem

thanks for some help


Staff member
What is your 820-xxxx board code?
Check against schematic if your board accepts NVME, or only SATA connection.



i checked the 820-3299 because i think no one has the 3481

J4500 has only SATA_SSD lines. so thats the reason isnt it? but may be the 3481 accept nvme?

and the 820-3299 has in the schematic a 1155 socket but my board 820-3481 has a 1150 socket. in the socket is the SR14D i5-4670
but by cheking the serial DGKLN2MAF8JC it shows can be i5 and i7 (i74771)

why the Intel Core i7-4790K dont run? of course may be its damaged. thats why i wanna ask, how i can be shure if this cpu can work in this board?


found that probably the efi dont support the 4790 only the 4771
there is no way to edit the efi that way?