iMac 27" 2011 does not turn on after upgrade

[igop]

Hi
After upgrading to replace the video card and processor, the iMac 27" 2011 does not turn on, the first LED on the motherboard blinks.
After I took out the memory and battery overnight, the first LED began to light constantly, but the iMac still wouldn’t turn on.
What could be the problem ?

 

[Chuckeee]

Double check internal cable connections

 

[DeltaMac]

The power supply, I think.
LED #1 should be on when the iMac is plugged in to AC power, as it shows that there is "trickle" voltage from the power supply. Should be on, and does not blink, even if the iMac is not powered on. It only shows the existence of the trickle voltage from the power supply. Flashing is not normal.
I think there can be power conditions that may turn the LED off, but not blinking. When you see the LED blinking, try disconnecting the power cord, then plug back in. That should put the power supply/logic board in the normal power configuration, which should make the LED #1 turn on, no blinking.
And, yes, re-check any internal connections that you disturbed...
Have you seen ANY video of any kind, even brief flashes, with the replacement video card? The "new" card might be defective -- but, again, the #1 LED should never flash. It's either on, or off, and usually will be on if plugged in to power.

 

[GidroGen]

Hello everyone!

I'm facing a serious issue with my iMac 27" from 2011 and can't seem to find a solution. Here are the main problems:


- All fans are running at maximum speed.


- The CPU is throttled to 1.7 GHz.


- The GPU is running at only 20% of its capacity.


- After waking up from sleep mode, everything works normally for about 10 seconds, but then the problems come back.


Here’s what I’ve already done:

- Replaced the CPU (with an i7).


- Replaced the logic board.


- Installed new RAM.


- Replaced the SSD (from a Samsung 840 EVO 500GB to a Samsung 870 EVO 1TB).


- Installed a special temperature sensor for the SSD.


- Reinstalled macOS High Sierra.


- Reset NVRAM/PRAM and SMC multiple times.


I’m using *Macs Fan Control* and *Intel Power Gadget* for monitoring, but nothing changes: the CPU stays at 1.7 GHz even under load, and the GPU runs at minimal capacity.

Has anyone encountered a similar issue or has suggestions on what else I can try? I would really appreciate any advice
or help! Thank you!