all these iMac ranges have diagnostics LEDs on the board. on this range they are on the lower left of the board.
If you are not getting the first LED coming on when plugging it in, then there is no trickle charge from the PSU. This either means the logic is shorted and damaged, or the PSU is dead.
If the LED is coming on, and the 2nd doesn't come on when the power button is pressed, check the power button to make sure you haven't squashed it between the speaker and PSU, as its really common to damage it in that way, then check the PSU or logic.
But if it was fully working beforehand, and you've fitted a non-standard CPU and proper antistatic precautions were taken during the rebuild, its likely the logic board at fault unfortunately.
it is difficult to say for sure without seeing the machine, but i hope this helps.
all these iMac ranges have diagnostics LEDs on the board. on this range they are on the lower left of the board.
If you are not getting the first LED coming on when plugging it in, then there is no trickle charge from the PSU. This either means the logic is shorted and damaged, or the PSU is dead.
If the LED is coming on, and the 2nd doesn't come on when the power button is pressed, check the power button to make sure you haven't squashed it between the speaker and PSU, as its really common to damage it in that way, then check the PSU or logic.
But if it was fully working beforehand, and you've fitted a non-standard CPU and proper antistatic precautions were taken during the rebuild, its likely the logic board at fault unfortunately.
it is difficult to say for sure without seeing the machine, but i hope this helps.