Logic board or power supply?

"Your assumptions are in order. A trickle charge is sent from the power supply to the first LED. If the first LED is not lit the power supply is not working. I would suspect a possible bad connection except a trickle charge doesn't carry much current or voltage. - Just enough to supply the LED.

The diagnostic LEDs to the right of the RAM slots (this may be covered by a piece of black electrical tape). - you noticed that.

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• Verify AC source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that MXM video card is seated properly

• Verify logic board

If no lights are coming on you need to replace your power supply. A trickle charge is sent from the power supply to the first LED. If the first LED is not lit the power supply is not working."

"Hello,

As my problem is quiet the same, I don't know if I have to open a new topic?

For sometime, I had more and more difficulties to switch on my iMac when it was cold (after a night or more, I had to reboot it up to 30 time to really wake it up!), and a lot of sudden ""sleep mode"" with anything to do but reboot it…

Few days ago, finally refuse to boot:

no gray screen, no sound, just the light on in the right corner… and a new weird sound when it tries to boot!

After trying everything I could (reset PMU, PRAM, change the ram etc etc), I decided to open it.

I cleaned it of all the dust accumulated in 2 years (when I changed the original HD) and checked everything I could, but I didn't notice anything suspect.

The 1st LED is lit when the cable is connected,and when I push the power button, the 2 and 3 are lighting up too (and stay lit), and that's all.

The fans are on, everything else seems to light on, but there is still the same unusual noise and the display stay black and the HD don't seem to start up.

Only the white light (on the right beneath the display) stay lit.

I strongly suspect the power supply, but before buying a new one, I'd like to know what you think of it : )

If some has a clue or an advice, it would be great!

And sorry for my english… I'm a f… French ;-)"

"Hi FAlbano! Hi Everyone!

FAlbano I'm curious to know about your iMac. Did it work fine after changing the power supply?

Or did you find out what the problem really was?

I have a 2013 iMac 27"" and it has started randomly going off. No warning it just goes black.

And refuses to turn back on for some time. Moreover, how long it stays on seems to depend on how long I've left it off

If I try to start it again after say 10 minutes, it turns on but shuts off again in a minute or even during startup.

If I leave it for a few hours then it might work for half hour or so the next time.

When I left it alone for more than 24 hours it worked for more than an hour before it shut off.

I'm quite puzzled and it's become impossible to work on this machine.

Thought I'd like to have some clues before I take it to the service people. Don't want them to dupe me into unnecessary part replacements.

All advice is much appreciated. Thank you all!"