iMac 2011 LED #1 lits for a second then goes off

[Tom.Tom]

Hi all,

Come across another problem. Got a 27 2011 iMac that wont boot. In the beginning, when connected to power, the LED #1 was lit and it went off after pressing the power button. I removed the MoBo battery, did SMC reset and now, when plugging the power cable LED #1 lits for a second and then goes off.

Checked with two other different PSUs - same thing.
Checked the MoBo with everything disconnected except PSU - same thing.

Anybody has ever come across such an issue? Any advices what to check further highly appreciated before I start digging into schematics trying to trace that myself having no former experience with this kind of problem.

Tom

 

[tgaillar]

Currently hunting down a similar issue on a 27" 2010 iMac motherboard:

Spoiler: A few technical hints at the investigation, for what it's worth... ;-)

• LED #1 lights up for a fraction of a second after plugging the power cord, then dies, then flashes even quicker a second time ~3s after that
• Stripped *everything* off the mobo (CPU included) and same behavior
  
  

Since LED #1 mirrors PP3V3_S5 (3.3V power line in halted mode), I traced the signal further with an oscilloscope placed at the output of the L7710 coil at the PP3V3_S5 generation stage

One can clearly see the 2 "chirps" ~3s apart (the latter much shorter)

A closer look at the left "chirp" shows 3.3V generator starts OK then ... dies

So PP3V3_S5 generation works and starts OK, to a point it suddenly (and unexpectedly) stops. No short seen so far (~96 Ohms measured @ L7710, like such any other iMac), PP12V_S5 is OK, I guess PP5V_S5 too, investigating (on and off...)

To be continued ;-)

EDIT: put everything in a spoiler frame, so not to hijack OP ...

 

[nikey22]

U7700 should turn on and generate PP3V3_S5
what's the resistance to ground on L7710?

If you don't have more than 1ohm to ground on PP3V3_S5, something has to get hot, feel for hot components. This is assuming your are using a known working power supply that will provide good amperage.

 

[Tom.Tom]

On both pins of L7710 I have around 100ohms. The whole PP3V3_S5 line tested on the TP537 has 100ohms to ground so that does not look like a short there. I can inject 3,3V into TP537 and getting 1,15A with LED#1 on and nothing seems to be heating up. However no 2nd LED after pressing power button.

Any clues where to look further?

Is it possible that there is something wrong with 3V3_S3 lines which is causing 3V3_S5 to shut?

 

[boombari]

Hi all,

Come across another problem. Got a 27 2011 iMac that wont boot. In the beginning, when connected to power, the LED #1 was lit and it went off after pressing the power button. I removed the MoBo battery, did SMC reset and now, when plugging the power cable LED #1 lits for a second and then goes off.

Checked with two other different PSUs - same thing.
Checked the MoBo with everything disconnected except PSU - same thing.

Anybody has ever come across such an issue? Any advices what to check further highly appreciated before I start digging into schematics trying to trace that myself having no former experience with this kind of problem.

Tom

Hi Tom, I have the same problem as you, how did you solve it?

 

[Tom.Tom]

Hi, haven't yet - I'm in the process of searching how does the MoBo boot sequence look like...

 

[Tom.Tom]

Interesting - have taken out the CPU and got LED#1 steady ON! Looking for some other CPU to test it...

EDIT: checked another CPU - same thing - with the CPU in, the LED#1 turns on for a second and then goes off, without CPU it's on. Examined CPU socket under the microscope and found some rubbish in there. Removed, but that didn't help.

 

[Tom.Tom]

ok, putting CPU into the socket gives me a short on the PPVCORE_S0_CPU - need to investigate the socket again - maybe there is something under one of the pins that makes the short when it's pressed by the CPU...

EDIT: well, no it's not a short - there is around 16 ohms which should be fine with such a low voltage and high power in that circuit...

 

[The_Croupier]

Have you tried removing the GPU. I have experienced similar symptoms with a short on the GPU.

 

[Tom.Tom]

Sure - that's on the motherboard alone - just the CPU and power cable - nothing else

 

[mapeai]

Hi Tom, any solution? I have exactly the same problem. LED #1 was lit and it went off after pressing the power button. After changing the battery of the motherboard, now LED goes on then off in less than a second

 

[Tom.Tom]

@mapeai try the following two procedures:

1. Press the power button and while pressing it remove the power cord still keeping the button pressed. Keep the button pressed for 10-15 more seconds, then release it. Plug the power cord back in and see what happens when you press the power button as you normally do.
2. Remove the CPU and try starting the board without CPU - if you get LED #1 and LED #2 on, it means this is the same problem as mine - probably some short in the CPU power rail.

Haven't had time to track this down as I got another mobo for cheap. Tracking down the short on the mobo should be doable, but if you have no experience in doing such analysis, it will be cheaper to get another board than trying to repair the one you have.