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Help!!!, I want to save this iMac 27 inch 2011

A

AlejandroPeralta2019

macrumors newbie
Original poster
Oct 31, 2019
16
3
Argentina
Hi everyone, it's been a while since I've been on the forum.
We're a group of friends who want to save this iMac from being forgotten. The iMac won't turn on, so we decided to carefully open it up, etc.

- We replaced the graphics card (for one compatible with "opencore" metal), and it worked.
- We checked all the connectors and found nothing unusual.

Until we discovered that when we plugged in the computer, none of the motherboard test LEDs lit up. We did some research, and with the help of ChatGPT, we read that when we plugged in the computer without turning it on, LED 1 should be lit, indicating that there are 5 volts on standby. That's not the case. When we plug it in, none of the LEDs light up.

As for the problem, the power supply is in the power supply. ChatGPT and a photo from iFixt showed us how to measure the power supply and run some tests. In the attached photos, we used a multimeter to test the power supply (we disconnected the MOLEX output from the motherboard). With the power supply connected to power, we discovered that it is delivering 12 volts (pins 1 and 4), but not delivering the standby 5 volts (pins 1 and 3). Therefore, the hot stage would be working, but the cold stage would not.

So far, we've discovered a fuse in the power input. We've measured it, and it works perfectly. We've disassembled it and inspected it for any explosion or other obvious problems, but we haven't found anything.

We would appreciate it if someone could help us and guide us through any steps we can take to measure, learn, and ultimately fix it.

We look forward to hearing from you. We're not engineers; we love electronics. But if anyone has any suggestions, please explain everything we need to do to fully understand it.

Greetings from Argentina.
 

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I would start by testing the individual capacitors on the board of the power supply. You could probably narrow it down with a schematic of the board. You can probably find one by Googling it.
 
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Do you think it could be a capacitor? I've read in some forums that one part of the board has a "diode rectifier bridge" and that it usually breaks. We're still searching. We should try the capacitors, but since they're 12V, at least we know that large capacitors can't be the problem. Right?
 
I'm not sure. I was just guessing. I guess it could be the bridge rectifier by your description of the lack of low voltage. You should test every component because there could be multiple faults. I think you have to isolate the circuit and then the component(s). Easier to just test every component.
 
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AlejandroPeralta2019 said:
Hi everyone, it's been a while since I've been on the forum.
We're a group of friends who want to save this iMac from being forgotten. The iMac won't turn on, so we decided to carefully open it up, etc.

- We replaced the graphics card (for one compatible with "opencore" metal), and it worked.
- We checked all the connectors and found nothing unusual.

Until we discovered that when we plugged in the computer, none of the motherboard test LEDs lit up. We did some research, and with the help of ChatGPT, we read that when we plugged in the computer without turning it on, LED 1 should be lit, indicating that there are 5 volts on standby. That's not the case. When we plug it in, none of the LEDs light up.

As for the problem, the power supply is in the power supply. ChatGPT and a photo from iFixt showed us how to measure the power supply and run some tests. In the attached photos, we used a multimeter to test the power supply (we disconnected the MOLEX output from the motherboard). With the power supply connected to power, we discovered that it is delivering 12 volts (pins 1 and 4), but not delivering the standby 5 volts (pins 1 and 3). Therefore, the hot stage would be working, but the cold stage would not.

So far, we've discovered a fuse in the power input. We've measured it, and it works perfectly. We've disassembled it and inspected it for any explosion or other obvious problems, but we haven't found anything.

We would appreciate it if someone could help us and guide us through any steps we can take to measure, learn, and ultimately fix it.

We look forward to hearing from you. We're not engineers; we love electronics. But if anyone has any suggestions, please explain everything we need to do to fully understand it.

Greetings from Argentina.
Click to expand...
Greetings Alejandro,
I cannot claim knowledge at the level of your electrical understanding but I have worked on the 12.1 model. I inserted a Samsung Evo 870 SATA SSD to run legacy software and was impressed with result.

If you have no trickle power, clearly the power unit is damaged.

In case you have not found full diagnostic LED information:

LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.

LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.

LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

Personaly, I would simply buy another 12 series iMac or another used power unit. As you may know, a new power unit will be disproportionately expensive relative to the iMac value.

If you seek to apply your electrical skills to the challenge of fixing the iMac I wish you well.

For what it is worth, I understand all the iMac 12.2, Mac 27" series use the same standardised power unit (325 watt if I remember correctly), and each 12 series model draws power according to its configuration needs - so you can use any 12 series iMac configuration to cannibalise power unit.

I am curious to learn which GPU you chose? How did you established the GPU worked without using the Apple start button? Did you simply power the GPU from an independent source?
 
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Getting 12V on your PSU should indicate your power supply is fine. I would take the logic board out and inspect that whole board itself and the CPU - in the same way you have done for the power supply, checking for any type of corrosion, or exploded components.

I would also make sure you have actually plugged the cable in around the back of the logic board connecting it to the power supply! It's an easy one to miss.
 
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As you have only 1 iMac 2011, it will be difficult to test and isolate the problems.
Assuming you have fixed the PSU, if the logicboard is still dead, you would see the same result: No LED.
And you don't really need the GPU to test PSU and logicboard: Without the GPU, the iMac still runs: the chime, the fans, the HDD, 2 LEDs etc... providing that the parts are working.
 
Nguyen Duc Hieu said:
As you have only 1 iMac 2011, it will be difficult to test and isolate the problems.
Assuming you have fixed the PSU, if the logicboard is still dead, you would see the same result: No LED.
And you don't really need the GPU to test PSU and logicboard: Without the GPU, the iMac still runs: the chime, the fans, the HDD, 2 LEDs etc... providing that the parts are working.
Click to expand...
Who said I don't have another iMac? I have an iMac exactly the same, I haven't tried putting in the power supply yet because I have that computer at work, I can't take it apart right now.
 
AlejandroPeralta2019 said:
Hi everyone, it's been a while since I've been on the forum.
We're a group of friends who want to save this iMac from being forgotten. The iMac won't turn on, so we decided to carefully open it up, etc.

- We replaced the graphics card (for one compatible with "opencore" metal), and it worked.
- We checked all the connectors and found nothing unusual.

Until we discovered that when we plugged in the computer, none of the motherboard test LEDs lit up. We did some research, and with the help of ChatGPT, we read that when we plugged in the computer without turning it on, LED 1 should be lit, indicating that there are 5 volts on standby. That's not the case. When we plug it in, none of the LEDs light up.

As for the problem, the power supply is in the power supply. ChatGPT and a photo from iFixt showed us how to measure the power supply and run some tests. In the attached photos, we used a multimeter to test the power supply (we disconnected the MOLEX output from the motherboard). With the power supply connected to power, we discovered that it is delivering 12 volts (pins 1 and 4), but not delivering the standby 5 volts (pins 1 and 3). Therefore, the hot stage would be working, but the cold stage would not.

So far, we've discovered a fuse in the power input. We've measured it, and it works perfectly. We've disassembled it and inspected it for any explosion or other obvious problems, but we haven't found anything.

We would appreciate it if someone could help us and guide us through any steps we can take to measure, learn, and ultimately fix it.

We look forward to hearing from you. We're not engineers; we love electronics. But if anyone has any suggestions, please explain everything we need to do to fully understand it.

Greetings from Argentina.
Click to expand...
Go to this site https://archive.org/details/appleservicemanuals/imac-27-2017/page/n71/mode/2up and you will find Apple technical repair manuals with diagrams and procedures.
I hope this helps solve your problems.
Cheers,
 
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iMac Status Update
We tried another power supply that we know works., and the computer didn't turn on, so we conclude that the motherboard has a problem.

The power supply works perfectly.
I've recorded some videos for you to see. When you plug in the power, LED 1 tries to turn on, but it turns off after a second, as if it has some protection mechanism due to a short circuit.

We replaced the graphics card with one we know works.
We don't know how to check the motherboard or how to find the fault. We appreciate your help. Greetings, everyone.

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We tried another power supply that we know works.
 
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AlejandroPeralta2019 said:
iMac Status Update
We tried another power supply, and the computer didn't turn on, so we conclude that the motherboard has a problem.

The power supply works perfectly.
I've recorded some videos for you to see. When you plug in the power, LED 1 tries to turn on, but it turns off after a second, as if it has some protection mechanism due to a short circuit.

We replaced the graphics card with one we know works.
We don't know how to check the motherboard or how to find the fault. We appreciate your help. Greetings, everyone.

View attachment 2510900

View attachment 2510901
Click to expand...
Dear enthusiastic Mac friends,
On Monday I sent you a link to a free library of Apple iMac service manuals to help you assess the task you are facing. It seems you have not used or read the link.

Also this thread deals with power supply.

Looking for schematic diagram of 27" 2009-2011 iMac power supply, PA-2311-02A (liteOn)

Hi all, I'm looking for a schematic diagram of the 27" 2009-2011 models power supply, my model number is PA-2311-02A (LiteOn branded). If you happen to have the one of model ADP-310AF, I appreciate this, too. I have repaired several standard switch mode supplies, but Apples is more complex, with...
forums.macrumors.com forums.macrumors.com

Diagnostic Lights
That Apple manual offers circuit diagrams and confirms diagnostic light 1 measure trickle voltage and should remain ON as long as power is supplied. If light fades off as you describe, that power board is NOT working perfectly but also has a fault.
The diagnostic lights work in analog sequence to check logic board, Power supply, CPU, GPU and backlight/monitor; read the Apple Service manual.

Logic Board
If you want to have fun keep at it, you may get lucky. If you want to repair the iMac, I would take the Logic board to a professional micro soldering technician who has the equipment and understanding to analyse and fix micro circuitry.

Good Luck.
 

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AlejandroPeralta2019 said:
iMac Status Update
We tried another power supply, and the computer didn't turn on, so we conclude that the motherboard has a problem.

The power supply works perfectly.
I've recorded some videos for you to see. When you plug in the power, LED 1 tries to turn on, but it turns off after a second, as if it has some protection mechanism due to a short circuit.

We replaced the graphics card with one we know works.
We don't know how to check the motherboard or how to find the fault. We appreciate your help. Greetings, everyone.
Click to expand...

The easiest and doable thing now is to
1. Remove the graphic cards and recheck the LED indicator.
 
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