Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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Pinout voltages, No 12 volt on standby

Just tested my dead iMac Intel dual 2.4Ghz PSU.

It currently doesn't boot, no signs of life. After looking at the pinout voltages on here I see pin 8 should read 12V. I have the 3.6V on pin 5 but no 12V rail.

Is it likely the PSU is shot.

I had somebody elese test it and they said it was OK but I am wary of their motives. i.e. Replacing the logic board as a repair instead of swapping out the PSU.

Any pointers would be great.

Thanks

Chris

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Answered! View the answer I have this problem too

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I did do the voltage test as per the linked article. The Voltage on pin 5 is actually 2.6 VDC not 3.6. When the power button is pressed nothing happens the voltage stays the same.

I saw the LEDs (The small bank of 4 in a row above and to left of SATA cable) but they definitely were not lit. Never seen an LED that small, no wonder I didn't notice them unlit.

Checked the cable too, it is a little confusing without a diagram as to which wire corresponds to which plug on the logic board side. However after going from pin to pin on each wire I can determine a reasonable resistance readout on all with whatever their corresponding connector is.

I have an image to illustrate the 2.6 VDC on pin 5 and unlit LED but uploading it seems a challenge. I'll see if I can do it in a bit.

It does sound like PSU but you guys may have more insight, the spanner in the works is the fact somebody 'tested' the PSU and said it was fine but I am not sure of their motives for that. I'd rather know 100% myself through testing.

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If LED 1 is not lit when the iMac is plugged into a working ac source of proper voltage then the PSU is bad. LED 1 should be on whether the iMac is powered on or not. Here is richdaves guide that should apply to yours for voltages. iMac Intel 20" EMC 2210 Power Supply Output Voltage Test Something for you to think about. You can't fully test one of these power supplies without it being under load. They can have all the correct voltages and be too weak to handle the load. How do you plan on testing for this? How did the person that told you the PSU was good test the PSU under load? Did they remove it from your system and install it in another iMac? The diagnostic LEDs are a good way to troubleshoot these. From everything discussed here, if you have given accurate information (I'm not saying you haven't) - you need to replace your PSU.

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I am inclined to agree, everything screams PSU to me. I have added an image of the pin5 Voltage and LED area of the board which shows no LED1.

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He said he swapped it out in to a known good iMac and it worked fine.but this is not consistent with what the PSU is telling me.

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Maybe he never tested it, maybe he thought I'd send him the complete unit and he could tell me it was my logic board, I am a little confused about that really.

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What are the last 3 characters of your serial number, please? That way we can define your exact machine. Weak/bad power supplies can have all the correct voltages, but not be able to handle the current draw. Your 12v rail is missing then LED 2 will not be lit. With a totally dead machine you need to start with what the diagnostic LEDs say

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

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. If the power supply is weak and can't sustain the system, the LED 1 will be lit, LED 2 should blink when the power button is pressed - but may not light at all.

I would look to the guide Richdave put together for your pinouts. - iMac Intel 20" EMC 2210 Power Supply Output Voltage Test

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Hi, there are no LED's visibly lit inside, I actually cannot even see where they could be. This makes me sound like a noob but I genuinely cannot see any LED's.

However if they were lit I am thinking I should be able to see them. I understand they are located by the RAM slots is that correct?

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The LEDs on many iMacs are located by the RAM slots. The LEDs are covered by black tape from the factory on many models. They also can be located by the battery on a few models. On the early 2006 they do not exist. Again, please give us the last 3 characters of your serial number so we can determine your exact machine and better address your problem.

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Hope you don't mind me chiming in... :) What happens on Pin 5 when you press the power ON button? does it stay at 3.6V or drop to 0? Not having LED 1 illuminated is a concern though. Step 22 of THIS GUIDE shows the location of the LEDS - to the right of the RAM slot and above the SATA connector. (assuming this is correct model iMac and you are referring to this Voltage Test )

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@Richdave chime in when ever you want to share your experience or point out something I have missed. If I am totally off, drop a bomb on me. Until I was sure of the machine I couldn't give more than a standard answer as this references the iMac with no screen size, etc... Unusually the standard diagnostic LEDs to the right of the RAM, could be covered with tape, works - for LED 1-3. On some newer models the LEDs are to the left of the RAM and on other newer models they are by the battery. On some older models LED 4 indicates a heat issue, on newer models LED 4 indicates the LCD is acknowledged. Some of the newer only have 3 LEDs.

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Appreciate your cooperative spirit. :) The 2.4G was the giveaway - AFIK there were only 3 iMacs with this speed processor 2007 20", 24" and 2008 20"

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If not done already - check the cable between the PSU and Logic board is seated correctly, and that no pins have been pushed out of the connectors. This could explain no LED 1.

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Also you may want test each wire in the cable with a meter - there are reports of this cable failing. (again assuming I have ID'd the correct model) Check the part number on the cable near the logic board connector - if its 922-8188 it's the new type cable. If it is not visible then it is likely the older (less reliable??) cable.

TBH the more I think about it the more convinced I become that no 12V at pin 8 is a terminal indicator for the PSU. Hope I'm wrong.

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+ I am hoping it is the PSU. Almost certain it is. It is one of the lessor evils.

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Agreed, between Logic board and PSU, the PSU will be the better one to have to replace.

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Chris will be eternally grateful.
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