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  1. #1
    Proper way to check voltage rails on removed logic board?

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    Proper way to check voltage rails on removed logic board?
    Experts,

    I had an A1278's 820-3115-B out yesterday, and the CPU was running and got (understandably) hot. I unplugged the magsafe/power immediately.

    Where is documentation on how to do this safely?

  2. #2
    Proper way to check voltage rails on removed logic board?
    chscag's Avatar
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    Welcome to our forums.

    Apple documentation is akin to the Atomic Secrets. It's proprietary and Apple keeps it close hold.

    Go on YouTube and search for procedures like yours. There are literally hundreds of troubleshooting videos you can scan through. I'm sure you'll find something there. We here at Mac-Forums do not have any special access to Apple documentation.

  3. #3
    Proper way to check voltage rails on removed logic board?
    iggibar's Avatar
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    The answer you are seeking is more specific than the generic info and question you stated.

    1. Define 'hot'.
    2. The are different voltage readings everywhere. What voltage reading are you trying to find? I'm assuming CPU?
    3. The CPU doesn't always get the same amount of power.
    4. What was the condition of the board when it was 'plugged in'?
    5. A CPU on a logic board that doesn't have an SSD plugged in can run SUPER hot under some conditions. This is normal. The CPU will run on MAX power if there's no controller.

    I can never recommend anyone play with electricity. You will NEVER find documentation from Apple that shows you how to do this. But if you are comfortable working with electronics, and are specifically looking for board diagrams for layouts so that you can find the contact points to read voltage ratings....here's how I go about it. Again, only if you feel comfortable....
    1. There's an app called OpenBoardView. Search it on google and download it.
    2. Search google for "820-3115-B boardview". Should come up within the first 3 hits. The file downloaded will be a '.brd" file and can only be opened with the OBV app. This will allow you to identify board-level components.
    3. Now, go back to google and search for "820-3115-B Apple schematic". You will see a website that has the file for download right away. This file is a schematic for the COMPLETE electrical pathway for all logic board related components and connections.

    Use them both together to trouble shoot problems and to help with your voltage checks.

    Now I'm feeling super nice
    This is the download page for OpenBoardView for Mac. This download is offered free under license from the Massachusetts Institute of Technology.
    This is the download page for the 820-3115-B boardview and schematic files. This download is offered for free by the amazing team at Apple Schematic.
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  4. #4
    Proper way to check voltage rails on removed logic board?

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    Quote Originally Posted by iggibar View Post
    The answer you are seeking is more specific than the generic info and question you stated.

    1. Define 'hot'.
    2. The are different voltage readings everywhere. What voltage reading are you trying to find? I'm assuming CPU?
    3. The CPU doesn't always get the same amount of power.
    4. What was the condition of the board when it was 'plugged in'?
    5. A CPU on a logic board that doesn't have an SSD plugged in can run SUPER hot under some conditions. This is normal. The CPU will run on MAX power if there's no controller.
    Thanks for writing @iggibar. I appreciate the level of detail in your follow-ups. Let me give your questions the detail that they deserve:

    1. When I took the logic board out, and plugged in the mag safe, because the fan wasn't starting when the logic board was installed, I did not expect it to power up the CPU and heat up the cooling mechanism. And when I mean that the logic board would not spin up in the case, I mean with absolutely no cables plugged into it. I only remove the logic board when it wouldn't power up when in the unibody case, but with nothing plugged into it except the mag safe and the mag safe board.
    By hot, I mean it was hot like the CPU is running and it burned my finger. That kind of hot, which I totally didn't expect, as basically the logic board being out of the case should have behaved the same as the logic board inside the case. When I remove the logic board, I did not keep the fan plugged into it and facing the cooling fins. I see now, that is something I'm going to do in future.

    2. As for the voltages, here's what I have: PPBUS_G3H is good. PP3V42_G3H is ok. PP5V_S5 is just 0.3v, and PP5V_S3 is 0vdc
    PP5V_S3 to ground is 104 ohms.

    U7200 after looking at the data sheet is still confusing me as to what the LDO outputs should be. Voltage in is good, but I haven't put my scope on it yet for the outputs. What's weird is that PGood is ok.

    My initial question was whether it was good enough to have the fan on the logic board and hope it was running when the CPU was powered, or if there was a better trick to having it removed for testing.

    I was hesitant to give more details because I don't know which forums and subforums are more appropriate for things, and didn't want to make an off topic post.

    Perhaps I'll just sum up things here with a response to the last part of your post regarding tools. I use Paul's FlexBV, and have a fairly extensive shop. I am new to doing mac logic board repair, but have a civilian and military background and electronics, and own a MSP business that does do break fix.

    I've been eating up all the videos I can get my hands on, especially those working on this 1278 board. The symptoms were that when the board is installed, green mag light but nothing else. No power on. I took the measurements above, and when I got to the u-7200, I realized that the board was getting extremely hot. It did not have an SSD, or a fan with the logic board. I naively assumed that the board would not power up and I would only have the always on voltage rails to check.

    This unit was extraordinarily filthy. And I'm talking a few levels above dirty. If this unit will not work while it's in the case, and the CPU is getting hot when it is not, I'm going to put the fan on it next, and see whether or not the fact that it is outside of the case is making it power up.

    I guess I was looking initially for advice on how to test one of the logic boards out of the case, as I've always seen the dead boards tested without the fans. Just wondering if this was something others have ran into.

    If, however, anybody can help me with this particular issue my customer has, that would be huge. I've had this unit a week, with only an A few hours to troubleshoot it. I have pictures of when I first got it, and it had so much stuff inside of it that you could not see the circuit board. Always willing to share!

    Thanks again for your response.
    Last edited by Holes Flow; 05-18-2020 at 04:27 PM.

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