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Short on PPBUS_HS_CPU?

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Hi

For the last while I have been working on a MacBook Pro that was lightly water damaged and I’m in the process it of repairing it

Something really wierd has just happend and I’m wondering if I have got the correct idea before to start removing things to get rid of the short that I have

So here’s what happend:I was replacing components quite normally and testing them as I went along until I put one component on and noticed it was shorted to ground ,so I thought it may have bridged to another component beside it ,so I replaced it and still has the same issue!

The thing is though it’s not just that component that’s shorted it’s the whole rail from what I can see

I can’t understand how it just randomly happened,

I did think it could be to do with one of the cpu mosfets as there are 2 that have one on the little pin/legs missing but that still doesn’t explain why it randomly started

Is there any thing I should shock before I remove the mosfets to see if the short persists like check any known components for a short

Perhaps I reflowed something like the cpu mosfets and there is abridge under it

any help is greatly appreciated

Thanks:-)

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@tech_ni just to quickly interject. Awesome support to @hellomacos! Very concise, factual and no frustration. Super job @tech_ni Thank you for doing that.

Just wanted you to know that.

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@oldturkey03 thank you

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@oldturkey03

Big thanks to @tech_ni,he has been a massive help:-)

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@oldturkey03 Just noticed he just passed 10,000 milestone, great job @tech_ni , congrats and thank you on behalf of the repair community, whatever that means ;) Anyone knows who's in charge to offer him a beer at iFixit ?

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@arbaman thanks

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how do you know that its a short and not a low impedance rail. a low impedance rail will seem short in continuity mode. you would have to check it in ohms

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I checked some components and they were very very low (about 0.04

By when I check things like capacitors both sides are on ground

I’m soooo confused as one minute it was completely normal next minute everything shorted to ground (in that area/rail)

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@hellomacos remove the component you replaced and check it again

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@tech_ni

I did that to all of the components in the area

And somehow even with the components off it was still shorting everything on the rail to ground

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@hellomacos what is the board number

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@tech_ni

The board number is 820-00850-A

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So, not to derail the ongoing discussion. But a few things I will mention just as food for thought.

  1. Any time you're working on a board, especially a liquid damaged one, there is the possibility that in introducing heat, flux or power to the board you're going to add variables. Maybe some corrosion shifted around, or something that was hanging on before, gave up the ghost. If the last component you replaced was the one which seemed to introduce the short, check for other components in that area on the same line.
  2. A thing to keep in mind with any of the PPBUS rails is that they are all essentially legs of PPBUS_G3H and are generally only separated by a small resistor. You will likely detect short on any of these if one is short. The board I was using for initial practice for board level MacBook repairs was massively liquid damaged. And short on PPBUS_G3H. I ripped off so much stuff trying to hunt it down. It ended up being U7760 (on an 820-00923 board). Which is on PPBUS_HS_OTH3V3.
  3. The other thing I will mention is, if there is any doubt that this short may be due to CPU mosfets, do some investigating before powering the board in any way (this includes injecting voltage). Especially in cases where there is liquid damage in that area. It is not uncommon for corrosion around those mosfets to bridge PPBUS (~13V) into CPU power (~1V), effectively murdering the CPU.

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@flannelist

Thanks for the info:-)

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At this point the cpu could be already toasted.

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