Retina Pro good SMC & power lights but no boot

[Flippers McCoy]

So I've got a 13" MacBook Pro Retina Early-2015, motherboard model 820-4924-A

Here's what I know so far

1. The magsafe light turns from green to orange if I have a battery plugged in, and just green if I have the motherboard alone.
2. I can do an SMC reset successfully by shorting R5101.
3. Powering on the computer has no effect, both by pressing the power button and by shorting R5116
4. Getting just over 1V on PPBUS_G3H on pin 1 of R7005, supposed to be reading 8.6V
5. Getting 3.42V on PP3V42_G3H, as expected

If anyone needs the schematics and/or boardview, I've got em, just let me know in the comments.
I'm not really sure what to check from here to be honest, but I'm gonna think on it and hopefully have some ideas later. I'll also keep this forum updated with the current situation!

Thanks for the help, everyone

[Th3_uN1Qu3]

Re: Retina Pro good SMC & power lights but no boot

You should figure out why you only have 1V on PPBUS_G3H. Without that, the machine cannot possibly start up. I can try to help with things to check if you attach the schematics and boardview.

[Flippers McCoy]

Re: Retina Pro good SMC & power lights but no boot

Gotcha, thanks for the tip! Here are the schematics & boardview, let me know if you have any ideas on things to check

(I am unable to add the .brd file as an attachment, so I created a dropbox shared folder which has both the schematics and boardview)

https://www.dropbox.com/sh/r28qzwy9m...jpf0vwEOa?dl=0

[thecomputercellar]

Re: Retina Pro good SMC & power lights but no boot

Okay, so I made a stupid mistake that turned out to be super helpful. I left the office before my day off after working on it for a bit, and accidentally left the bare board plugged in the whole time, about 38 hours. When I got back in, I saw it and disconnected the power, then later noticed that a component(C7140) had been reduced to ashes! I replaced it with a good part from a donor board I had, and when I tested it the computer worked fine!
While I'm happy that it works, I'd really, really like to know more about why this component failing caused those symptoms.
If anybody has any insights, I'd greatly appreciate them!

[Flippers McCoy]

Re: Retina Pro good SMC & power lights but no boot

(Sorry, I was logged into my work account at the time of making the above update!)

[Th3_uN1Qu3]

Re: Retina Pro good SMC & power lights but no boot

Gotcha, thanks for the tip! Here are the schematics & boardview, let me know if you have any ideas on things to check

(I am unable to add the .brd file as an attachment, so I created a dropbox shared folder which has both the schematics and boardview)

https://www.dropbox.com/sh/r28qzwy9m...jpf0vwEOa?dl=0

Use a Zip or Rar archive next time where you can put both files, archives are accepted by the forum. Windows (or Mac) by itself without any extra programs will allow you to make Zips.

I saw it and disconnected the power, then later noticed that a component(C7140) had been reduced to ashes! I replaced it with a good part from a donor board I had, and when I tested it the computer worked fine!
While I'm happy that it works, I'd really, really like to know more about why this component failing caused those symptoms.
If anybody has any insights, I'd greatly appreciate them!

Well that is easy. Component C7140 is a ceramic capacitor. These are notorious for failing short circuit, especially if subject to shock (physical or thermal) or moisture (does not necessarily have to be a liquid spill). It was also straight across the PPBUS_G3H rail, so it going short was the reason it was sitting at 1 volt.

The reason why you had that 1 volt is that the circuit making PPBUS_G3H from the adapter has quite high current capability. A component this small, while it would have measured as a short with no power applied, cannot possibly go so low resistance as to drag that down to zero volts, the power that the circuit was safely able to make was enough to generate 1 volt over the resistance of that damaged capacitor. Learn to use your multimeter in resistance and diode mode, with the power turned off, to check for short circuits on a voltage rail.

If you have a bench power supply that allows you to set both voltage and current output, that will help greatly in detecting such parts, because sometimes they go so low resistance that you will have zero volts there. Since there are so many capacitors on one voltage rail, it is useful to find out exactly which one is shorted.

What you do is set the power supply to the voltage that rail is supposed to have while working (or a little lower, sugest you start lower), set the current limit to 1A and attach the power supply to the shorted voltage rail on the board, without the adapter attached to the board and of course paying attention to connect positive to positive and negative to negative. Feel around the board for anything getting hot. If nothing's getting hot after a minute or so, start increasing the current slowly. Something will get hot or put out smoke eventually, and then you have found your shorted component.

[Flippers McCoy]

Re: Retina Pro good SMC & power lights but no boot

Well that is easy. Component C7140 is a ceramic capacitor. These are notorious for failing short circuit, especially if subject to shock (physical or thermal) or moisture (does not necessarily have to be a liquid spill). It was also straight across the PPBUS_G3H rail, so it going short was the reason it was sitting at 1 volt.

The reason why you had that 1 volt is that the circuit making PPBUS_G3H from the adapter has quite high current capability. A component this small, while it would have measured as a short with no power applied, cannot possibly go so low resistance as to drag that down to zero volts, the power that the circuit was safely able to make was enough to generate 1 volt over the resistance of that damaged capacitor. Learn to use your multimeter in resistance and diode mode, with the power turned off, to check for short circuits on a voltage rail.

If you have a bench power supply that allows you to set both voltage and current output, that will help greatly in detecting such parts, because sometimes they go so low resistance that you will have zero volts there. Since there are so many capacitors on one voltage rail, it is useful to find out exactly which one is shorted.

What you do is set the power supply to the voltage that rail is supposed to have while working (or a little lower, sugest you start lower), set the current limit to 1A and attach the power supply to the shorted voltage rail on the board, without the adapter attached to the board and of course paying attention to connect positive to positive and negative to negative. Feel around the board for anything getting hot. If nothing's getting hot after a minute or so, start increasing the current slowly. Something will get hot or put out smoke eventually, and then you have found your shorted component.

Thanks for the tip about the archive, I can't believe I didn't think of that >.>

I thought that C7140 is an electrolytic capacitor? The ones next to it in the schematics (C7148 and C7145) would be ceramic, but I think that C7140 is electrolytic because it is polar, and has a very high capacitance. Correct me if I'm wrong, though, I'm here to learn!

I've used my multimeter to check for continuity in DC jacks in the past, would this be one way test for shorts? The way I usually use it is I set it to beep at me when it detects continuity, that's the best description I've got.

So, what you're suggesting is basically what I did by accident, right? Except in a more controlled environment. I left voltage on the board for a day and a half, and that just totaled the bad capacitor, and the idea behind your suggestion is put voltage on the rail and poke around for evidence of a capacitor becoming totaled and then replace it

[Th3_uN1Qu3]

Re: Retina Pro good SMC & power lights but no boot

Without a picture of the board i can't tell. If it's a round capacitor it could be an electrolytic or a polymer. If it is rectangular it is more likely to be a tantalum. Tantalum caps also fail short. They also have a habit of catching fire.

Yes, beeper mode is what you would normally use for detecting shorts. It's just that some meters have the beeper on resistance, others have it on diode.

[Flippers McCoy]

Re: Retina Pro good SMC & power lights but no boot

Without a picture of the board i can't tell. If it's a round capacitor it could be an electrolytic or a polymer. If it is rectangular it is more likely to be a tantalum. Tantalum caps also fail short. They also have a habit of catching fire.

Yes, beeper mode is what you would normally use for detecting shorts. It's just that some meters have the beeper on resistance, others have it on diode.

I don't have access to that board anymore, but I've attached a picture of what was there I think it is tantalum, as in the schematic it said POLY-TANT and it was rectangular, like the one pictured.

So I would just keep one test lead on ground and poke around (coordinately, using the schematics) with the other lead until I found the part that was shorted to ground, right?

[Th3_uN1Qu3]

Re: Retina Pro good SMC & power lights but no boot

The way it is recommended to do this is diode mode on your multimeter, RED test lead goes to ground, BLACK on the parts you are measuring. The reason why you do it this way can be found with a little research and some studying of basic electronics theory.

[Flippers McCoy]

Re: Retina Pro good SMC & power lights but no boot

Gotcha Thanks so much for helping, I've learned a lot!