Samsung Chromebook Pro missing 1.2V power rail and no boot

[theflyingsquirrel]

I am trying to repair my Samsung Chromebook Pro. There is no boot. Upon turning on, the screen is blank and there is no backlight. Power LED is blue. I tried refresh with power and esc refresh with power with no avail.

When I measure the 1.2V rail, it is pulsing. It will jump from 0 to 1.2V then 0 again. I replaced the 2 MOSFET and even power management IC and it is the same. Resistance from midpoint between mosfet to ground is 0.5 kOhm but it will show 250 Ohm first before jumping to 0.5 kOhm. So it feels like it was charging up some capacitor. Since MOSFET and IC was replaced, I was thinking it may be current limit? It is using the ROHM992 IC, but I cannot find datasheet anywhere so I really have no idea how to check the current limit.

5V and 3.3V rail is present and stable.

CPU also measure 1V.

I tried using thermal camera to see if there is any hot spot and there is none. Even the CPU is cold.

Any help is appreciated.

[Sephir0th]

From my experience what you have described can't be true. When the 1.2V is pulsing, then it should be the same with other supplies coming up later in the sequence, like 3VS, 5VS, VTT, VCCST, VCCSTG, VCCIO and so on, up to VCC Core.

So I strongly recommend to observe all supplies from 1.2VS again to have a better understanding of the situation an share your findings her again in a structured way.

[theflyingsquirrel]

From my experience what you have described can't be true. When the 1.2V is pulsing, then it should be the same with other supplies coming up later in the sequence, like 3VS, 5VS, VTT, VCCST, VCCSTG, VCCIO and so on, up to VCC Core.

So I strongly recommend to observe all supplies from 1.2VS again to have a better understanding of the situation an share your findings her again in a structured way.

I will try to find more information tomorrow.
Is there possible that the sequence starts from 5V? The battery is 7.84V so may be it works it way down?

[DeXXter]

I haven't worked on one of those modern samsung, but if you have all the cpu power supplies then is most probably not an issue with the 1.2v power supply but probably with the ram itself or even the cpu, have you used an oscilloscope to analyze the data on the bios (pin 1 or 2). Is your cpu crystal oscillator working okay?

[theflyingsquirrel]

From my experience what you have described can't be true. When the 1.2V is pulsing, then it should be the same with other supplies coming up later in the sequence, like 3VS, 5VS, VTT, VCCST, VCCSTG, VCCIO and so on, up to VCC Core.

So I strongly recommend to observe all supplies from 1.2VS again to have a better understanding of the situation an share your findings her again in a structured way.

I heated up the EMMC and moved it a bit then the 1.2V comes back stable. I have no idea what that has to do with the 1.2V, but it seems that voltages are back. Unfortunately, performance wise, it is the same. No display with the blue power led.
I've measured the voltage on the board and marked it accordingly. I've measured the voltage when pc is on. And then turned it off and measure the resistance to ground for each choke. I also measured in diode mode with +ve to ground to choke.

I haven't worked on one of those modern samsung, but if you have all the cpu power supplies then is most probably not an issue with the 1.2v power supply but probably with the ram itself or even the cpu, have you used an oscilloscope to analyze the data on the bios (pin 1 or 2). Is your cpu crystal oscillator working okay?

This is not a modern machine by the way. It was from 2017. I just now replaced the RAM and it is the same. 1.2V is back after I reheat and moved the EMMC for some reason. The EMMC is good though. I am not sure about the CPU. It is possible that it is bad but is there a way to check? The CPU does heat up when I turn on the machine. I do not have an oscilloscope, but I do have a ET828 that can be used to some extended. I think it can measure from 5Hz to 5MHz. I am not sure it is enough.
Is BIOS chip the SPI flash that store UEFI?
It does has a 25Q127C SPI flash under the RAM.
datasheet is here.
https://www.mouser.com/datasheet/2/8...XPSsVXZ8v2kR_m
So I should measure SO pin 2?

Update: I measured with my ET828 but pin 2 seems to stay at 3V. My min time range is 2.5uS so may be that is not enough?

I am not sure which component is the CPU crystal. I think it is this one?

It is SMD. I am not sure how to measure it.

[DeXXter]

Pretty modern for a samsung tho, this is the cpu oscillator (check pic), you should have a value (on diode mode) on this resistor on both sides and a wave (using oscilloscope), sometimes you lose connection under the cpu and the laptop turns on, but with a dead cpu it does nothing, it can be the oscillator, but the only way to know is using an oscilloscope.
And on the bios pin 1 or 2 you can check the cpu activity, again using an oscilloscope, this is mostly experience but first data you see there is stuff like EC and pch, after that ram and cpu. In other words a lot of waves, that's a healty laptop.
Now if you got issues on the emmc that can be the issue too. I replaced one of those on a small acer tablet (32gb-->128gb) just took one and solder the other and is still working fine (couldn't install windows).

[theflyingsquirrel]

Pretty modern for a samsung tho, this is the cpu oscillator (check pic), you should have a value (on diode mode) on this resistor on both sides and a wave (using oscilloscope), sometimes you lose connection under the cpu and the laptop turns on, but with a dead cpu it does nothing, it can be the oscillator, but the only way to know is using an oscilloscope.
And on the bios pin 1 or 2 you can check the cpu activity, again using an oscilloscope, this is mostly experience but first data you see there is stuff like EC and pch, after that ram and cpu. In other words a lot of waves, that's a healty laptop.
Now if you got issues on the emmc that can be the issue too. I replaced one of those on a small acer tablet (32gb-->128gb) just took one and solder the other and is still working fine (couldn't install windows).

What is the oscilloscope frequency range you need for measuring this? Do I need a few hundred MHz?
I can check in diode mode for voltage on that resistor.
EMMC is good. I replaced it with another one. When I put the original one into a socket and test it. It is working fine.
I suspect when I heat up and move the EMMC, it reflowed some other component around the EMMC. Yeah 128GB is the sweet spot. It is cheap and a lot more usable than 32GB.
When I measure BIOS chip pin 2, it is 3.3V. Since my scope is only max 5MHz. I think there is activity, but I just cannot see it.

I did some digging and I think there is a a servo v2 connector pads and STM32 in the picture.


I am thinking to connect the UART TX, RX and GND pin to see if I can connect to the EC UART and gain access to the EC console.

Since the CPU is heating up and BIOS pin 2 is 3.3V. I think the CPU is working and on.
But it is heating up faster than a working machine, so I bet it is doing intense work.

[DeXXter]

What is the oscilloscope frequency range you need for measuring this? Do I need a few hundred MHz?
I can check in diode mode for voltage on that resistor.
EMMC is good. I replaced it with another one. When I put the original one into a socket and test it. It is working fine.
I suspect when I heat up and move the EMMC, it reflowed some other component around the EMMC. Yeah 128GB is the sweet spot. It is cheap and a lot more usable than 32GB.
When I measure BIOS chip pin 2, it is 3.3V. Since my scope is only max 5MHz. I think there is activity, but I just cannot see it.

I did some digging and I think there is a a servo v2 connector pads and STM32 in the picture.


I am thinking to connect the UART TX, RX and GND pin to see if I can connect to the EC UART and gain access to the EC console.

Since the CPU is heating up and BIOS pin 2 is 3.3V. I think the CPU is working and on.
But it is heating up faster than a working machine, so I bet it is doing intense work.

50mHZ should do fine, I got a multimeter oscilloscope with 10mHZ (zoyi 702s) and it also can read 48m (a little noizy but still works). I usually set the fastest my oscillocope goes to measure crystals, otherwise it won't work (50ums something like that), to measure bios data I use around 250ms up to 500ms (half a second).
These little motherboards are sometimes a nightmare, can't tell much without looking at the bios and see what is doing. I don't think that port has something to do with it tbh, I mean the EC seems to work, it turns on the power supplies and that's usually as far as it goes. If you got all the power supplies it can be a few things, like the ram not processing data the right way, the same goes for the cpu, it can also be a bios issue, you said that you replaced the emmc, I'm not sure, but I guess some devices might need a clean ME region, but again you should measure a complex failure like this with an oscilloscope, that's the thing when you got no shorts.
A question the cpu looks funny, is that dirt or it looks like that, have you checked the die closely?

[theflyingsquirrel]

50mHZ should do fine, I got a multimeter oscilloscope with 10mHZ (zoyi 702s) and it also can read 48m (a little noizy but still works). I usually set the fastest my oscillocope goes to measure crystals, otherwise it won't work (50ums something like that), to measure bios data I use around 250ms up to 500ms (half a second).
These little motherboards are sometimes a nightmare, can't tell much without looking at the bios and see what is doing. I don't think that port has something to do with it tbh, I mean the EC seems to work, it turns on the power supplies and that's usually as far as it goes. If you got all the power supplies it can be a few things, like the ram not processing data the right way, the same goes for the cpu, it can also be a bios issue, you said that you replaced the emmc, I'm not sure, but I guess some devices might need a clean ME region, but again you should measure a complex failure like this with an oscilloscope, that's the thing when you got no shorts.
A question the cpu looks funny, is that dirt or it looks like that, have you checked the die closely?

I turned to the lowest setting but it still show constant 3.3V on pin 2 on bios chip.
May be it is time to get a better scope.
The thing on top of CPU is just some remain of the thermal pad. It looks normal.

I am just thinking may be the EC UART will give some insight into what is happening when the screen is blank.

[DeXXter]

I turned to the lowest setting but it still show constant 3.3V on pin 2 on bios chip.
May be it is time to get a better scope.
The thing on top of CPU is just some remain of the thermal pad. It looks normal.

I am just thinking may be the EC UART will give some insight into what is happening when the screen is blank.

Well a constant 3.3v on pin 1 of the bios indicates that the cpu is doing nothing or not recognizing the ram. In any case it should have a variation at the beginning of the wave because the pch should communicate with the bios. In any case this is not an easy fix, it reminds me of one lenovo legion I'm fixing and it shuts down ONCE, ONLY ONCE. Already ordered the cpu fom this sombitch. Good luck tho.

[theflyingsquirrel]

Well a constant 3.3v on pin 1 of the bios indicates that the cpu is doing nothing or not recognizing the ram. In any case it should have a variation at the beginning of the wave because the pch should communicate with the bios. In any case this is not an easy fix, it reminds me of one lenovo legion I'm fixing and it shuts down ONCE, ONLY ONCE. Already ordered the cpu fom this sombitch. Good luck tho.

When you say pin 1, do you mean the SO pin or the CS pin? Or they both should have lots of activities?
It is hard to hold on to the probe to measure the pin and turn on the laptop at the same time, so I usually turn on and start measuring. As a result, it is hard to see the starting wave.
Do you solder a wire to the pin and clip on to it?
I agree. This is not an easy fix for sure. I will see what I can see from the EC console if I can get it to work.
I read that if it shut down randomly, it may be due to leaky current from capacitor. I think you need dedicated equipment to find the leaky cap.

[theflyingsquirrel]

Connected through UART and I actually got something from the console!

https://pastebin.com/eHpGQb3b

Here's the log.

[DeXXter]

When you say pin 1, do you mean the SO pin or the CS pin? Or they both should have lots of activities?
It is hard to hold on to the probe to measure the pin and turn on the laptop at the same time, so I usually turn on and start measuring. As a result, it is hard to see the starting wave.
Do you solder a wire to the pin and clip on to it?
I agree. This is not an easy fix for sure. I will see what I can see from the EC console if I can get it to work.
I read that if it shut down randomly, it may be due to leaky current from capacitor. I think you need dedicated equipment to find the leaky cap.

Hey, yeah pin one is chip select CS (some use pin 2) it should work both ways, I just use the number 1 and yeah I do solder a wire to watch the oscilloscope. You can also check ram request on data pin on smb_sda, but that's on regular laptop with ram slot, with this I wouldn't know where to check that (this proves if the cpu is communicating with the ram).
If it is a capacitor I'm pretty sure you should have an inconsistent voltage, but I don't think that could be the issue, I mean cpu issues are way more common than capacitors in my experience, unless u have water damage.
I would like to help you with the uart log, but I don't understand that tbh, haven't used that tool.

[theflyingsquirrel]

Hey, yeah pin one is chip select CS (some use pin 2) it should work both ways, I just use the number 1 and yeah I do solder a wire to watch the oscilloscope. You can also check ram request on data pin on smb_sda, but that's on regular laptop with ram slot, with this I wouldn't know where to check that (this proves if the cpu is communicating with the ram).
If it is a capacitor I'm pretty sure you should have an inconsistent voltage, but I don't think that could be the issue, I mean cpu issues are way more common than capacitors in my experience, unless u have water damage.
I would like to help you with the uart log, but I don't understand that tbh, haven't used that tool.

I came across this cheap logic analyzer with 8 Channels and 24 MHz.
https://lygte-info.dk/review/Equipme...0both%20worked.
Do you think this will be able to see the communication between BIOS chip?

If I want to find out if the CPU is shorting, what can I measure?