Lenovo T14 gen1 no power, 9v on power rail instead on 15v

[mrfree2ita]

Hi all,

I'm trying to fix a Lenovo T14 Gen1 laptop motherboard I bought to replace the dead one in mine

If I plug the usb-c charger nothing seems to happen. The LED stays off and pressing the power button doesn't have any effect. I'm attaching an excerpt of the schematics with my measurements in green.

I made some measurements on the board (I also read the nice guide I found in this forum for ideas):

• The 20v reach the charging circuit, and thus I'd say that the voltage is correctly negotiated.
• The total current sensing line measures about 10 ohm and so it matches with the schematics.
• PGOOD on the chip is low (kinda expected)
• VSYS15 is at 9v, instead of 15v I'd say considering the name on the power rail
• I can find the same 9v on the battery connector too
• The low sides gates on both dual-mosfet are at 6v, while the high sides are both at 0v
• I didn't find any 1.8v, 5v or 3.3v on the small coils I measured

Voltage doesn't flow trough the first dual-mosfet so I'm also a bit surprised to find even 9 volts on the power rail

I didn't find any short to ground while measuring the coils or VSYS15.

Any ideas on where should I look next?

PS: I didn't upload the full page of the battery circuit schematics for better readability, but if deemed useful I can add it.

[rumpumpel1]

Assuming that no battery is connected, the measured 9V for VSYS15 are ok. The mystery of how this can be, when the high side gates have 0V, becomes clear when you use an oscilloscope. The gate drivers are PWM signals, here probably with an abnormally small duty cycle. They probably look more like needle peaks than real PWM signals. This is why a multimeter shows 0V.
How to search for the error? I would look in this forum for the circuit diagram for an X270. There is a sequence diagram on the last page. Since many signals and voltages are the same or similar on the T14, I would work through the diagram from top to bottom. The first missing signal or the first missing voltage will then give you an indication of where to look for the fault.

[mrfree2ita]

Many thanks for the tip!

I found the schematic you are referring to and the power sequence. I'd say that in my case it doesn't go very far



In fact, I can see that VCC3SW goes high, but that's it. M_OK remains low.

I'm not sure why VCC3M and VCC5M have a different color, but from the schematic I can see that M_OK goes to the EN pin of the chips taking care of the respective power rails. Therefore, I'd say that no M_OK means no VCC3M and no VCC5M.

Moreover, while looking at the board I also noticed whomever previously worked on the board removed the BIOS chip. Is the latter needed for M_OK to go high? More in general, should the main power rails come alive even without the BIOS chip? I'll obviously find a replacement if needed, but it'd be great to understand if there's any hope to get this board working first

[rogfanther]

You probably need it, yes. And if you are lucky, that is your only problem. Was the board bought as "non-functioning" ?

Also, if you saw the bios chip is missing, search the board for other componentes that may be missing also.

[mrfree2ita]

I doubt the missing BIOS chip could be the only problem at this point, otherwise why they would have removed it in the first place?

I bought the laptop as untested, but the seller didn't say anything about a failed repair attempt or missing parts from the board.

The main reason why I'm not sure if the BIOS chip has anything to do with the missing M_ON is the fact that at this stage the BIOS chip is simply not powered (i.e. 0v on its VCC pin).

[mrfree2ita]

After checking all the power rails without noticing anything completely wrong, I decided to inject 3v to bring the M_OK signal high... and the board gave a sign of life 🙂 LED lit up and power rails VCC5M, VCC3M, VCC1R05_SUS, VCC5M_PD are now on.

As I mentioned, the BIOS chip is not present on the board but I can see that the CPU is trying to speak to it (I assume so at least). In fact, sniffing the SPI communication on the missing BIOS, I can see some initial activity as soon as the M_OK goes high then nothing much happens, but since there's no chip there I'd say it's kinda expected. But at this point the BIOS chip would get its 3v.

Do you think I have a chance to see this board working? 🙂

[Lifeisfun]

After checking all the power rails without noticing anything completely wrong, I decided to inject 3v to bring the M_OK signal high... and the board gave a sign of life 🙂 LED lit up and power rails VCC5M, VCC3M, VCC1R05_SUS, VCC5M_PD are now on.

As I mentioned, the BIOS chip is not present on the board but I can see that the CPU is trying to speak to it (I assume so at least). In fact, sniffing the SPI communication on the missing BIOS, I can see some initial activity as soon as the M_OK goes high then nothing much happens, but since there's no chip there I'd say it's kinda expected. But at this point the BIOS chip would get its 3v.

Do you think I have a chance to see this board working? 🙂

Hi there, I have similar problem with my E15, the VSYS15 is also only 9.3V
Did you get it working?

Thanks

[mrfree2ita]

Hi there, I have similar problem with my E15, the VSYS15 is also only 9.3V
Did you get it working?

Kinda I made some progress, I think I fixed the power issue, but at the end the laptop was not working (probably an issue linked to a bad EC-BIOS combo).

You can find details of my troubleshooting adventure on this post https://www.badcaps.net/forum/troubl...roubleshooting