Advanced help for troubleshooting CPU voltages on a Lenovo L490

[rddube]

Hello everyone,

This is the same laptop that SMDflea helped me diagnose a problem with the memory 1.2v power supply. Turns out the laptop now boots and everything works fine except 2 things. The CPU is either undervolted because it throttles @ 390mhz. Aida 64 reports it as overheating detected so throttling which makes it go very, very slow....... Another problem is their doesn't seem to be any sound chip or the laptop doesn't detect it but the chip is there.

Not sure if the 2 problems are related, but anyhow the most important problem I need to pin down is the CPU voltage. See photos attached. When the motherboard starts, I get 1V on the 2 inductors shown in the photos and after maybe 15 seconds, both inductors go down to .7V although on the gate of one of them I have 1 volt and on the other mosfet gate I have .7V. Not sure what's knocking down the voltage but the problem I think is around there somewhere.

Links to the schematic and board view is attached

Schematics: https://www.badcaps.net/filedata/fetch?id=2077260

Boardview: https://www.badcaps.net/filedata/fetch?id=2105881

I checked PRT401 to PRT405 and they look ok. Also checked PR1 to PR9 and they also look ok.

Not sure what to check next, so any advice would be more than welcome! Thank you!

[mon2]

Hunt for any thermal sensor / thermistor that should be onboard for the CPU temp gauge. Confirm it is present and properly mounted to measure the CPU temps.

[rddube]

Hunt for any thermal sensor / thermistor that should be onboard for the CPU temp gauge. Confirm it is present and properly mounted to measure the CPU temps.

Hello Mon2, thank you for your response.

I think I got them all as I checked PRT401 to PRT405 and they look ok. Also checked PR1 to PR9 and they also look ok. I don't think there are any other sensors, but I might be wrong.

How can you tell in a schematic what is a sensor / thermistor? I found that PR7 is the main thermistor under the CPU and since I have a donor board I did change that one. Also measured it while the motherboard was on and it seems to be working ok.

Any other ideas?

Thanks!

[m1ch43lzm]

May be related to PROCHOT, at least on Intel CPU you can try using ThrottleStop to disable BD PROCHOT as a workaround/troubleshoot the issue

https://www.techpowerup.com/download...-throttlestop/
I can't test it as I don't have any PC/laptop here with Intel CPU (program won't run on AMD CPU)

Also download HWINFO64 and select Sensors only on the dropdown box
If disabling BD PROCHOT works, follow VR_HOT# in the schematics to try to find the root cause, either it's being pulled down, or the pullup resistor is faulty
Find a suitable point to measure it, should be 1.05V (+VCC_IO, same as +VCC_STG), check R0801 and R0802

[rddube]

May be related to PROCHOT, at least on Intel CPU you can try using ThrottleStop to disable BD PROCHOT as a workaround/troubleshoot the issue

https://www.techpowerup.com/download...-throttlestop/
I can't test it as I don't have any PC/laptop here with Intel CPU (program won't run on AMD CPU)

Also download HWINFO64 and select Sensors only on the dropdown box
If disabling BD PROCHOT works, follow VR_HOT# in the schematics to try to find the root cause, either it's being pulled down, or the pullup resistor is faulty
Find a suitable point to measure it, should be 1.05V (+VCC_IO, same as +VCC_STG), check R0801 and R0802

Thank you for your help!

I did try throttlestop version 9.0 but it does absolutely nothing. The clock is stuck at 399.0. I downloaded HWINFO64 and selected sensors, I get a very long report of the sensors. Measured after R0802 and I get 1.035V at that point. Both R0801 and R0802 are as per specs in the datasheet.

When I trace VR_HOT# I come across PRT401 to PRT404 which are thermistors I think (NCP15WF104F03RC) and I'm not sure how to test those, when I check them no power applied, here is what I get:

PRT401 16.1K ohms
PRT402 36.2K ohms
PRT403 43.6K ohms
PRT404 38.24 ohms

When power is applied and when the fan starts spinning:

Power applied Fan starts
PRT401 16.52 Kohms 16.35 Kohms
PRT402 38.4 Kohms 46.8 Mohms
PRT403 44.1 Kohms 54.4 Kohms (this is Kohms)
PRT404 38.3 Kohms 43.5 Mohms

There is no voltage on PRT401 to PRT404.

What do you think ?

[Sephir0th]

I think you could finally share the voltage of PROCHOT# respectively VR_HOT# to get a idea whether the issue really originates from this signal.

This all is digging in the mist so far.

Edit: 1.05V foe VR_HOT# is okay. Hownis it on the other side, VR_HOT_P# ?

[rddube]

I think you could finally share the voltage of PROCHOT# respectively VR_HOT# to get a idea whether the issue really originates from this signal.

This all is digging in the mist so far.

Edit: 1.05V foe VR_HOT# is okay. Hownis it on the other side, VR_HOT_P# ?

Hi and thanks for chirping in!

I get 1.035V for VR_HOT# and 1.184V for VR_HOT_P# ..

[Sephir0th]

So PROCHOT# and everything which is connected to it definitely isn't the source of the limit, so we need to think about other options which could result into the current behaviour.

The charger is OEM, known good and of correct wattage?

I would check PR103/104/113/114 for sanity.

Ultima ratio would be to conclude the CPU being internal fried in this way after everything else was ruled out.

And history please! What was initially wrong with the 1.2V supply (for RAM?)? The statement alone is a strong indicator for something wrong with the CPU itself.

[rddube]

Yes the charger is OEM original Lenovo.

Checked PR103/104/113/114 and all show good.

I got the laptop it was turning on but no display. Turned out the RT8231 in the 1.2V supply was fried. Replaced it and it refried again. SMDflea helped me out to identify there was a blown capacitor in that area which I changed and the 1.2V supply has been working fine since.

The other problem is the one of this thread.

I have a donor board that I can take parts from.

I should mention that I have no voltage on PL403, but I think that is for the graphics chip which is not present on this board - or maybe it's related to my problem?

[Sephir0th]

Doesn't sound promising. Did you test from charger only or from battery too?

[rddube]

I actually tried both, charger and battery, charger only and battery only. When it boots, it gives me the message that BIOS recovered from self-healing. Here's the weird part - I did a test. Removed PRT401, thinking this is probably not needed since there is no graphics chip. What do you know, it's zipping away - still showing that it throttles, but the CPU goest to 4090 mhz ?? Is it giving it's last signs of life, I don't know but this is somewhat weird. I have it on charger only right now.

[Sephir0th]

It's part of the iGPU, so i would not remove it since we don't know how this all is connected together.

What i am more interested in now is this one. A typo or really in Ohms? Additionally with NTC thermistors the resistance should decrease and not increase when the temperature rises. So the previous measurements of the four thermistors are somewhat confusing.

Maybe you can measure again after the board was running for a while after 10 - 15 mins or so

It would be great to finally find a faulty one, but even four faulty thermistors doesn't sound right to me.

[rddube]

No that is a typo, should be a K in there, sorry! System seems to be running normally with PRT401 removed. But I will replace it as you suggested and will see how it behaves. Will leave it on for around 15 minutes and then measure each one. Will report back.

Should I replace it with its original or from the donor board I have?

[rddube]

Ok, so I reinstalled the original PRT401. Turned it on and left it on for approx 10-12 minutes before taking the following measurements (double checked):

PRT401: 15.5K ohms
PRT402: 46.2M ohms (yes Mega)
PRT403: .775K ohms
PRT405: 42.5K ohms

Couldn't measure PRT404 because it is on the other side of the motherboard which is in the laptop.

[Sephir0th]

I would say its worth trying to replace PRT402 with the one from the donor and report. Be careful, these thermistors are indeed somewhat heat sensitive afaik

[rddube]

Ok, so installed PRT402 from the donor board, same behavior with the PRT's. PRT402 goes from 38K ohms when not powered to 42 MegaOhms when powered.

Any other ideas would be welcome. Starting to think that maybe the EC is not functionning properly as besides this problem of throttling there is the BIOS message every time I boot that says it has healed from the backup BIOS and the board has no sound, as if no sound chip is installed but it is there? I have one on the donor board??

[m1ch43lzm]

Resistance should always be measured with power removed from the board, otherwise you get a wrong reading
38K is normal, you have PR418 61.9K in parallel, according to parallel resistor formula you get (100K * 61.9K)/(100K + 61.9K) = 6190000000/161900 = 38.23K
You can heat those thermistors with your hot air station set to 100 deg.C to check if they're working properly, resistance should become lower

Any other ideas would be welcome. Starting to think that maybe the EC is not functionning properly as besides this problem of throttling there is the BIOS message every time I boot that says it has healed from the backup BIOS and the board has no sound, as if no sound chip is installed but it is there? I have one on the donor board??

For the "No sound"/recovered from backup issue, it could be that someone flashed a random BIOS from another laptop (same model), that could cause random issues related to the Intel ME region
You can go to the BIOS Requests ONLY! subforum, search for the thread with your laptop model (here it is: https://www.badcaps.net/forum/troubl...-thinkpad-l490 ), and post there:
- BIOS backup attached as .zip or .rar (you need a programmer to dump the BIOS chip)
- Laptop model, serial number, machine type/MTM, board model (you have to type that info, per forum rules)
- Description of the issue (No sound/"healed from the backup BIOS" message every boot)
The people there will help with a fixed BIOS

[rddube]

Thank you m1ch43lzm , taking your advice in seriously. Will start with trying to get the laptop to stop throttling so will do as you suggested measure everyone (no power) and heat them up a little and re-measure. Will report back.

[rddube]

Ok so did the tests on every thermistor as suggested by m1ch43lzm .

Here are the results, on the left reading not heated, on the right heated at 100C for 10 seconds

PRT1 .646Kohms .606Kohms
PRT2 .622Kohms .601Kohms
PRT3 .611Kohms .590Kohms
PRT4 .662Kohms .649Kohms
PRT5 - Not installed near the graphics chip with is not present
PRT7 71.0Kohms 61.8Kohms (this is the one right under the CPU)
PRT8 .611Kohms .584kohms
PRT9 .632Kohms .601Kohms

PRT401 16.8K0hms 14.6Kohms
PRT402 37.8Kohms 29.7Kohms
PRT403 43.9Kohms 36.2Kohms
PRT404 33.32Kohms 28.6Kohms
PRT405 16.5Kohms 14.2Kohms
PRT601 and PRT602 not installed also near the graphics chip

So I guess the thermistors are working as they should?