Lenovo Thinkpad T480 no charging

[jamesportman]

Hi,

Short version: laptop works normally but won't accept charge from AC adaptors.



Longer version:

Lenovo Thinkpad T480.
USB-C/thunderbolt style charging port, you can use the main "power" connector or the thunderbolt port next to it to power the laptop normally.
Motherboard is NM-B50

Laptop was working fine, left switched off but charging.
Came back to it and the internal and additional batteries were dead.
No longer charges on either of the ports.
Got the laptop to boot by using a spare additional battery.
It does seem to run normally in all other respects.

I tried all of the reset tricks advertised for these laptops - power button, emergency reset button, taking batteries in and out, taking cmos battery off.

I got sidetracked and ended up connecting to the batteries on the bench using i2c/smbus commands and charging them a bit with bench power supply, all seems fine with regards to the batteries.

I've done some general circuit design and repair but nothing this small before.
I found schematics for a T480 but a different motherboard so seems different.

I did some probing on the current board and checked everything immediately obvious to me, loads of caps to ground or voltage, didn't find any shorted, do the fail open sometimes? Checked resistors that seem to be mostly in spec.

The 4th pin of the USB-C/thunderbolt is showing 5v which I think is normal for having just connected the power adaptor, but then should it talk to a controller of sorts and jump up to 20v to charge the laptop?


The big thing that jumps out is it seems like the tps65988 chip ("Dual Port USB Type-C® and USB PD Controller") might have an issue,
from the pinout of that, the ground pins (pin 20 and pin 51) have something like 10k resistance to ground.

I feel like the ground should basically never read 10k, shouldn't there be direct traces or a ground plane? Unsure how this could even happen unless some traces are burned, but I can't see any.

I did check loads of other ground points around the motherboard and they all come out at basically zero to each other, or maybe heading towards 1 ohm but that could be my test leads.

I'll try to attach a picture,
any advice?

[mon2]

Re: Lenovo Thinkpad T480 no charging

Locate U202 - see attached.

This is a single device ESD / surge protector for the USB-C connector.

The power delivery negotiations may be halted if this component is damaged.

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Thanks,
I did see one of those next to each of the ports, just marked 8S30 but I guessed they would be TPD8S300 like you say.

I checked the FLT (fault) pins of both of those chips with power cable connected and they are both still high at 3v due to pull up resistors. Apparently if faulty these pins are pulled low.
They both seem to be passing through whatever is on the four input pins to the output pins.

There is approx 1.7v and 0.1v on the CC pins depending which way around the cable is plugged in, nothing on the SBU pins that I can see - maybe they are the higher power pins - "sideband use"?

Are those voltages sensible?
Edit: read a USB-C doc that basically says for 20v charging circuit this CC pin voltage should be 1.67v, so seems correct

I think after that it jumps to the other side of the board so I'll keep tracing.

Thanks so far

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Checking further into the tps65988 chip,

Pin 51 which should be ground is showing floating voltage of up to 30mv between it and a normal ground elsewhere on the board.
The other ground pin on the TPS chip is showing 0 mv.
Would that mean something is dumping/shorted to ground near pin 51? Or that chip is damaged?

[mon2]

Re: Lenovo Thinkpad T480 no charging

Pin 51 should be grounded.

Remove all power -> meter in (lowest) resistance mode -> meter probe on pin 51 (either color) -> meter probe to ground.

What is the resistance ?

It should be ~0 ohms. If not, check for PCB traces that may be broken / damaged.

Plan B is to use wirewrap wire to solder directly from ground to pin # 51.

Be 110% sure that you are truly on pin # 51 of this device. As this is a QFN package, each side can look the same as the others.

See attached for the full datasheet -> locate the dimple for pin # 1 and then confirm again, that you are truly on pin # 51.

Edit: read a USB-C doc that basically says for 20v charging circuit this CC pin voltage should be 1.67v, so seems correct

Not quite. The adapters with Type C connectors are now very common and are PD style (Power Delivery type). This means that each external adapter can spit out a mix of power delivery contracts (voltage & current as a pair).

A very popular PD contract offers 20V @ 5A = 100W to power the laptop.

There are other common values but regardless, the logic board chats over the CC1 or CC2 (CCx in general) lines which are used for communication between the host and the external adapter. The external adapter relays back the database of PD contracts it can support. This has nothing to do with the final contract that will be selected but low voltage communication between the 2 connected parties.

Once the host is happy with the best PD contract, one is selected and the logic board continues to boot with usually 20V @ 5A. The lower power adapters will use 20V @ 3A = 60W, etc. as required.

There are some nice USB Type C meters from offshore (RD is the one we like to use) that will sniff the details of the communication flying by and display a summary on the small LCD. For full USB PD details, you will need a USB Type C PD Sniffer (TeledyneLecroy / ST / Infineon / Cypress).

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

You have me doubting myself now but top view being - a component installed onto the board, looking down on top of it?
I am definitely looking at the same datasheet, and assume the central pads are under the chip.
Dot in the corner for pin 1, also makes sense being bottom-left of the text on the chip, then anti-clockwise around it.
It looks like they've tried to make it clear by marking partial pin numbers but they are actually for a 48-pin chip?
Picture attached.

However, I see the two pairs of caps top and bottom go to the 6th pins in from the right, and those are definitely grounds.
Is this a horizontal mirror image of the datasheet? e.g. upside down like you hinted.
Are the chip markings wrong? Surely dot should be top right of the picture if this is the case?

[mon2]

Re: Lenovo Thinkpad T480 no charging

See attached.

1) PCB designer cannot count - the silk screened pin numbers are not correct.

2) Confirm again that you are indeed on pin # 51 which should be ground. Is the solder on the actual pin #51 intact ? You can confirm it with a resistance check to ground.

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Sorry yes that pin 51 and pin 20 are the ones I meant in first post that seem wrong.
And yes I went for the pin 51 you marked. You can see I marked above it by probing.
One of those two pins is 70k to ground, one is 130k to ground.
I think we both have the wrong datasheet or the datasheet pinout is a bottom view, or the chip is installed flipped horizontally?

If you look at what would be pin 23 and pin 48, they are both 100% grounds, 0 ohms.
There are even a few caps using the trace/areas as their ground side.

I’ll try checking voltages tomorrow with the view of the datasheet being a bottom view and see if it makes more sense

[piernov]

Re: Lenovo Thinkpad T480 no charging

TPS65988DH and TPS65988CE pinouts are not compatible. Refer to schematics.

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Ah thanks for clarification, it's strange that the datasheet shows presumably the DH pinout but no mention of CE, can't find datasheet for it anywhere.
I'll ask for schematics in the requests area..

Edit: actually I do have the right mobo schematics already, will work from that

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Maybe a sidetrack but I found the attached in motherboard schematics, "Workaround for PD burn-out issue".
The 0 ohm resistor is already there on my board but the mosfet is not.

I'm not sure what installing the mosfet would achieve, it is already bridged with the 0 ohm resistor, and that is still there in the workaround.
If the mosfet turns on is it just going to connect the same things that the 0 ohm resistor already is?

[SMDFlea]

Re: Lenovo Thinkpad T480 no charging

Maybe a sidetrack but I found the attached in motherboard schematics, "Workaround for PD burn-out issue".
The 0 ohm resistor is already there on my board but the mosfet is not.

I'm not sure what installing the mosfet would achieve, it is already bridged with the 0 ohm resistor, and that is still there in the workaround.
If the mosfet turns on is it just going to connect the same things that the 0 ohm resistor already is?

It has an @ so its not installed

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

Ah thanks, my board is already set up like the workaround in that case.

So originally they might have installed the mosfet without the 0-ohm, then had issues with it burning out? So now the workaround fix as a revision?

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

OK think I have found an actual issue now, the 998CE chip above was a red herring.

Now I see that VCC for U198 is at 0.6v rather than 3v - pin 13, ts3ds10224 chip.
Checking the resistance from VCC3M to ground is about 180 ohms which seems very low.
Is there a way to narrow down without randomly removing components?

I tried to check resistance and voltage across caps nearby but I guess it doesn't measure them individually, it just shows the same for everything.
I suppose this is when heat/IR cameras help, or I have seen someone use a vape to lay flux powder down and look for heat

[jamesportman]

Re: Lenovo Thinkpad T480 no charging

OK "fixed" it.
In case it happens to someone else..

I went off looking at the 20v lines since they were not working but the comms chips seemed OK in general.
Had to inject voltage and use iso alcohol to see it evaporating much faster.

It was a cap from the 20v line to ground that was almost 0 ohms resistance.
This showed up as the 20v input from charging circuit "VINT20_IN" towards the main power section showing as almost no resistance to ground.
Picture attached.
Both of those were getting hot so I removed both, tested, put the good one back.
Datasheet shows 6 caps from that area, so I've lost one.
It was one of 4 that were the same size so I think it must be 10 microfarad.
There are 2 more smaller ones that will be the 0.010 uF.

Powers up just from the AC adaptor now, will check batteries now charge also.

Thanks for help

[Pasha_Lu]

Re: Lenovo Thinkpad T480 no charging

Thank you

[gsoft]

Re: Lenovo Thinkpad T480 no charging

OK "fixed" it.
In case it happens to someone else..

I went off looking at the 20v lines since they were not working but the comms chips seemed OK in general.
Had to inject voltage and use iso alcohol to see it evaporating much faster.

It was a cap from the 20v line to ground that was almost 0 ohms resistance.
This showed up as the 20v input from charging circuit "VINT20_IN" towards the main power section showing as almost no resistance to ground.
Picture attached.
Both of those were getting hot so I removed both, tested, put the good one back.
Datasheet shows 6 caps from that area, so I've lost one.
It was one of 4 that were the same size so I think it must be 10 microfarad.
There are 2 more smaller ones that will be the 0.010 uF.

Powers up just from the AC adaptor now, will check batteries now charge also.

Thanks for help

I think I have the same problem on a T480. Please can you help me?
Where are located the 2 caps?

Thanks