DELL Latitude 7280 does not start

[PowerAmpFreak]

Hello
I have a DELL Latitude 7280. When press start button it turns white for a few seconds and turns off.
This happens both with battery connected and not.
I dont know where to start troubleshooting. Any advice would be highly appreciated.

[mon2]

Re: DELL Latitude 7280 does not start

Hi.

Start with the schematic page with the charger IC @ PU901.

Measure the voltages at the following points:

+CHARGER_SRC

+PWR_SRC

ACIN_ISL88738

ACOK_ISL88738

LG1_ISL88738
UG1_ISL88738

LG2_ISL88738
UG2_ISL88738

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

Thanks. I am searching for PU901 charge controller but unable to find it. It is perhaps on the bottom side of the board.
I need to remove the MB from the chassis.
Is it safe to remove the MB and the heatsink / fan assembly all together?

[mon2]

Re: DELL Latitude 7280 does not start

If possible - leave CPU heatsink on so we can test while it is outside.

For logic board removal, review this webpage:

https://www.parts-people.com/blog/20...-installation/

The PU901 charger IC will be near the DCin adapter connector and should be a square looking black in color IC. Pins are each side of the black body = QFN (quad flat pack).

[Udemzy]

Re: DELL Latitude 7280 does not start

This is a very common issue on most dell laptops,. especially the recent Gen.

This is what i always do and it works 70% of the time.

Remove the heat sink, there's a possibility that the cooling paste has dried up, clean the old one and apply a new thermal paste.

If that didn't work, reprogram the BIOS chip, if it uses two BIOS chip ,EC and main BIOS, reprogram both.

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

This is a very common issue on most dell laptops,. especially the recent Gen.

This is what i always do and it works 70% of the time.

Remove the heat sink, there's a possibility that the cooling paste has dried up, clean the old one and apply a new thermal paste.

If that didn't work, reprogram the BIOS chip, if it uses two BIOS chip ,EC and main BIOS, reprogram both.

Hello and thanks for responding. Please advice the proceduren of flashing the bios ships and where to find the binary?

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

After pressing the power button repeatedly I saw the white light in the button stay on longer and longer, and suddenly it booted up BIOS.
I was able to run some Dell diagnostic and the only thing it had to report was cpu fan speed to be zero rpm. But when cpu heats up the fan spins correctly.
So this tells probably to BIOS is ok but could it be a capacitor issue making the cold start of the charger IC to not start correctly when not warm?

[mon2]

Re: DELL Latitude 7280 does not start

Review the voltage of FAN1_TACH when the fan is OFF. Then measure the voltage while the fan is ON - the voltage on FAN1_TACH should change.

Pin # 3 of the FAN connector = FAN1_TACH feedback.

Confirm it. Possible that the fan is defective - happens over time.

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

Can a defective tach circuit really stop the pc from boot ?

If I disconnect the fan it should not boot then. A test I can do.

[mon2]

Re: DELL Latitude 7280 does not start

No, I don't think so since the tach feedback does not appear to be working yet you did get the logic board to boot so you could run the diags.

The flaky random chance of booting up is likely a bad cap on the board. Do you see any bulging caps (that are not on viagra) ? The top side may be domed shaped rather than flat.

When the logic board does NOT boot -> use your meter in DC voltage setting -> check the voltage to ground of all grey coils. That is, each coil (inductor) is linked to a local switching buck power supply (takes as input a high voltage; outputs a lower voltage).

If there is a borderline shorting cap on the respective power rail, the cap will cause an excessive current draw -> cause that power rail to shut off, hence causing your observation.

If you find a coil with no voltage, make a note of it and post the details here.

[Udemzy]

Re: DELL Latitude 7280 does not start

After pressing the power button repeatedly I saw the white light in the button stay on longer and longer, and suddenly it booted up BIOS.
I was able to run some Dell diagnostic and the only thing it had to report was cpu fan speed to be zero rpm. But when cpu heats up the fan spins correctly.
So this tells probably to BIOS is ok but could it be a capacitor issue making the cold start of the charger IC to not start correctly when not warm?

Remember i posted about applying a new thermal paste?

like i said,the latest dell board are very sensitive,system can stop powering on due to thermal paste issue,just clean the old one and apply a new one.

if there's anything wrong with the fan line,the fan won't spin at all.

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

Yes but there can't be a thermal paste issue, the PC has been running properly for several hours and passes all the diag tests including CPU benchtest.
Fan starts up correctly when needed.
I have no clue what caused the odd behaviour and refuse to boot. Now I have installed Win10 and everything seems fine.

[Udemzy]

Re: DELL Latitude 7280 does not start

Yes but there can't be a thermal paste issue, the PC has been running properly for several hours and passes all the diag tests including CPU benchtest.
Fan starts up correctly when needed.
I have no clue what caused the odd behaviour and refuse to boot. Now I have installed Win10 and everything seems fine.

I've solved a lot of dell dead cases by just applying thermal paste because the old one dried up and probably increase the CPU thermal temperature especially on newer models.


In fact that's what I do first on Dell laptop whenever I get such cases.

[Udemzy]

Re: DELL Latitude 7280 does not start

It sound weird but that's just it.

[PowerAmpFreak]

Re: DELL Latitude 7280 does not start

It sound weird but that's just it.

Yes its really strange.
What I did not mention before was that this model has two charging options. It has the standard Dell connector and USB-C.
The standard connector was damaged, It had broken pins inside the connector.
I simply removed the connector put it aside.
But that should not affect anything?

[Geo-fr]

Good morning/afternoon Mon2

I am writing to get an expert opinion

Here is the problem:
- I have a dell latitude 7280
- by mistake yesterday I connect it to the wrong charger (both chargers with same pin plugs on the same power strip)
- the wrong charger is for my bike battery : output voltage 42VDC
- after that: computer runs slowly
- today : doesn't start
- I had an old battery, I reinstall it
- computer restarts but runs slowly
- conclusion: PC runs only on batteries, but as batteries were very low charged, it runs low, and when fully discharged, stop.
The power with external AC charger should be burned somewhere.
the question which component ? How to diagnose ? How to repair ?

thanks for your highlighting

Geo

[mon2]

Hi. Welcome. Ouch. < In this order >

The 42vdc hit to the board is exceeding the rating of the DCin mosfets (act like solid state relays) so this is of concern.

The fact that the logic board powers on using the pre-charged battery is a positive direction.

The issue will be for sure with the PSID line which is a 1-wire communication line and it features a TVS diode that has a trigger voltage of 5 volts. Anything higher and the part will trigger.

There may be an issue with the DCin mosfet @ PQ9 - AON7409. Aside from the debugging process, the parts are very inexpensive.

Remove all power. Meter in resistance mode. Measure the resistance to ground of inductor @ PL3. Post pics of the DCin adapter area if not sure of the location of PL3.

Also interested in some measurements of PQ9 mosfet. Post pics if not sure of PQ9.

Requesting:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Post each measurement in ohms. From this review, we can confirm if PQ9 is defective or not. Each measurement should be hundreds of k ohms or higher if ok.

Can you confirm which board you have in your box?

See the list below.

[Geo-fr]

Thanks for quick reply

For the board type, need to check the exact model (where it is written ?)

I believe I need to dismantle the board to check the components you are mentioning? They are on the « back » side because was not able to locate them on the visible side.

I will try tomorrow. Too late for today.

Note that I connected just few seconds (5-10s) to the bad charger (I know only few milliseconds of overvoltage can create damage). The PC was switched off. No start button triggering.