Laptop Computer Repair.

[ChrisGreece52]

Hello everyone i had my laptop going dead yesterday. Below is my post from Tom's hardware. I have decided to post it again here because of this forum's members experience with electronics.

I had a weird day. Due to a power cut my laptop shut down suddenly (i have no battery installed because I only use it on my desk connected to 2 monitors).
Anyway after the power came on I turned it on again to have the power being cut again.
When I tried powering it up the power led came on but no display.
Being an undergrad electronics engineer i decided to tear it down (that is after I tried swapping the memory modules and eliminating the hard drives)

I tore the thing up to its motherboard which I tried measuring some basic stuff (input voltage, Ic supply voltage) trying to find the cause.

When I saw that everything looked normal I tried powering the thing up and voila... it booted telling me that there was no available media (i had only the screen and power plugged in to the mb)

I thought it was a random hiccup and closed the thing up. It booted normally and then I powered it off and went to sleep.

Now the problem occurred again and I have no idea on what the cause might be. Really I have no clue or reason to suspect a specific component since I tested it even without the fan plugged in (I don't know why I have seen some weird things in electronics in the past)

I hope someone who looks at this with a fresh pair of eyes can point me somewhere.
Edit 2 : I forgot to mention that in this state I have unplugged most things from the mb trying to eliminate a faulty component but the pc won't boot

Also when it is in the state where only the power led is in the cpu gets a bit worm (with the fan inactive) like its doing something. I also tried the button on the side that gets the laptop into the boot menu /bios etc.

The laptop specs :
Lenovo G50-70
CPU :i5 4200 @ 2,4ghz
RAM :12gb of ram ( one original Samsung stick 4gb and one hyper x one I added later 8gb (same frequency))
GPU :Radeon 230m graphics (don't quote me on that It may be another series but definitely AMD Radeon discreet graphics card)
Edit : forgot the HDDs
HDDs : Samsung 840 Evo 120gb SSD (main drive)
HGST 1tb 7200 RPM mechanical (as a replacement for the cd reader)


Anyway thanks in advance.

As stated above thanks in advance. Extra info I was powering up an esp8266 with an external power supply while programming it from the laptop's USB port. The thing is that the Vin pin of the mcu is isolated from the USB port so I have nothing else to suspect but the sudden power losses.

Thank you so much for your time.

[ChrisGreece52]

 Btw here is the main board schematic

[PKTKS]

Before you start weird things on the MOBO itself

Check **IF** your  PC will work fine using another PSU
prefer a LAB PSU with controlled CC/CA to stress the
MOBO with things... like .. say FURMARK and Uningine...

If that works ...  check your PSU with a proper LOAD
using the rated maximum AMPs. Should pass the test
or it just gone wild with the shortage.

Paul

[ChrisGreece52]

I don't have a proper bench supply to test the motherboard. The only thing available is a 18,5 V 3,5A (65W) laptop adapter and some cheap Chinese boost converter pcb's

[PKTKS]

Nop. This can make things even worst

You have one possible alternative of using a KNOWN GOOD PSU.

I mean it.  It must be good because if the original has
some sort of current weird fault you will not spot that easily

Alas.. the bench supply with proper CC/CA ConstantCurrent/Amperes
limitation is not expensive.. and mostly required to do any thing repair.

Worth 1000 times any money on itself.

Otherwise..  you will fallback to change this change that.. recap this
try that  methods...

Also valid but can be even more expensive..

Some PSU faults can drop the voltage below threshold on
some faults when current reaches above certain value

common fault on that cheap external notebook PSUs
dropping things adhoc

Paul

[ChrisGreece52]

Thanks for the answer I will send the laptop to a proper repair shop and invest in a bench power supply that I wanted for some time now.

[PKTKS]

A last resource may be kindly ask to power your note
with some well good known PSU (even at the repair...)

It may work normal enough to suspect the PSU was indeed damaged

A last tip on Bench Power Units target to MOBOs is that:
- mundane notebooks require a higher voltage with (comparable) lower current
- on the other hand desktop MOBOs require smaller volts (12V) but insanely
  higher current - at present some GPUS+CPUs would require a quite expensive
  bench power supply capable of powering (controllable and measured) at least
 and safely 30 AMPS.  (12V @ 30A or 50A roughly 500W regulated bench unit)

The last one is not cheap. But the notebook bench units are rather simple
and quite affordable.

Paul

[ChrisGreece52]

I told the technician to try powering the laptop from a known good power source to check the motherboards integrity. As for repairs I don't really do many but a good bench supply would be handy! Now that I am thinking about it the power supply was outputting 20v when the label said 18,5 v but I assumed it was due to the lack of load on the computer side.

[geggi1]

You have to start simple with the most likely cause for failure the PSU. Borrow a PSU from a friend.
If you have tried with another PSU and the laptop is still dead and will not boot it most likely the BIOS.

[rob77]

laptop is supposed to work between 3 cell lipo voltage and whatever the charger voltage is. don't place too much emphasis on the power supply it has to be good for sure, but it's definitely not critical.

check the voltages - go from charger input , check B+ rail - that's your combined rail supplying the whole laptop (either coming from charger sitting at around 19V or coming from battery and is much lower ). if B+ is not present check eround teh BQ circuit PU301 in the schematic. if B+ present then check the 3V LDO - it's in PU904 , this a is a voltage which powers the KBC (keyboard controller) which in turn is responsible to enable the always on rails - (signal EC_ON to PU904 and PU905) if it reacts to power on then check your always-on voltages +3VALW and +5VALW if those are present and it's not powering on then it might be some other supply rails are missing or shorted (check further by schematic) or you might be having a "rotten" bios.

[Bicurico]

How about putting the battery back in?
You probably drained the battery cell on the motherboard by now and that puts the laptop in an undefined state when power is cut off.
Laptops were designed to have their battery inside. I don't understand why people insist on removing them. They will age with or without use and modern battery management will not charge the battery forever or let it drain.

[ebastler]

Gentlemen, this laptop probably does not exist anymore. This thread dates from 2019, and was just resurrected by new user "Barathrum", who most likely plans to add a spam link next. Reported.

[Simon]

he did, banned!