Dell Inspiron 14-5448 - fan turns off and mobo keeps running

[Felipe_Serra_83]

Hi, everyone. First post of an actual repair. I'm a technician but a complete newbie with laptops.

So I got this Dell Inspiron to play with.

No charger, so I connected to my bench PSU.

The laptop also didn't include: hard disk (I added one of my own for testing, see below) and wireless PCB (but I doubt this could have anything to do with this particular failure).


------What it did (and didn't do)------

1) I turned it on and it powered on. Powered down in 2 seconds by itself.

2) Removed RAM, changed slot. Same issue. Removed RAMs, cleansed slots. Turned laptop on. It stayed on and it gave the two intermittent beeps alarm.

3) Put RAM again. Removed battery.

4) Checked CMOS battery. Removed it, resetted BIOS (pressing power button for 30 seconds)

5) Removed heatsink and replaced thermal paste.

5) Added the DDE. Powered on, and it stays on. BUT... The fan (this mobo has only a single fan) stops and the laptop keeps running. Touched the heatsink. Noticed that CPU and GPU temp go up, I shut it off manually. No POSTing, and a single intermittent beep.

This is the pattern that repeats all the time.


------Some checks I made------

Checked mobo for water stains. None, looks ok to me.

No component seems visually damaged (such as diodes, mosfets, capacitors and the like).

No idea if it was ever tinkered with (as in attempts of repair, if it ever had other issues, etc.)

The chassis and other boards (such as USB and card PCB) are fine.

I made voltage measurements of inductors. Here's a pic with the numbers:





------Machine specs------

Machine model number --> Dell Inspiron 14-5448

CPU --> no model specified in the chip itself. Here's a pic:



In Aliexpress it says the CPU could be either a i5-5200U or a i7-5500U

GPU --> AMD 216-0855000

RAM --> 4GB, PC3L, 12800S

Storage device --> I plugged in a spare DDE I had, a Hitachi 5K320-160 (SATA, 160 GB)

Motherboard model number --> LA-B015P


----------------------------------

I supossed the problem could either be: in the BIOS. Or maybe the Temp sensor or any other part of the temp monitor is dead.

Hope I'm not missing anything in the description above. If so let me know.

Thanks in advance, I really appreciate your help.

[mon2]

No charger, so I connected to my bench PSU.

Stop. Did not read the rest (yet).

Dell uses a single-wire communication line which is often named PS_ID. This communication line is to the Dallas Semiconductor -> Maxim -> Analog (in the order of who owns whom), specification. If you are using a PSU, you are not able to emulate this mandatory PS_ID line so the EC controller (8051 based) does not get pacified on the expected specs of the power adapter.

Can you source a true and suitable Dell power adapter and test again?

[Felipe_Serra_83]

Hi, mon2.

Yes, I can try and get a compatible charger. I'll let you know.

Thanks

[mcplslg123]

Btw, your cpu setails is indeed there on soc.Look carefully. and its SR23W

https://www.cpu-world.com/sspec/SR/SR23W.html

[Felipe_Serra_83]

You're absolutely right, mcplslg123, and I missed that because of all the other numbers behind the "SR" :P. Thanks for the heads up.

[Felipe_Serra_83]

I got the charger, original one, from the guy who gave me the laptop.

Changed the thermal paste again, installed heatsink and fan. Connected screen and battery. Single RAM in its slot.

Plugged in the charger and turned on. Neither the power nor the battery light turn on. The screen flashes once, very lightly, but shows nothing. And like before, the fan turns off after two seconds, the single beep warning sounds, the CPU and GPU get hot.

Does anyone know the terminals of the battery, so I can check the voltage with a multimeter? Perhaps the charger works, even thought the LED doesn't turn on.

Thanks in advance.

[mcplslg123]

This mb seems to be a discrete model. Share the mb markings so that discrete graphics can be disabled or not-can be ascertained.

[Felipe_Serra_83]

Hello, mcplslg123. I suppose that by markings you mean the characters printed there.

I attached here some snapshots.






Thanks.

[mcplslg123]

LA-B015 is mb marking. Try disconnecting graphics coil(PL1002/1003). If it doesnt help
01 Shift RC99 to RC100
02.Remove RE3
03.Stuff RE5(20K)

This will make it UMA and if the problem lies in graphics card,you may get display.

[Felipe_Serra_83]

Ok, thank you very much. I'll do these and let you know how it goes.

[Felipe_Serra_83]

Hello, all. Couldn't find either the RC nor the RE components. Searched for the mobo's schematic, but no luck either. There's only a manual service for the laptop, but it wasn't of much use.

I did find the coils.

But in desoldering one of them, I accidentally blew a tiny resistor and now I can't find it.

Those d-mned little things. But in any case, fat chance of ever reinstalling it, as I have no microscope. :P

[mcplslg123]

The only resistor near those coil is a thermistor. After disconnecting both the coils,what is the status?

[Felipe_Serra_83]

Tested, and laptop did the same thing: screen flashes, then shows nothing. One beep, which repeats itself continuously. Fan turns off, CPU and GPU heat up.

[mcplslg123]

You can put the coils back as graphics chip doesnt seems to be the issue here.
Whats the status of DRAMRST#? Measure it on ram slot pin30.

[Felipe_Serra_83]

DRAMRST# equals 1.34 V

Hope I measured this one correctly :P. I did it in the back view of the RAM, where pair numbers are printed in the PCB right next to the pins.

BTW, mcplslg123, I appreciate the time you are putting into this. Thank you!

[mcplslg123]

Dramrst# is correct. What about PLTRST#?
Do you have a dc power supply and DSO? If yes, then we proceed to check smbus communication.
However, if you dont have these tools,measure voltage on all pins of QC1.
Are you sure bios you flashed is good and me cleaned?

[Felipe_Serra_83]

PLTRST# = 3.3 V

(BTW, I found a schematic of the mobo)

No oscilloscope here, so I measured QC1. All six pins = 3.3 V

"Are you sure bios you flashed is good and me cleaned?" Yes, I resetted it by unplugging and then plugging the CMOS battery,

[mcplslg123]

So,you didnt flashed the bios chip and did only a cmos reset. Correct?
If so,strongly recommend to flash bios with an spi programmer like CH34A/RT809F etc.
Cmos reset will reset the bios but wont fix a corrupt bios.

[Felipe_Serra_83]

Correct, I only did the cmos reset. Like I said, I'm a noob in laptop repairs, but I learnt quite a bit in this experience. Thanks to you and mon2 who replied to this

I'll get the programmer... It's pretty cheap and will sure come on handy for other laptop repairs.

Let you know when I have it. Have a good day!

[mcplslg123]

Yeh,flash the bios chip first and then if issue persists we can dig further. However,i'm very hopeful it will be resolved with bios flash.

PS: I wont be available for next 4-5 days as heading for a short vacation but keep the forum posted about progress,if any.

Good luck.