Dell inspiron 17 (7786) - DP/N 09P7JP - odd power issues, starting with no power LED

[zlr8r]

hey guys, hoping for some pointers to repair an eBay bought "faulty" laptop.

Ill try and give as much info as I can.

Dell Inspiron 17 (7786) Laptop
Service Tag: 5J0NHT2
Bios: 1.9.0

When I received this , it would only flicker on the KB backlight for a second, then off and on.
Then it started getting weirder!
Sometimes when attaching the AC power supply, and pressing various pre-boot diagnostic keyboard keys... it did try to boot, in fact out of a hundred attempts, the laptop did actually boot into the Bios ! but never has since...
At present, when i connect the AC power supply, it turns the Fan on full power, 3x amber flashes, and 5x white flashes can be seen (with or without anything being attached to the mobo (just power button) and with or without ram installed...?
I am able to use the power button when it does this, to turn it off. But it doesnt power on again unless i disconnect the power supply and reconnect it?

Now, i would call this 'progress' since it was doing "less" when i received it.... lol

Even though there is no power LED I am seeing various voltages around the board, 19v, 13v, 3.3v, 1.3v, 0.7v?? . not sure if these are exact....

I am not seeing any hotspots under a thermal camera (at all) ?

Here is how i have only been able to get the status LED to come on.;
1. disconnected battery and bios battery (lets say for argument that the main laptop battery has been removed completely for all experiments)
2. disconnected mains power supply.
3. press and hold power button for 30 seconds
4. reconnected the mains power supply

at this stage the white light (beside the jack) comes on after 10 seconds, stays lit for several seconds and goes out again, and stays that way no matter what is pressed. Again the only way to get it to do anything again, is to reconnect the bios battery, disconnect and reconnect the power supply, and it goes back to full fan running, KB backlight on, 3x amber and 5x white lights come on.

I have tried multiple combinations of pressing and holding keys during boot but nothing works. even tried to do the bios recovery method but again nothing happens.

Its very strange that I did manage to get the laptop to just one time boot into the BIOS so surely that tells me everything is (mostly) working to some degree ?

I have been able to extract the current bios from it using a programmer , which I can attach if allowed? it doesn't look to be corrupted but then again i dont have anything to compare it too. I dont want to programmer-flash anything back to the bios chip just yet until im certain the bios could cause these startup problems .

Would be grateful for any tips on where to look for likely faulty components and next steps to pinpoint exactly whats wrong.

Here is a pic of the back of the board showing the part number etc etc



















































































































many thanks

[mcplslg123]

Resistance to GND measurement on all coils(no power attached) to begin with.
Voltage on all coils after powering it on.
Link schematic,if available.

[zlr8r]

Resistance to GND measurement on all coils(no power attached) to begin with.
Voltage on all coils after powering it on.
Link schematic,if available.

- sorry i dont have the schematic, i know it would be useful...

Resistance to GND with NO power going to motherboard.















V at coils when power supply connected (still with laptop battery removed).

Note- Laptop powers itself on to the error LEDs, once power supply is connected.

[mcplslg123]

The resistance to GND readings seems good and likely there is no shorting.

Going by the voltage measurements,you have 3V/5V/1.05(PCH Power) only. 13V on coil indicates its NVDC design and main power rail is fine. The motherboard is stuck in S5 stage.

Missing Ram voltage on coil is sure indicator that mb has not reached even S4 stage.

Look for Ram IC,download datasheet or any other schematic with same IC, and check the enable pins.

Missing VCORE/GPU is not the issue here as they will rise only after ram voltage is ready.

So i'll suggest to check why ram voltage is not ready and that may give you some clue.

Fighting it out without schematic is bit tough and may not be always successfull but majority of them can be solved with experience and patience.

Good luck.

[mcplslg123]

I see that the mb is still sitting pretty in casing. Take it out from housing and visually inspect the mb under microscope and you may be in for a surprise.

[zlr8r]

I see that the mb is still sitting pretty in casing. Take it out from housing and visually inspect the mb under microscope and you may be in for a surprise.

thanks. where would i get the schematic for the board ?

I already scanned the board under the microscope before posting on here, but maybe i missed something. ill take another look.

thanks for info

[zlr8r]

The resistance to GND readings seems good and likely there is no shorting.

Going by the voltage measurements,you have 3V/5V/1.05(PCH Power) only. 13V on coil indicates its NVDC design and main power rail is fine. The motherboard is stuck in S5 stage.

Missing Ram voltage on coil is sure indicator that mb has not reached even S4 stage.

Look for Ram IC,download datasheet or any other schematic with same IC, and check the enable pins.

Missing VCORE/GPU is not the issue here as they will rise only after ram voltage is ready.

So i'll suggest to check why ram voltage is not ready and that may give you some clue.

Fighting it out without schematic is bit tough and may not be always successfull but majority of them can be solved with experience and patience.

Good luck.

Hi, looking at my mobo image, which IC is the RAM IC ? thanks

[zlr8r]

The schematic is a bit above my abilities to decode the fault with, can you please advise where i should be looking and checking on the board to isolate the faulty component ?

Ive spent days looking at this motherboard and am simply lost now in finding the fault. Any pointers would be great. many thanks.

ps. I spent some time looking at page 54 on the attached schematic but couldn't draw on any conclusions.... i just dont know where im losing the RAM voltage....

[MOD EDIT] Link to schematic -> https://www.badcaps.net/forum/troubl...ic-plz?t=86068

[mcplslg123]

PU5101 on Pg 51 is your ram ic. Pin5/6 are enable pins.Check voltage on pin9/10/21 also. Note that enable will come after switching only.

[zlr8r]

PU5101 on Pg 51 is your ram ic. Pin5/6 are enable pins.Check voltage on pin9/10/21 also. Note that enable will come after switching only.

Hi again, thanks for pointing out the IC.

So, i checked the voltages on pins 9/10/21 (see diagram below).

(Note:The power button on the laptop is basically useless after connecting the charger, as the laptop goes straight to S5 mode, but the button does switch it off ok).

Okay, so on Pins 5,6 I wasn't seeing any change when the charger was plugged in (am i looking for a quick on and off voltage here ?)

Where do i go from here ? many thanks for your help with this.

[mcplslg123]

Absence of PM_SLP_S3/S5 is not a good sign .Does the power button has 3V and on pressing it goes to zero and bounce back to 3V?
The power button press goes to SIO and in turn SIO sends this feedback to PCH and then PCH will get activated. Here the PCH is not being asked to activate/not getting activated after receiving the signal from SIO. Can confirm after you give me the measurement.
Page 102 has the timing diagram.Check where you get stuck.

[zlr8r]

MEC140X.PDF

Absence of PM_SLP_S3/S5 is not a good sign .Does the power button has 3V and on pressing it goes to zero and bounce back to 3V?
The power button press goes to SIO and in turn SIO sends this feedback to PCH and then PCH will get activated. Here the PCH is not being asked to activate/not getting activated after receiving the signal from SIO. Can confirm after you give me the measurement.
Page 102 has the timing diagram.Check where you get stuck.

(at the PWR BTN)
I checked the power button voltage (the button itself) , and it drops to 0v when pressed.

(at the SIO)
I then checked the SIO PIN-2 when the power button is pressed and it drops to 0v also. (and is 3.1v before button is pressed).

I checked a few other voltages at the SIO (first row only);

PWR BTN Not Pressed vs PWR BTN IS pressed

PIN 01 - 3.3v - 3.3v
PIN 02 - 3.3v - 0.08v
PIN 03 - 3.14v - 3.13v
PIN 04 - 3.14v - 3.13v
PIN 05 - 0v - 0v
PIN 06 - 0v - 0v
PIN 07 - 3.14v - 3.13v
PIN 08 - 0v - 0v
PIN 09 - 3.13v - 3.10v (dips briefly when pwr_btn is pressed, then returns to 3.13)
PIN 10 - 0v - 0v
PIN 11 - 3.3v - 3.3v
PIN 12 - 0v - 0v
PIN 13 - 3.3v - 3.3v
PIN 14 - 3.3v - 3.3v
PIN 15 - 3.3v - 3.3v
PIN 16 - 0v - 0v
PIN 17 - 0v - 0v
PIN 18 - 2.24v - 2.24v
PIN 19 - 3.3v - 3.3v
PIN 20 - 1.3v - 1.3v
PIN 21 - 3.3v - 3.3v
PIN 22 - 3.3v - 3.3v
PIN 23 - 3.3v - 3.3v
PIN 24 - 3.26v - 3.26v
PIN 25 - 3.3v - 3.3v
PIN 26 - 3.3v - 3.3v
PIN 27 - 0.13v - 0.13v
PIN 28 - 0.13v - 0.13v
PIN 29 - 0v - 0v
PIN 30 - 3.3v - 3.3v
PIN 31 - 3.3v - 3.3v
PIN 32 - 3.3v - 3.3v

if you need more SIO pin measurements please let me know. cheers.

** On a random check with the LED flashing (S5) , I tested the voltage on the very last pin of the RAM slot #2 (nearest to PU5101) and I'm getting 3.3v ?? on the outside and 2.5v on the inside ??

Surely that cant be right for SODIMM ? Shouldn't I just be getting 1.2v ? (note: On RAM socket 1 I do not get any voltage present on the last RAM pin ?)






























What's next area I can check ? thanks

[mcplslg123]

Ram slot does receive 2.5V(VPP),3.3V(SPD)-so perhaps nothing wrong there.Pin27-RSMRST# is pulled low and should be 3.3V instead. Its pulled low means its in reset mode.

Measure resistance to GND on RSMRST# pin. Should be 10K or higher.

Have you flashed the bios?

[zlr8r]

Ram slot does receive 2.5V(VPP),3.3V(SPD)-so perhaps nothing wrong there.Pin27-RSMRST# is pulled low and should be 3.3V instead. Its pulled low means its in reset mode.

Measure resistance to GND on RSMRST# pin. Should be 10K or higher.

Have you flashed the bios?

Thanks ,

Tried to measure for any resistance on SIO Chip Pin #27 - GPIO123/SHD_CS# to GND, but I got no value at all on the multi-meter (also tried another meter just to be sure).

No, I haven't tried to re-flash the bios, but I did take a copy of it before I started doing anything, using a programmer.
I was thinking of re-flashing with Dell's latest bios but it extracted (using a Github script which i have tried) but it produced a 12mb file and not a 16mb, and am not 100% sure how to "merge" or manipulate my old bios into the newest version to make it a "complete" 16mb file.

Here is my Original Bios Copy (link below). Maybe you can see a problem with it ?
(attached links to Dell's Bios EXE file; Recovery Bios file as well, as the 12mb extracted .bin)

[MOD EDIT] "Bios requests ONLY!" ->
https://www.badcaps.net/forum/troubl...quests-only

[mcplslg123]

Meter not showing any reading means meter is set to alower scale. Set your multimeter to 200K scale and then measure RSMRST# pin resistance to GND.

I'm not a bios expert though i do manage my own requirements of building a fresh bios from manufacturer's update files. You should make a request in "BIOS SECTION" and there are very capable people in that subforum who will build a fresh bios for you.

[zlr8r]

Meter not showing any reading means meter is set to alower scale. Set your multimeter to 200K scale and then measure RSMRST# pin resistance to GND.

I'm not a bios expert though i do manage my own requirements of building a fresh bios from manufacturer's update files. You should make a request in "BIOS SECTION" and there are very capable people in that subforum who will build a fresh bios for you.

ok, thanks. i will try that.

meantime, something has changed after all the poking around this laptop. The original fault evolved from just the laptop flashing the KB backlight and nothing else, then it went into a state of flashing 3 amber and 5 white leds. Now the CPU has decided to get quite hot. Not perfect but seems like progress lol. All i need now is the GPU to come on and i will have picture ... i wish.

Also, i had another quick view over the board with a thermal camera and noticed a very slight temp increase on one of the mossfets on the other side of the board. There are 4 in a group and one displayed as hotter than the others. Maybe its meant to ? im not sure , but i would have thought all of them would be warm when activated ? Its not hot or even warm to the touch, but the camera did pick it up none the less, so maybe another piece of the puzzle found ?

image attached

[mcplslg123]

Is that mosfet PU4405? If yes, then it is part of charging circuit. This fet is expected to be warm if battery is connected and its charging-else should not be warm. Time to remove the fet from motherboard and do resistance check between
01.Pin1-4
02.Pin1-8
03.Pin4-8

Meter should be in Mohms range and not Kohms.

[zlr8r]

Is that mosfet PU4405? If yes, then it is part of charging circuit. This fet is expected to be warm if battery is connected and its charging-else should not be warm. Time to remove the fet from motherboard and do resistance check between
01.Pin1-4
02.Pin1-8
03.Pin4-8

Meter should be in Mohms range and not Kohms.

Hi, I know i should remove mosfets for proper measurement but i thought id just test them in situ to see if any readings were way off and one of them was , and it wasn't the one that gets "warm" (red square) when charger is plugged in?

As you can see in the image below, I'm getting a very odd reading (26 Ohms) between all the contacts on the lower-right mosfet (no matter which way round the probes are positioned). Whereas, readings are "normal" on all the other 3 mosfets (including the one that just warms (in red square)...?

Could the lower-right mosfet be the culprit , and not the top-left one as first thought ?

[mon2]

Start with fluxing and removing the nearby cap so it is not lost. Check the resistance again. Then flux and remove this likely shorted mosfet. Allow it to cool on your table test the resistance again.

[zlr8r]

Start with fluxing and removing the nearby cap so it is not lost. Check the resistance again. Then flux and remove this likely shorted mosfet. Allow it to cool on your table test the resistance again.

Thank you for advice. Just before I try that, I just noticed one other thing on a continuity test. Both Source contacts on these two mosfets, have a direct short to GND ? is that normal? thanks