Dell E7440 no signs of life

[massaaris]

Hello everyone,
I've got some dell motherboard (LA-9591P) in an incomplete case, missing most of the components, keyboard, trackpad, screen, battery, etc. Looking for an opinion from someone more experienced with this model and maybe some common issues. CMOS battery was dead so I replaced it. Also, added one stick of PC3L Ram.
When I plug in the charger (original dell) there are no signs of life, no LEDs, power button does nothing. Charger connector has blue light on it. Measured 19.5V on input and also on some other places on the board. Measured some random chips looking in the schematics and found 5V, 3.3V at the correct pins. Tested all the capacitors (I think) for shorts and found none.
Please note that I have no experience with board level repair, but I would like to learn basics. Could replace some resistor, capacitor or a fuse, and that's about it (for now )
For starters, I would like to know if this motherboard could be tested outside of the case, connected to HDMI monitor? What is the minimum stuff to connect for it to work?
What would be the first steps to measure trying to find why there is no power light?

[Stevedb]

Re: Dell E7440 no signs of life

Yes you can you need , ram ,cmos battery and powerbutton with lidswitch to test it out the case.

[massaaris]

Re: Dell E7440 no signs of life

Thank you, I should have everything you mentioned. Power button is soldered on to the board. My main concern is that there is no light on the board when I plug the charger in. LEDs are good, I tested them with multimeter.
I might have picked very complex board to start with. Case has some liquid damage traces, but there are no evidence on the board as far as I can see. Maybe someone already cleaned it.

[massaaris]

Re: Dell E7440 no signs of life

I have tried to diagnose this problem but with no success. What is strange to me is that both bios chips (64Mb and 32Mb) have 3.3V on all pins except ground pin when charger is plugged in. There is no short between pins, I read between 1k and 14K resistance. Maybe someone with more experience can tell is this normal behavior?

[mon2]

Re: Dell E7440 no signs of life

Start with the charger section. Often the first 2 mosfets are the fault but remove all power.

No battery.

Meter in resistance mode (2k scale is ok).

Measure and post the resistance readings for:

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

Checking for any low resistance values which will indicate that the mosfet is defective and will need to be replaced.

If the resistance readings are ok then consider to use your meter in voltage mode (20v scale is ok).

Apply your external adapter only.

Carefully measure & post the readings on the charger IC @ PU700 (BQ24715RGRR_QFN). This is a popular charger IC so documents can be reviewed on how and when it works with respect to the external mosfets.

[massaaris]

Re: Dell E7440 no signs of life

Here are some measurements:

Input jack across + and - : 71 kOhm
PQ700
G to S : 4.5 MOhm
G to D: 5 MOhm
S to D: 40 kOhm and rising
Voltages: G 0 V, S 19.5 V, D 19.5 V

Similar resistance measured for PQ4, PQ704, PQ706

PL500 0 V
PJPDC1 13.5 V
PJP1 19.5 V
PL4 19.5 V
Cap below PL4 19.5 V and 9.2 V
PQ4 G 9.2 V, S 19.5 V, D 19.5 V
PL700 19.5 V
PR700 19.5 V
PL701 13.5 V
PR716 13.5 V
PL100 13.5 V
PL101 3.3 V
PC113 3.3 V
PL102 5 V
PC115 5 V
PL200 0 V
PC207 0V

PQ704 G 13.7 V, S 13.5 V, D 19.5V
PQ706 G 0 V, S 0 V, D 13.5 V

PU700:
01. ACN 19.5 V
02. ACP 19.5 V
03. CMSRC 19.5 V
04. ACDRV 25.2 V
05. ACOK 3.25 V
06. ACDET 3.12 V
07. IOUT 0 V
08. SDA 3.3 V
09. SCL 0.18 V
10. CELL 5.6 V
11. BATDRV 13.5 V
12. SRN 13.5 V
13. SRP 13.5 V
14. GND 0 V
15. LODRV 0 V
16. REGN 6 V
17. BTST 19.5 V
18. HIDRV 13.5 V
19. PHASE 13.5 V
20. VCC 19.25 V

DC input connected, no battery, one stick of RAM, no additional cards, flat cables anything.

[mon2]

Re: Dell E7440 no signs of life

08. SDA 3.3 V
09. SCL 0.18 V

Confirm the voltage of the SCL pin. It is low and appears to be stuck low.

Then, remove all power. No battery.

Meter in resistance mode (2k scale).

Measure the resistance to ground of the SDA pin.

Them repeat the measurement for the SCL pin.

Post your values.

[massaaris]

Re: Dell E7440 no signs of life

SDA resistance to GND 10.6 kOhm
SCL resistance to GND 180 Ohm

SDA 3.23 V
SCL 180 mV

[mon2]

Re: Dell E7440 no signs of life

SCL resistance to GND 180 Ohm

This line is being pulled down and causing problems.

Locate the ESD tvs diodes that are often on these lines near the battery. When found, remove this part.

Then check the resistance again. If no longer a low reading, power up and test again.

Post the voltage on the SCL line without the tvs ESD dual diode on this line.

[massaaris]

Re: Dell E7440 no signs of life

I'm not really sure where to look at. There is a diode "S510 M44" near PU700, or maybe something else?
Here are some pictures of the board from both sides.

[mon2]

Re: Dell E7440 no signs of life

Look for a 3 pin part with label PD1. If not silk screened on the logic board, show some pics near the battery connector. It should be close by.

See attached.

Update - I think I found the location - see the area of the battery.

[massaaris]

Re: Dell E7440 no signs of life

@mon2 I forgot to say, Thank you for all the effort
I found PD1 and removed it, but unfortunately no change, measurements are the same.

[mon2]

Re: Dell E7440 no signs of life

My bad...this logic board has multiple I2C (SMBUS) masters.

The charger bus master, CLK pin is at fault.

This could be:

1) due to a faulty KBC port pin that is responsible to drive the SCLK signal

or

2) the required pull-up resistor for the SCL line is missing (RP19) or corroded(?)

See attached. Inspect the resistor network @ RP19 which will be a 8 legged part (4 on each side). Does it look clean to you ?

If in doubt, apply flux and allow this part to 'dance' with the hot air so that the solder joints settle once again. Be careful to use low air pressure so that the part or neighbors do not go flying in the process.

[massaaris]

Re: Dell E7440 no signs of life

I think RP19 looks normal. I measured resistance between leg pairs and all are 10 kOhm, except third pair with 490 Ohm, not sure if that's relevant.
This is the part of the board where I found signs of flux (on two big chips), but it's cleaned now.
I don't have hot air station

[mon2]

Re: Dell E7440 no signs of life

I think a hot air tool is mandatory for such repairs. The large chip may have corrosion underneath that is not visible. For such cases, apply flux on top and around the pins of this part and hot air away till the part starts to swim into the solder. Use tweezers to gently nudge it - make it 'dance' so that the joints settle again.

If that does not work then likely this large IC is defective - not sure if it is a programmable device. If it is, then the part needs to be programmed with the proper firmware. Check on the internet or wait on some feedback from others on this KBC.

[massaaris]

Re: Dell E7440 no signs of life

I know it's basically mandatory, but I still don't have it or the skills to do such precision work (for now). My idea was to try, maybe if it was something simple, and also to learn some basics.
I measured yellow pin you marked on the RP19 and it has the same voltage as SCL 180 mV. Other pins have 3.3V. I suppose it comes from the chip, so it's probably defective or something.
Thank you a lot for the help!

If anyone else has any suggestions I'm ready to try it. If not, this could be a good practicing board for the future

[mon2]

Re: Dell E7440 no signs of life

Confirm that the LEFT side of the RP19 pads are each @ +3v3.

See attached.

If yes, then it is all down to the large IC and its soldering.

You can try to measure the DIODE MODE readings as follows:

Meter in DIODE mode setting.

Red meter (yes RED) probe to GROUND (shield is ok).

Black meter probe to the SCL line on the RP19 that is showing the low voltage.

What is the reading ?

Then repeat for the SDA line on the RP19.

What is the reading ?

As noted, you have a low resistance to ground on the SCL line and this is preventing the I2C (SMBUS) master (the large IC) from communicating with the other devices using this 2-wire communication interface.

RP19 plays an important role and it must be pulled up to the ~3v3 value but we cannot see what is under the large IC so a reflow is recommended. If that fails to work, then the large IC must be replaced. At this time, I am not sure if this large IC is a programmable device or not. Sometimes such parts feature a small microcontroller (like 8051) inside the device and then will fetch firmware upon power up from a flash device. The flash device may be external to the KBC controller or could be internal - again, if at all programmable.

[massaaris]

Re: Dell E7440 no signs of life

Yes, all yellow marked pins are 3.3V
Diode mode on SCL 0.13V (same on the RP19 pin)
Diode mode on SDA 0.5V

[mon2]

Re: Dell E7440 no signs of life

So to summarize,

1) the issue is most likely linked to the large KBC controller

2) with a hot air tool & flux, you can reflow this device to see if the low resistance value changes to allow for the SCL line to toggle as it should and settle @ 3v3 when not active

3) with a hot air tool & flux, you can replace this part, if deemed to be defective. However, see attached - the part is a programmable device and does require firmware to function. You can program it yourself if you have the rather expensive programmer or purchase from a reliable vendor (perhaps Ebay) that can sell this part pre-programmed.

The hot air tool is about $200 USD on Aliexpress - we use and recommend the BEST-863 but there are other devices on the market in this price range.

[massaaris]

Re: Dell E7440 no signs of life

Thanks again. I will save this board to mess with later when I have the proper tool. It's an old board, not worth much, so ideal for practicing.
I looked up some hot air stations a while ago, think I stumbled upon this model from Best. Also considered Atten ST-862D which I can get locally for a similar price.