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

[mon2]

It is fine. Most likely they are the low side switching mosfets. Of concern is if the main power rail is shorted but you will know once the high side mosfet is removed. Often due to a shorted capacitor.

[zlr8r]

ok, I removed the Cap nearest to the lower-right mosfet. The resistance now on that mosfet is now, 42 Ohms (was 26 Ohms before removing the nearby Cap).

I also see a direct short between Source and Drain on that lower-right mosfet ? Should I now remove it and retest ?

cheers

(with Cap removed beside lower-right mosfet, resistance is now 42 Ohms) . And is also shorted between Source and Drain ?

[mon2]

Start with fluxing and carefully removing PU4403 (lower right mosfet). Allow it to cool and measure the pins on the removed mosfet while it is on your work bench. Do not power up this logic board without the mosfets.

Trying to locate the reason for the short. Be patient and allow for the solder to melt.

[zlr8r]

PPU403 mosfet removed , and getting a direct short on it between Source and Drain.

[mon2]

To clarify, the short is being measured on the removed component?

Proceed now to check the pads on the logic board where this component was originally soldered. What are the measurement readings in this region? Have they improved to a higher resistance value?

[zlr8r]

To clarify, the short is being measured on the removed component?

Proceed now to check the pads on the logic board where this component was originally soldered. What are the measurement readings in this region? Have they improved to a higher resistance value?

Indeed, lower-right mosfet removed and getting a short on it between Source and Drain.

Good reading on its motherboard pads and other 3 mosfets (around 520 Ohms).

[mon2]

That is still a very low measurement if you are measuring across the mosfet pins. In your post, you must supply how you are measuring.

Specifically, best to post,

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

Do this for each of the remaining mosfets on the logic board for this region under test.

Mosfets should be hundreds of K ohms if they are not shorted. Post each measurement and the label of the mosfet being tested.

For sure, the removed mosfet is leaky and must be replaced. You likely have other mosfets that are also defective.

[zlr8r]

Purple squares illustrate removed Cap + Mosfet.
(Lower-right red+white values box reflects measurements with both Cap+mosfet removed)

many thanks

[mon2]

The attached in highlight, to me are perhaps also defective mosfets. Best to flux and remove and measure each while they are out of circuit. If the readings remain low then they are also leaky and considered to be defective.

Measure the mosfet pads as well after the parts are removed.

Check each mosfet while it is off the logic board.

[zlr8r]

many thanks. If im replacing 1 i may as well do the 4 since its going to take 3 weeks for them to come from China....

While im waiting on those , is there anything else I should be checking on the board for possible issues ?

thanks

[mcplslg123]

PU4404/4406 pin 1-2-3 is GND and clearly mentioned in the schematic. Remove PU4403 from board and check resistance readings on the mb PCB pads/pins of that mosfet.Is the reading same or it has climbed much higher?

Sorry,didnt read that you have already removed it.

[zlr8r]

ok, bit of a delay waiting on replacement mosfets coming from China.

Okay, I installed all 4 mosfets just to be sure. Reconnected the 19v charger.

Same thing happens as before, power light comes on for about 2-3 seconds and then goes off.

Or, the amber light flashes 3 times followed by 5 white lights. At same time, the CPU is getting extremely hot , and I can hear a whine from a coil somewhere, and then the same mosfet on the lower-right of image ends up blowing (shorts itself out after getting extremely hot - so hot I burned a square into my thumb lol)...?



ok so im right back where I started I guess.... how do i proceed to locating the component thats burning out that lower-right mosfet ?


thanks to all

[mon2]

Remove all power -> measure the resistance to ground of each pin on this mosfet. It is burning up due to excessive loading.

Measure source (1-2-3) to ground.
Measure gate (4) to ground.
Measure drain (5-6-7-8) to ground.

[mcplslg123]

I beleive this mosfet is part of charging circuit. If yes,then follow @mon2's advice and give measurements. If still low resistance on PCB pads,remove the charger IC and measure again. I think this mosfet is getting shorted 'coz of bad charging IC.

[zlr8r]

Remove all power -> measure the resistance to ground of each pin on this mosfet. It is burning up due to excessive loading.

Measure source (1-2-3) to ground.
Measure gate (4) to ground.
Measure drain (5-6-7-8) to ground.

Hi, sorry for the delay.

with bare motherboard on bench, no power, no battery, on that lower right mosfet I am seeing the following values. (it may be already burned though).

80 kΩ - S
OL - G (meter value rises to a MΩ point then goes to OL)
155 kΩ - D

[zlr8r]

ok, for all 4 of those grouped mosfet's, the values with the Charging IC removed (D,S,G to GND), are ;
















(top left)
11 MΩ - D
0.8 MΩ - S
none - G

(top right)
300 kΩ - D
0.2 Ω - S (shorted?)
none - G

(lower-left)
7.8 MΩ - D
0.2 Ω - S (shorted?)
none - G

(lower-right)
5 MΩ - D
300 kΩ - S
none - G

[mon2]

Hi. No - the 0.2 ohms to ground is likely due to the wiring of those mosfets being mated to the ground. That is, it is the 'low side' mosfet. On switching power supply designs, there will be a high side and a low side mosfet that will switch on / off to dial up the desired voltage which is being monitored by the feedback pin on the regulator. Much like steering a boat to dock - a bit to the left / bit to the right till the boat is docked into place. These mosfets will switch on / off as necessary as to not overshoot nor undershoot from the desired target voltage.

Remove all power. Meter in resistance mode. Measure the resistance across these same locations.

So source / drain resistance? source / gate resistance ? gate / drain resistance ?

[zlr8r]

whoops! apologies, ignore previous values, i forgot i had removed a big coil directly from the other side of the board from those 4 mosfets.

i soldered it back on and the correct values are ;

(top left)
1.1 MΩ - D
1.5 MΩ - S
none - G

(top right)
1.5 MΩ - D
0.2 Ω - S
none - G

(lower-left)
1.5 MΩ - D
0.2 Ω - S
none - G

(lower-right)
2.5 MΩ - D
1.1 MΩ - S
none - G



do you still want me to complete your last request ? i.e.
Remove all power. Meter in resistance mode. Measure the resistance across these same locations.
So source / drain resistance? source / gate resistance ? gate / drain resistance ?

thanks

[zlr8r]

Values ; source / drain resistance, source / gate resistance, gate / drain resistance.

(top-left mosfet)
0.6 MΩ S - D
none - S - G
none - G - D

(top-right mosfet)
2.5 MΩ - S - D
none - S - G
none - G - D

(lower-left mosfet )
3.2 MΩ - S - D
none - S - G
none - G - D

(lower-right mosfet)
0.9 MΩ - S - D
none - S - G
none - G - D

[mon2]

The posted mosfet measurements look ok. They do not appear to be shorted.


====

Yes, this is normal. You are reviewing the 4 mosfets which work together to provide the same power rail. Respectively, the mosfets you are measuring are each a low side mosfet so it is ok for them to be connected to the ground via the source pin group 1-2-3.

NB: Posted this reply out of sync to an earlier question but will leave it here for study.

See attached.