Help with board LA-9872P short to earth

[ex250]

Hello, I have a Dell M6800 that power supply goes off every time you try to plug in. Power supply is on good condition, working Ok in other devices.

The board is a LA-9872P, could not find a proper scheme, maybe 9871P is similar?

The board has almost 2 mosfets dead, PQ5 and PQ701 (MDU3603 both ones). Changed them but plugging the power supply, the PQ5 died again, PQ701 still live, but short is still present.

Tried to inject 2v, the PQ701 heats a lot, other components not seem to increase temp to my fingers "resolution".

Totally lost, some one could point me?

[mon2]

Re: Help with board LA-9872P short to earth

Hi.

1) Yes, my database reports to use LA-9871P for this chassis.

2) What part was used to replace PQ5 ? PQ701 ? MDU3603 for each ?

Is it possible they were soldered in backwards ?

3) Remove PQ5. Meter in resistance mode (2k).

Check and post the resistance for the following with this mosfet off the logic board:

source to drain
source to gate
gate to drain

Checking for low resistance which will indicate the part is likely defective.

4) With this mosfet off the logic board, meter in DC volts (20V should be ok for the scale).

What is the voltage on PQ5 pads for pins 1-2-3 ? pin 4 ?

Reference:
http://electronicsbeliever.com/how-t...-is-defective/

See the table on this website.

[ex250]

Re: Help with board LA-9872P short to earth

Hello, thanks for answer!

1) Oks, will use that one

2) Replaced with the same fresh MDU3603, maybe others could be better for the board
properly soldered, the layer on the board I think offers no doubt about position

3) The PQ5 shorted was removed a throw away to waste, not fitted any other again yet (just to not burn other) The Source to Drain reading was continuity and discarted it as failed, not tested source to gate and gate to drain... I could solder a new one and let it burn to check. EDIT, maybe I'm just too stupid and throw away 4 good ones... (shit)

4) with the PQ5 mosfet off and the power supply plugged, the pads 1,2,3 read is 19,57v and the pin 4 read is 9,29V, too high this one, not?, this not should be powered with the power button with 3.3v?

[ex250]

Re: Help with board LA-9872P short to earth

Some update about, I'm totally nerd.

I had desoldered all the coils, and remove the PJPs on board to try to asislate the circuit in short. Well seems the +VCC-CORE could be the problem, but seems that I cant find what is shorted. The PQ5 still heats up will all the PJPs jumps removed.

Also there are a set of 2 lines of capatitors that seem in short line PC905 PC908 PC915 PC916 and all the small ones near cpu socket, but I'm not sure where to inject some voltage/amps to see which ones heats up without do more damage.

Also I found that one of the bios eprom chips was in wrong position, reversed, just likely a previous try to repair by some one that could not want to any one try to repair the board. I removed both ones and put in the correct Pin 1 positions.

I don't know it this board could be repaired and maybe is better to go for a new / repaired functional one.

[ex250]

Re: Help with board LA-9872P short to earth

Any one has a .brd file of this board? reached a PU701 with short on pin 1,2,3 and a near cap shorted too (perhaps the 701)

Edit: removed PU701 and the supposed PC701 but short still on board pads.

Added schematic of the board

[mon2]

Re: Help with board LA-9872P short to earth

Have the boardview file but is tied to a paid sub.

See attached to see if it helps.

[ex250]

Re: Help with board LA-9872P short to earth

Have the boardview file but is tied to a paid sub.

See attached to see if it helps.

Many thanks, will check now.

[ex250]

Re: Help with board LA-9872P short to earth

Well, the PU701 pads are all shorted, maybe a noob question, the PQ701 source pins are earth, not?

The PR701 should give continuity? it’s earthed too on both sides

I'm hitting the jackpot, any idea where to continue?

[ex250]

Re: Help with board LA-9872P short to earth

Some update, found the PC535 (10uF 805) and PC529 (100uF CEEEFK1E101YP Panasonic) shorted, will change all the row of de 100uF capacitors for sure.

Almost one of the shorts is found, fingers crossed.

[ex250]

Re: Help with board LA-9872P short to earth

Found also the PC908 a 330uf in short.

Seems that all shorts are found. Now was trying to check if the mosfets 87350D, removed all from the board, any easy way to check if they are on spec?

[Tjp444]

Re: Help with board LA-9872P short to earth

Hi, just curious if you had any luck with this repair? I have the same board and same problem. Thanks!