Dell 1537 shorting power supply

[spleenharvester]

Got a Dell 1537 motherboard here. Minor water damage and corrosion to the PCMCIA controller, on the opposite side of the board to the southbridge, and a small power area. Liquid appeared to be coffee of some sort.

Gave it a good scrub with IPA.

Described fault was jack lights up, but doesn't power.

It did initially light up and the fault appeared to be near the small power area where the damage was.

But coming back to it today, it is completely dead, and the short-circuit protection kicks in on my Dell PA-10 every time it's plugged in (works fine in another laptop).

The jack's resistance does not test as short circuited until the supply is plugged in.

Tested a few diodes on the board but nothing has come back yet.

Schematic here: http://www.mediafire.com/view/?vdtrc28t628duzi (just click the save button at the top right, the built in viewer is crap)

For the lazier, this is the area around the DC jack. I don't currently have a working DMM but is it worth replacing PQ25?



Any help would be appreciated!

Cheers

[momaka]

Re: Dell 1537 shorting power supply

If the + and - on the input jack do not appear to be shorted, then PQ25 is probably fine and the problem is likely somewhere after it (possibly on the DC_IN_SS line). You will need a multimeter for that, though.

[spidey]

Re: Dell 1537 shorting power supply

i'm agreed with momaka. this is the vin detector circuit. go to charging circuit, at the vin side there are 2 sop8 mosfet connected together. This is the power supply circuit and drived by protection circuit. Adjust your power supply to 200ma and 3v then short 2nd fet's S and D. then check the component which is hot.if cant find slowly raise voltage and currrent while sensing. most probably you will find a capacitor shorted.

[siddiq]

Re: Dell 1537 shorting power supply

Yes possibly a capacitor.

[spleenharvester]

Re: Dell 1537 shorting power supply

Done some testing, the multi does work but the leads are damaged, so it's too risky to use for anything other than resistance testing.

The resistance of most of the components surrounding the charge IC (MAX8731A) all tests as fine. But a huge amount of components closer to the jack are testing as dead short (1ohm, same resistance as the leads). So clearly something is making all the components test as shorted.

All components marked PL test shorted. PC88 and PC93 test shorted. What both of these two components have in common? They are both connected to +CPU_PWR_SRC. Am I right in thinking the problem is on this rail?

More schematic screenshots.



@spidey - am I right in thinking the 2 sop8 mosfets you speak of are PQ24 and PQ23?:



I have no variable power supply to test with though, I wonder if I could build one using a potentiometer and a 5v 500ma supply I have lying about.

[spleenharvester]

Re: Dell 1537 shorting power supply

+CPU_PWR_SRC is generated by ISL6262A. There is one cap in between it and where it meets the other components. Could removing this cap prove the fault is on that line?

Also, I remember while testing some caps around the MAX17020ETJ before it started shorting the power supply, there was a really high pitched brief squeal noise. Could this be related?

EDIT: Hang on, is a cap that tests as same resistance as the leads shorted, or one that tests infinity?

Honestly I think I might just start removing components and seeing if the board still shorts, it's probably faster.

Also, upon plugging in the power supply, voltage in +DCIN rises to 1.3v then falls. The power in PSID is 0v.

[momaka]

Re: Dell 1537 shorting power supply

Let's start in order.

Check resistance to ground before PQ25 (i.e. check resistance between Drain and ground). If none, check resistance after PQ25 (i.e. between Source and ground).

Report your findings.

EDIT: Hang on, is a cap that tests as same resistance as the leads shorted, or one that tests infinity?

A good capacitor should appear open-circuit on a multimeter when tested out of circuit. In circuit, it depends where the capacitor is. If it's a filter cap for CPU/GPU/SB, it may appear shorted or with a low reading, but this is sometimes normal as CPU/GPU/SB all appear as low resistances to ground.