Compaq cq71 power or mosfet problem ?

[interzone]

Hi everyone,

I'm trying to repair a compaq cq71 (same motherboard than cq61) who does not power up.

I test all the mosfet, and found one that seems dead (not visually), but testing it with diode test and a mutimeter, give a reading of 20 instead of between 100 and 500 which is a good reading...

this mosfet is PQ26 on motherboard, it is a 4468 on my motherboard..

I saw that other people as problem in the same area of the motherboard, like in this post:
https://www.badcaps.net/forum/showth...ht=compaq+cq71

in this post the guy as problem with the mosfet just next to mine, the PQ24..
In the same post there is the board schematics, and the zone is on page 33
PQ24, PQ26 and PU8 wich are related to VCCP1.05V & +1.5V

I try all the other mosfet on the board, all seems fine, except this one...
so...
I looked at my dead motherboards lying around, fond some good 4468, test them before off course...and unsoldered mine, replace it..

The problem is, that once soldered, the 4468 is shortcut, and multimeter reading read 20 or 17...
I unsoldered it, test it , and the reading is something like 467,
I re-soldered it , and same problem...reading too low..around 20...

So....
Looking at the page 33 on schematics, (https://www.badcaps.net/forum/attach...4&d=1394625058)

from where can come the shortcut on the mosfet PQ26??

could it be PU8 which is dead?

Or where must I search to find the problem???

If anyone as any suggestion, it will be great...

Thanks everyone..

[rievax_60]

Re: Compaq cq71 power or mosfet problem ?

I have seen many times a low resistance reading between Gate and Source where it is caused by the driver stage of the control IC. In most cases there has been no fault. The low resistance reading changes to hundreds of K ohm on its own after a while.
It must be a capacitor staying charged up somewhere.

[interzone]

Re: Compaq cq71 power or mosfet problem ?

Well how can I have a good reading so ?

Does a long press to power button will discharge capacitors?

For me it look like there is a kind of shortcut on PU8
(page 33 of schematics)
maybe between pin 8 & 10...

dont know, I will try another 4468 today, just to see...

[interzone]

Re: Compaq cq71 power or mosfet problem ?

Well..
this morning felt lucky.

I unsoldered the bios chip, flash it,
resolder it, than.... laptop starts...

It looks like electric problem, but it was bios corruption..

So this thread is closed...

(anyway I still have strange reading on this mosfet...but probably cause of a capacitor as you said...)

thank you for your help.