Acer 5755g motherboard / CPU issue.

[Dantestcr]

Hello everyone.
I have an acer 5755g that was working fine up until yesterday morning. I tried to boot it, and a strange noise came from the speaker. Like a single beep, but distorted ( removing both ram's doesn't make any sound) . Everytime I tried to boot it, the same thing happened. I dissasembled everything. Now there's only cpu , cpu fan, internal gpu and the nvidia one.
Before that problem occured , if I tried to boot the laptop like this, it tries to POST ( fan starts spining and stops after 2 seconds cause there is no ram ).
Booting the computer results after the problem occured : Front power led lights up and stays lit, fan is not spinning. No signs of POST.
Tried it only with the battery, same.
Another PSU same volt less amper, same.
Reset CMOS, same.
It's obvious I have some sort of short in the MB or the cpu is failing.
Now let's go to the weird thing.
Sometimes, it tried to POST. From yesterday, it happened like 10 times. I immediately put ram and hdd, and it starts booting. Next time I shut it down, I am on square zero. I try to boot and nothing happens.
My motherboard's schematics : Link

Any help appreciated. Should I start searching for some kind of short, or... ?
Thanks.

[dellxps15]

Re: Acer 5755g motherboard / CPU issue.

no fan spinning = cpu problem. check if 5volt at cpu ic are present. they are 2 mosfet neat mouse pad botton on board. one is for 3.3 volt one is for 5 volt. one side is 19volt and the others are 3.3 or 5 volt.i had that problem on a similar board and was a resistor.

[Dantestcr]

Re: Acer 5755g motherboard / CPU issue.

Sorry but I am kind of new when it comes to the motherboard.
Are these , PL20 and PL21? Or if you can link me a pictured to recognize so I can recognize them by eye?

[dj_ricoh]

Re: Acer 5755g motherboard / CPU issue.

mosfet are marked eder with "pq" "q" that is a transistor, eder a supply with U.

i`m guessing you`re looking for U

[dellxps15]

Re: Acer 5755g motherboard / CPU issue.

check pu12 pin 23 for vin and check if 5vs are present on u22

u22 voltage must be 5volt pin 1-2-3-5-6-7-8 and 19volt pin 4 gate.

[Dantestcr]

Re: Acer 5755g motherboard / CPU issue.

PU12 pin 23 recieves 18.82 volts.
U22 Pins 1-2-3 recieve 2.30 volts.
Pin 4 recieves 0 volts.
Pins 5-6-7-8 recieve 5 volts.

[dellxps15]

Re: Acer 5755g motherboard / CPU issue.

u22 ... there are 2 resistor on the right side. check if first from left r372 has 19volt up and see if it has 19volt down. if not change it . 20k resistor

[Dantestcr]

Re: Acer 5755g motherboard / CPU issue.

If i checked the correct, 18.67 recieving upper side, 0 output down side.
You are a big help friend.
Could something else be broken too?

Also , is the resistor completely broken? Cause the laptop tries to POST sometimes.. maybe the resistor doesn't make a solid contact or something?

Thanks for your help and the quick replies.

[dellxps15]

Re: Acer 5755g motherboard / CPU issue.

check if resistor is 20k or is dead.

[Dantestcr]

Re: Acer 5755g motherboard / CPU issue.

I measured the resistance is 0. Today volts are 2 recieving , 0 output.