No power new70 5891p motherboard acer 5742

[John McGivern]

Hi any help would be appreciated. This laptop shows no power on Leds but there is 19v at adapter and first connector on PCs motherboard. Could anyone direct me where to next check for voltages and or signals.
Many thx in advance, John

[selldoor]

Re: No power new70 5891p motherboard acer 5742

Where does the "NEW" bit come in - is it still under guarantee?

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi, Selldoor thanks for replying.
The new bit is part of the motherboard serial number it is out of warranty.

[selldoor]

Re: No power new70 5891p motherboard acer 5742

Will need some laptop wizards to help- does it work on battery, can you post pictures of the area around the socket - can you spot any fuses?

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Yes, there are 2 fuses both ok nothing appears burn or discoloured will post pics tomorrow.
Unfortunately battery flat as no power on laptop to charge it.
Cheers, John.

[lezsimply]

Re: No power new70 5891p motherboard acer 5742

you can try this schematic. read page 44 dcin (pjp1) thru page 47 VIN/charger. hope this helps

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi lessimply, thanks for replying.
Took some random voltages as I am not an expert just learning will try to attach photos.
Voltages at components on schematic.
PD2 = 19VDC
PL24 = 19VDC
PD1 = 0VDC

MOSFET
PQ51 = DRAIN = 19VDC
SOURCE = 0VDC
GATE = 0VDC
MOSFET
PQ52 = DRAIN/SOURCE/GATE ALL = 0VDC.







Thanks lads, John.

[lezsimply]

Re: No power new70 5891p motherboard acer 5742

If pq51 pin number 4 (gate) is 0v, try checking pq56 and pq58 which gives voltage to gate. You can also check pc249 for short and the resistance of pr353

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi again,

PQ56 Base = 0vdc
Emitter = 18.6vdc
Collector = 18.6vdc

PQ58 Base = 13.13vdc
Emitter = 0vdc
Collector = 0vdc

PC249 not marked on PCB cannot find any capacitor 25volts.
PR353 also not marked on PCB.
Hope this helps.
Cheers, John.

[whaldsz]

Re: No power new70 5891p motherboard acer 5742

Check pin 1,2 and 3 of PU18A.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi whaldsz, thanks for your input.

PU18 is a LM393dg soic

Pin1 = 0vdc
Pin2 = 3.31vdc
Pin3 = 0vdc
Best regards, John.

[whaldsz]

Re: No power new70 5891p motherboard acer 5742

pin 1 is abnormal, it should be higher than 3.31vdc.

1. Check PR297 and PR298, they could be open.
2. Check PC211, it could be shorted.
3. Check the resistance of pin 1 to ground of PU18 - could be shorted. Replace if needed.

[spidey]

Re: No power new70 5891p motherboard acer 5742

didnt read the whole posts but change lm393, usually burns on this board.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi Lads, hard to identify resistors and cap as they are so small.
Pin 1 = 9.7Kohmns to gnd.
Sent for LM393DG will be a few days before it arrives.
Added another photo. PU18
Regards, John.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi lads, sorry for taking so long was waiting for parts and solder paste.
Replaced Lm393 regulator.
Pin1 = 0v
Pin2 = 3.3
Pin3 = 0
Pin4 = 0
Pin5 = 2.8
Pin6 = 2.8
Pin7 = 0
Pin8 = 18.4

[whaldsz]

Re: No power new70 5891p motherboard acer 5742

Hi lads, sorry for taking so long was waiting for parts and solder paste.
Replaced Lm393 regulator.
Pin1 = 0v
Pin2 = 3.3
Pin3 = 0
Pin4 = 0
Pin5 = 2.8
Pin6 = 2.8
Pin7 = 0
Pin8 = 18.4

Check why pin 3 is 0V on PU18A:

1. Is PR298 open? it should read 22k ohms
2. Is PR297 open? it should read 84.5k ohms.
3. Is PR301 shorted? it should read 20k ohms.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi whaldsz, thanks for your help.
Nothing is shorted, resistances are shown on attachment lm393 reg.
Many thanks, John.

[whaldsz]

Re: No power new70 5891p motherboard acer 5742

Pin 3 of PU18A is connected to PR298. You said pin 3 reads 0v and PR298 measures 49k which is not normal. When tested in circuit this resistor should measure no more than 22k ohms. Measure the voltage at both ends of PR298. You should expect a voltage readings of 0v at one end and around 3.6v at the other end.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi whaldz, took voltages at resistors shown on attachment.

[John McGivern]

Re: No power new70 5891p motherboard acer 5742

Hi again lads, was measuring voltages at
Pq51 gate source and drain between 19vdc to 18.6vdc
Pq52 had gate 18.6 pins 1,2,3 18.6
Pins 5,6,7,8. 360mV.
Does this need replacing.
Cheers