ACER 5742 Dead M/Board

[John McGivern]

Replaced Pq51 desoldered pj18. Used power supply 18.5v 1amp. Turned on 18.5v on pq51 drain/gate /source also on pq52 source then nothing on drain of pq52. Moved power supply to drain of pq52 injected power as above sparks at pq52 drain power supply drawing 550 mAmps, put alcohol on board no obvious heating of components, turned off PS. What is the best way to devide and conquer this motherboard?

[John McGivern]

Correction post21 should be soldered pj18, apologies. Cheers.

[techtiger]

Originally Posted by John McGivern

Correction post21 should be soldered pj18, apologies. Cheers.

Yep PJ18 must be soldered

[techtiger]

Originally Posted by John McGivern

Replaced Pq51 desoldered pj18. Used power supply 18.5v 1amp. Turned on 18.5v on pq51 drain/gate /source also on pq52 source then nothing on drain of pq52. Moved power supply to drain of pq52 injected power as above sparks at pq52 drain power supply drawing 550 mAmps, put alcohol on board no obvious heating of components, turned off PS. What is the best way to devide and conquer this motherboard?

If you get sparks their is a short, test the PQ52 without power help of the multimeter Drain source Gate for resistance

[John McGivern]

gate to source 2.109v red lead gate/ black lead source. O/L when leads swapped.
gate to drain O/L both ways.
source to drain 0.514v black lead on source red lead on drain. O/L when leads swapped.

[techtiger]

Originally Posted by John McGivern

gate to source 2.109v red lead gate/ black lead source. O/L when leads swapped.
gate to drain O/L both ways.
source to drain 0.514v black lead on source red lead on drain. O/L when leads swapped.

that fet is ok chek the voltage of PU7 pin 19 20 24 and find out the voltage of pic

[John McGivern]

19v on adapter

PU18. Pin voltages.

1= 0V. 8= 18.33v
2= 3.3v. 7= 124mV
3= 0v. 6= 2.886v
4= 0v. 5= 2.886v
No current draw.
Difficult to view picture unable to zoom in.

[John McGivern]

Can anyone let me know what to expect if I measure diode mode at all coils what is good reading and what is bad, many thanks.

[mcplslg123]

Vcore coils will show low ratings-might even beep in diode mode but will not show absolute zero if vcore circuit is not shorted. Rest of the coils should show high values. Say a rating of 100 in diode mode on 3v coil is wrong. However, will suggest to measure in resistance mode.

[John McGivern]

techtiger

pu17
pins 19,20= 403mV
pin 24= 0v

PD1 = 0v anode and cathode.

mcpisig123
thanks for diode measurements I thought I may be able to narrow down fault area with diode measurements thats why I asked the question.

[John McGivern]

Some voltages.
PQ58 base = 13v
PQ59 drain = 13v
PQ63 source = 18.2v
PQ56 collector/emitter = 18.54v

[techtiger]

Originally Posted by John McGivern

Some voltages.
PQ58 base = 13v
PQ59 drain = 13v
PQ63 source = 18.2v
PQ56 collector/emitter = 18.54v

this is how it works DCIN & charging circuit diagram look at the pic

[John McGivern]

techtiger, sorry for delay in replying to your circuit. I have marked voltages on attached drawings please advise how to proceed, many thanks.

[techtiger]

Check DCIN PU17 pin 24 Resistance to GND
PR351 both side Resistance to GND
PQ53 in DIODE mode S/D/G in DIODE mode
Apply 1V increase amps to PR351 and see what is getting hot near PU17 PQ53 PQ46
Do a continuity test DCIN PU17 pin 24 to PQ51 DRAIN
Voltage PR297 both side Voltage 296 both side

[techtiger]

https://www.youtube.com/watch?v=AOALg_rDIZk&t=4s
check this out

[John McGivern]

PU17 PIN24 to GND = 3.72Mohms.
PU17 PIN24 to PQ51 Drain = O/L.
Pr351 both sides Ohms to Gnd = 25.7.
PQ53 diode test good.
Applied power to board sprayed with freeze spray. PQ38 got hot Si7686DP. (S08).
Voltage test on Pr296/7 could not identify for sure due to circuit configuration.
Tried to find ohm values but not that good with parallel circuits.
Applied 1.4 volts to board, on 2 resistors near PU18 and PD1 only one side of each resistor had 1.4v other side 0v all other resistors nearby had 0v both sides, many thanks.

[John McGivern]

Hi all, can anyone suggest an alternative MOSFET for SI 7686DP? Many thanks in advance.

[John McGivern]

Waiting for Si7686DP from China will be a while.
Tried to desolder above chip from board could not get enough heat into board.
Can anyone advise method as there is a very large area of copper to heat around the chip and what is best temperature for preheat? Many thanks.

[John McGivern]

Same message as above how to desolder this chip. Best preheat temp, tried hot air not enough heat. Has anyone tried heating the chip through main body of chip with an iron as it is shorted anyway. I will still have the problem soldering new chip when it arrives please advise, John.

[John McGivern]

Sorry for delay guys, been awaiting part from China. I removed cpu heat sink and cpu also memory. After great difficulty removed shorted chip I am now ready to test for short again but would like some advice as I don't want to short anything else out if possible. Should I reinstall memory and cpu + heat sink? What voltage and current should I apply? I haven't soldered in the new chip yet as I want to see if short is gone. I notice a sort of plastic gasket that surrounds cpu has curled up and hardened with preheat to board, does anyone know what this material is and where to buy a new one or can it be left out? Many thanks.