Working on a dell LA-9104P Motherboard

[NickyD-]

I have attached a picture of the schematic i am looking at.

this motherboard when turn it on the power led lights up for .5-1sec and turns off.
while i was checking the 19.5 volt ac adapter power rail i notice on the two p channel mosfets PQ100 and PQ101 there are both p channel mosfets, and from my understanding when a p channel mosfet gate has voltage the mosfet should be off and turns on when the gate is grounded.

when i check the gate voltage for both of these mosfets the gate voltage is 19.5 , which i would assume turns them off not allowing current to flow. but when check the source and drain they both have 19.5 volts. am i missing something???

[piernov]

Re: Working on a dell LA-9104P Motherboard

Are you sure you are checking the correct pins? If PQ101 has exact same voltage on drain/source/gate it's probably shorted.

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

appreciate the response. i solder on two new mosfets. pq101 is acting as i suspect it to. pq100 has 19.5 volts on the gate and still has 19.5 volts on both source and drain.
in pic circled in green... pq103 i measured pin 3 at at 19.55v, pin 2 at 0.035v, pin 1 at 0v
pq104 pin 1 0.035v, pin 2 5.27v, pin 3 0v

circled in purple pq114 pin 6 5.27, pin 2 0v, pin 1 0v, pin 3 19.55, pin 5 0v, pin 4 0v

[piernov]

Re: Working on a dell LA-9104P Motherboard

PQ103 pin 1 is connected to PQ100's gate so something is not consistent there.

[Ikung]

Re: Working on a dell LA-9104P Motherboard

I have attached a picture of the schematic i am looking at.

this motherboard when turn it on the power led lights up for .5-1sec and turns off.
while i was checking the 19.5 volt ac adapter power rail i notice on the two p channel mosfets PQ100 and PQ101 there are both p channel mosfets, and from my understanding when a p channel mosfet gate has voltage the mosfet should be off and turns on when the gate is grounded.

when i check the gate voltage for both of these mosfets the gate voltage is 19.5 , which i would assume turns them off not allowing current to flow. but when check the source and drain they both have 19.5 volts. am i missing something???

Voltage on gates PQ100 and PQ101 should be around 8.36 V. Make sure PQ114 turns on and ACIN in high level (should be around 3 V).

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

just for the heck of it i removed transistor pq103 from the board and then checked the gate voltage again, they were still around 19v .
then i removed pq114 and check the voltage again and it was still 19v at the gates. and where pq113 is marked on the board there is no mosfet there just blank pads. i there is no voltage at the current sensor. do you think power can somehow be leaking in from what ever p2 goes to.

if it helps the schematic i am looking at can be found here:

https://cdn.badcaps-static.com/pdfs/...8f64d7dba6.pdf

[Ikung]

Re: Working on a dell LA-9104P Motherboard

do you think power can somehow be leaking in from what ever p2 goes to.

if it helps the schematic i am looking at can be found here:

https://cdn.badcaps-static.com/pdfs/...8f64d7dba6.pdf

Normally, 19.5 V goes through PQ100 because of the body diode (even if it's turned off). Then the resistors PR101 and PR105 (a resistor divider) cause the voltage at the gates of PQ100/PQ101 to be 8.36 V (with PQ114 turns on). That turns them both on (PQ100 & PQ101).

Better to do as much measuring as possible. Measure all pins on PU100.

[piernov]

Re: Working on a dell LA-9104P Motherboard

If you randomly remove parts without even measuring at least some voltage&resistance and reporting them here we can't help you.

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

I follow. My intention was to see voltage dropped off, but after reading last couple post, I now understand why not to do that.

I didnt realize power always flowed thru p0100 due to the body diode.

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Ok I'm measuring pu100 and on pin 26.. isnt the power input for the chip in order to work at all. Ac in is 2.62v.
ACOK is 0v and icout is .7v. Isnt icout also called vcc and is the power input for the chip to work

[Techtiger]

Re: Working on a dell LA-9104P Motherboard

dont remove randomly parts if you need any help as piernov instructed

If you randomly remove parts without even measuring at least some voltage&resistance and reporting them here we can't help you.

Ok I'm measuring pu100 and on pin 26.. isnt the power input for the chip in order to work at all. Ac in is 2.62v.
ACOK is 0v and icout is .7v. Isnt icout also called vcc and is the power input for the chip to work

check the resistance to GND on PR102 make sure not short SGD on 1st 2 Fets ( check resistance 1st 2 Fets PQ100/101 ) whats is the voltage on PU100 pin 22 26? voltage on PR139 ? do you have a DC POWER LAB ?

[Ikung]

Re: Working on a dell LA-9104P Motherboard

Ok I'm measuring pu100 and on pin 26.. isnt the power input for the chip in order to work at all. Ac in is 2.62v.
ACOK is 0v and icout is .7v. Isnt icout also called vcc and is the power input for the chip to work

Can you measure all pins on PU100?

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Pin 22: 19.55volts
Pin 26: .177 volts
Pin 2: 2.62 volts
Pin 4: .140 volts
Pin 13: .004 volts
Pin 27: .106 volts
Pin 28: .106 volts

All other pins not mentioned were 0v


I measured voltage on the gates of pq114 and didnt have any voltage and when measuring resistance from pin 2 of p0100 to the gates of pq114 its OL. If I'm not mistaken I would of expected to see 200k ohms due to pr139.

Pr139 isnt marked on the board either

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Appreciate the assistance. Just really started digging into board repair to try and learn

[Ikung]

Re: Working on a dell LA-9104P Motherboard

Pin 22: 19.55volts
Pin 26: .177 volts
Pin 2: 2.62 volts
Pin 4: .140 volts
Pin 13: .004 volts
Pin 27: .106 volts
Pin 28: .106 volts

All other pins not mentioned were 0v

Something not right here. Can you turn on the laptop with charged battery (without adapter) only? Check both resistors value on PR116 and PR232.

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Measuring resistance I found short to ground on pr121 current sensor. All three caps pc129 pc129 pc129 etc

When I test the battery pins on the board positive and negative terminals are shorted togather. Working on that path now

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Tell me if bad Idea. But something is shorting would pluggingac adapter in and spraying 91% isopropyl alcohol do damage

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

Pc129, Pc130, pc139 were the caps I measured. My bad auto correct.

But looks like everything on the circuit connecting to battery positive is shorted

[lapfix]

Re: Working on a dell LA-9104P Motherboard

desolder PL100 and with diode mode test the impedance on the two sides ,side 1 from PU100 ,and side from pr121

[NickyD-]

Re: Working on a dell LA-9104P Motherboard

PL100 removed
Resistance measurement: 0.2

In diode mode measurement: 0.0000 volts