Dell Latitude 3550 not powered on

[usyusy]

Del latitude 3550 (LA-B071P) not powered on,
no shortage on any gray coil
Some measurements are :

1st and 2nd mosfet 20.4v
CLR 20.4v
(BQ24777)
Acdrv 25.1v
Acdet 3.14
Acok 3.1
SDA 0
SCL 0
B+ 0
so no voltage on 3v/5v ic
(also when board came no step down ic was there (TPS51285B) so temporarily used TPS51225 (only have 3v ldo always-on instead of required 3&5v Ldo always-on)
As per schematic step-down ic got B+ from charging ic, so it means charging ic faulty? It's upper and lower mos changed also but same result)
Any guidance please

[mon2]

Re: Dell Latitude 3550 not powered on

Remove all power -> meter in resistance mode.

Measure the resistance to ground on PL100, pin # 1 (+3VALWP). Reading ?

Measure the resistance to ground on PL101, pin # 1 (+5VALWP). Reading ?

The mosfets mated with the charger IC need to be reviewed. While they are passing through the voltage to the logic board, you may have defective mosfets.

Start with, remove all power / no battery.

Meter in resistance mode.

Measure the resistance of each of the 2 x dcin mosfets (PQ701 & PQ702) as follows:

Source (1-2-3) & drain (5-6-7-8)
Source (1-2-3) & gate (4)
Gate (4) & drain (5-6-7-8)

Post each measurement and the meter scale used.

Repeat for the next mosfet.

[usyusy]

Re: Dell Latitude 3550 not powered on

Resistance on pl100 (2k settings) 358
On pl101 384

Pq701 1st mos, S-D. 377
Pq702 2nd mos S-D 428
Could not get any value on S-G and G-D

[mon2]

Re: Dell Latitude 3550 not powered on

Can you confirm the measurements?

Meter in 2k scale. Remove all power to the logic board.

Meter lead to the PL100 inductor; other meter lead to ground.

Then you are reading 358 ohms ? Maybe share a pic of the meter? That is low.

Same process and question for the next power rail.

What is the resistance scale for the mosfet testing?

The readings are low if on 2k scale. Share a pic again to confirm the settings.

No power when checking resistance.

[mcplslg123]

Re: Dell Latitude 3550 not powered on

I'm afraid if the mb was powered on without 3/5V PWM ic in place, there could be multiple damages on the mb.

[usyusy]

Re: Dell Latitude 3550 not powered on

I'm afraid if the mb was powered on without 3/5V PWM ic in place, there could be multiple damages on the mb.

this was done by some professional engineer, yes sir, but i checked for shortage no coil showed shortage, 3v line not shorted (checked on bios pin 8)

also this board using NVDC charging configuration, if m right i.e. B+ is the output of charging IC. but no B+ i saw there, 0v.