Toshiba P855 and P850 LA-8391p AC adapter goes into protection

[zioricci]

Hi everyone,

I had two identical motherboards, LA-8391p coming from a Toshiba P855 and P850 laptop

one has been repaired and i would like to do the same with the other one that has a power problem.


Schematic: https://www.badcaps.net/forum/showth...914#post558914

The problem: without the battery inserted, if i connect the AC adapter, the power LED lights up white normally, while if i connect the adapter with the battery already inserted, the power LED does not light up and the adapter supplies 0v, perhaps it goes into protection (?), but if for a few seconds i unplug it from the wall socket and plug it back in, then it supplies me with 19v normally.

I would like to understand if this behavior can be caused by the short between drain and source on the first mosfet PQ203 measured with the multimeter, while the second mosfet is ok.

Thx !

[mon2]

Re: Power problem

The linked schematic URL is broken.

See the above sticky by @Piernov. Often, it is the 2nd DCin mosfet that gets nuked but best to check both.

You can check their resistance readings:

(remove all power / no battery)

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. Repeat for both, the first and 2nd DCin mosfet.

Review the charger IC onboard -> is it the TI BQ series? If yes, review the voltage of REGN, ACDRV, ACOK, ACDET.

For the DCin mosfets to enable, the gate voltage must be ~25 volts. A shorted mosfet can tank the ACDRV voltage.

[zioricci]

Re: AC adapter goes into protection

Near the mosfet (1 cm) there is the small power connector which is made of plastic, i covered it with kapton tapei but i don't know if it is enough to keep it from melting.

Edit:

It worries me to leave the adapter connected together with the battery for the time of the measurements, i don't want some other component to get damaged.

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Disconnect the battery and use only the power adapter. In this case, the ACDRV should be ~25v which will be used by the gate pins of the 2 x DCin mosfets to enable both to power the logic board.

Respectively, you can still check if the mosfets are damaged or not by removing all power and checking the resistance between the mosfet pins as posted above.

A low resistance across the mosfet pins will confirm that the component is defective.

For resistance checks, no power must be applied - no adapter / no battery and is completely safe to process.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

I wanted to use this MB as a spare, and at the moment the CPU is missing and also the PR221 (ACDRV) and PR222 (CMSRC) resistors that i had put in the other MB and also the PL151 ferrite is missing. In these conditions,

Mosfets 1 - PQ203

Source - Drain 0Ω
Source - Gate 46Ω
Gate - Drain 46Ω

Mosfets 2 - PQ204

Source - Drain open
Source - Gate 46Ω
Gate - Drain open


REGN 6.04 v
ACDRV 0.32V
ACOK 3.38V
ACDET 2.69V

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Remove the first mosfet and then check the mosfet resistance again. At least one of the mosfets in the power path is defective.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

As said, the first mosfet is very close to the plastic power connector and i'm afraid it doesn't resist heat, i covered it with kapton tape but i don't know if it's enough to protect it.

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Locate if you have a metal plate which works well to protect such parts. Try aluminum foil as well.

Kapton tape, real or fake brands are useless here.

Practice on some other donor parts to check if the part can remain shielded.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

I put some aluminum foil, perfect and finally the first mosfet off the board, it is shorted, while no short on the pcb.

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Excellent. Proceed to replace this mosfet which likely was making the 2nd mosfet look bad. After soldering in the fresh mosfet, do check the resistance across the mosfet pins again. If necessary, remove and test the 2nd mosfet.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Without the first mosfet, if i connect only the adapter i could see 25 V at least on the gate of the second mosfet, ACDRV pin ? Can i check if the battery is charging ?

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

This is a good start for sure. The charger IC is powered by the raw voltage from the dcin connector so the logic board, that is, even the charging mosfets do not yet have power to charge the battery. This is because the first mosfet is missing. Ideally you need to replace the first mosfet.

Alternatively, remove all power and confirm the resistance across the only mosfet in the power path which is the 2nd mosfet. If the resistance is ok then at your discretion (ie. risk), you can bridge the source and drain pins on the PCB pads to bypass the first mosfet for testing. Again, we will get flamed for this suggestion as every part serves a purpose else the bean counters at the factory would have left this part off the board.

Aside from the above warning, it should be ok to bypass the first mosfet for testing but a better suggestion is to use a current limited power supply, set to say 3A and 19v. Do confirm the ratings of your power adapter before proceeding. This will be on your power adapter label.

PS: you will need to connect the 4k resistor on ACDRV back if it was removed.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

I had already seen bypassing the first mosfet as you said, what's the risk ? Not the same thing as if it was shorted ?

The 4k resistors that I had removed, on the one hand have lost their tinning, i guess I can put two of the standard non-smd type.

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

The regular leaded resistors will be ok but suggest smd since they will not tear up the pcb traces or pads. Do not flex the leads on the leaded part else the pcb pad will lift.

Lots of flux and smd part with a single strand of flexible wire will work. The gate pin is low current stuff.

The mosfets are like solid state relays but protecting against reverse feed of the power rail. If the polarity of the power adapter did occur, the mosfets would block it.

Just be careful and proceed.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Ok, better smd but i'll have to wait because in addition to the 2 4.12k resistors, i also need a ferrite bead HCB1608KF-121T30_0603 but i would like to buy them both in the same shop for a maximum of 10 units, to avoid disproportionate shipping costs.

For the moment i have looked on Farnell, Mouser, eBay and others but either they have one or the other, on Aliexpress yes but if I'm not mistaken large quantities like 1000 units.

By the way, will the 4.12k be enough at 1/4W ?

[mon2]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

Yes, it will be fine. Not at my desk but do review if the size is 0402 or 0603 and buy the correct size. Guessing it will be 0402 smd size.

[zioricci]

Re: Toshiba P855 and P850 LA-8391p AC adapter goes into protection

0603