Toshiba A300-211 completely dead no lights at all

[BlueMidnight]

Re: Toshiba A300-211 completely dead no lights at all

Those readings look good. Unfortunately :/

Some other possible causes off the top of my head:
-A MOSFET is bad but just not bad enough to be detected by a simple multimeter test
-Bad feedback resistors
-Broken trace (no continuity) between U23 and the feedback resistors
-Bad R278
-No continuity from U23 pin 20 to the MOSFETs and L20
-No continuity through L20
-Output capacitor has lost too much capacitance (i.e. C260 should be near 330uF)
-U23 itself is bad (Actually, since you didn't find anything catastrophically wrong with the MOSFETs, it's safe to try replacing U23 if you have another one)

[Andreasbest]

Re: Toshiba A300-211 completely dead no lights at all

Those readings look good. Unfortunately :/

Some other possible causes off the top of my head:
-A MOSFET is bad but just not bad enough to be detected by a simple multimeter test
-Bad feedback resistors
-Broken trace (no continuity) between U23 and the feedback resistors
-Bad R278
-No continuity from U23 pin 20 to the MOSFETs and L20
-No continuity through L20
-Output capacitor has lost too much capacitance (i.e. C260 should be near 330uF)
-U23 itself is bad (Actually, since you didn't find anything catastrophically wrong with the MOSFETs, it's safe to try replacing U23 if you have another one)

I'm gonna test them all in a few hours that i'll return home.
The Q29 that gives only 0.133v in Red on Source, Black on Drain it's ok?

[BlueMidnight]

Re: Toshiba A300-211 completely dead no lights at all

Yeah, Q29 sounds okay to me. It's actually kind of a special MOSFET. It's a different part number than the other one and it contains a Schottky diode. Schottky diodes have a lower voltage drop than regular diodes. Because of how a buck converter works, some power is lost due to the body diode of the "lower" MOSFET (Q29 in this case) for a split second during every cycle. Adding a Schottky diode, either a separate one or one inside the same package as the MOSFET, improves the efficiency of the circuit.

This will make much more sense after reading some buck converter tutorials though.

[Andreasbest]

Re: Toshiba A300-211 completely dead no lights at all

The C260 is 409.6uF.But being higher is not a problem.Right?

[BlueMidnight]

Re: Toshiba A300-211 completely dead no lights at all

Yes, a bit higher is fine.

[Andreasbest]

Re: Toshiba A300-211 completely dead no lights at all

Still haven't found the resistors on pcb, it's the last to check before the replacement of U23.
No markings on pcb.

[BlueMidnight]

Re: Toshiba A300-211 completely dead no lights at all

They are usually right next to the 3V/5V chip. Just put one probe on pin 2 and then touch both sides of all the resistors (resistors are usually black in color). You should find 2 resistors connected to that pin. Likewise, you should find 1 resistor connected to pin 1.

[highpulse]

Re: Toshiba A300-211 completely dead no lights at all

can you please chek both coils of the main regulator , to ground please ? put 200ohm scale on your multimeter , and chek both coil to ground and tell us wich values you got ! if it does display number 1 in both then increate multimeter for 2K and tell us exact values on each coil of the 3.3 v and 5 volts .
if you wont have linear and output from tps51125 , to things , short nio 3volts line , or you killed entrip circuit . its a not easy machine to fix somethimes , but first of all give us values for the following pins of the 51125 to GND and tell us values

1 6 7 8 13 17 and 24

and also chek if pin 16 is exactly 19 volts

[Andreasbest]

Re: Toshiba A300-211 completely dead no lights at all

Something weird happened, the laptop worked for a while (i even found the 3.3v at L24) but after about 10 minutes of testing (power off and on) it died suddenly again. Is that a clue?

[Andreasbest]

Re: Toshiba A300-211 completely dead no lights at all

can you please chek both coils of the main regulator , to ground please ? put 200ohm scale on your multimeter , and chek both coil to ground and tell us wich values you got ! if it does display number 1 in both then increate multimeter for 2K and tell us exact values on each coil of the 3.3 v and 5 volts .
if you wont have linear and output from tps51125 , to things , short nio 3volts line , or you killed entrip circuit . its a not easy machine to fix somethimes , but first of all give us values for the following pins of the 51125 to GND and tell us values

1 6 7 8 13 17 and 24

and also chek if pin 16 is exactly 19 volts

The l801 gives 600Ω to ground in both pins.What other coil you mean to measure?
Also i measure the Ω between the pins you said and ground.
1= 7.6κΩ
6= 103κΩ
7= 12ΜΩ
8= 46κΩ
13= 103.6κΩ
17= 40κΩ
24= 150κΩ

[BlueMidnight]

Re: Toshiba A300-211 completely dead no lights at all

If it's still acting the way it was before (i.e. the 3V output doesn't work unless you remove Q34 to disable the 5V output ), then the problem still must be in the area from U23 to PAD8. And now we know it is an intermittent fault. The most likely causes for something intermittent are a broken trace or a bad solder joint. Unfortunately, a component with a strange issue can't be completely ruled out either.

You can keep hunting and measuring all the components and traces around there. But it's possible it's only failing when it's got power going through it. In that case, you might just drive yourself crazy trying to find it. Another possible alternative is to just start replacing things (e.g. the two MOSFETs, U23, the capacitors, and so on), but you only need to worry about U23 and the components connected to that 5V rail in the area between U23 and PAD8.

[Taruga]

Re: Toshiba A300-211 completely dead no lights at all

I got one today, completely dead no lights at all and the problem was a faulty C260.
I didn't have any voltage on L20 and L24