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Old 05-11-2018, 08:32 PM   #21
shelby5041
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Default Re: Toshiba laptop C855 no power dead

love your photos that you lableled it will really help me ....i greatly appreciate it

photos help a lot
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Old 05-11-2018, 08:38 PM   #22
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Default Re: Toshiba laptop C855 no power dead

i can tell you from testing other day that when power plugged in i tested Q6011 and Q6010

and if my memory is correct
Q6011 i got 0v on all 8 pins
Q6010 i got 19v at pins 5678 and 0v at 1234

i will double check when i can
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Old 05-11-2018, 09:53 PM   #23
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Default Re: Toshiba laptop C855 no power dead

It looks like there is a pre-charge circuit on this board also, made up of R6015 and D6002. So there really should be voltage on the main power rail regardless of whether the input FETs turn on or not.

Given your confirmation just now of which exact FETs you had measured before, I'm inclined to change my instructions on what to check first...

Do confirm your readings of course, but then I suggest measuring between R6000 and ground. Do it with the power unplugged and with the meter in "diode mode". Place the red probe on ground and the black probe on R6000. In diode mode, readings will be in volts (or may even show no continuity at all).
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Old 05-12-2018, 05:25 AM   #24
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Default Re: Toshiba laptop C855 no power dead

hi blue

im going to do some testing in a few..ill get back with results

thanks a lot...your detailed help is very nice
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Old 05-12-2018, 05:56 AM   #25
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Default Re: Toshiba laptop C855 no power dead

ok here are results..i could not really test Q6000 or Q6001..too small my probe too big...would have to grind end down to do that...but i dont believe those are issues per doing my research i tried to run my probe as best i could along the pins and saw no voltage but not really sure

unplugged... using diode test R6000 as u stated 001 reading

plugged in.... pin 1 2 3 4 5 6 7 8
Q6010 0v 0 0 0 19 19 19 19
Q6011 0 0 0 0 0 0 0 0
Q6012 .41 .41 .41 .19 0 0 0 0
R6000 0v
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Old 05-12-2018, 05:58 AM   #26
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Default Re: Toshiba laptop C855 no power dead

sorry the chart i sent lined up as i was typing but not when i sent it...
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Old 05-12-2018, 06:02 AM   #27
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Default Re: Toshiba laptop C855 no power dead

i assumed you wanted laptop plugged in on the 2nd part of tests
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Old 05-12-2018, 06:45 AM   #28
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Default Re: Toshiba laptop C855 no power dead

You don't need to measure the last 2 FETs. What you've found out so far is really good so we won't need those. And by good I mean bad. There is indeed a really nice short to ground on the PVBAT rail.

But just for any FETs you measure in the future.. as long as pins 1-3 are connected together and 5-8 are connected together (you can usually clearly see they are because the trace on the board touches all of them), you don't need to be too careful with the probe. Just jam it in between a couple pins. The only one you have to be really careful about is pin 4. But since that one is on the corner, it's usually not too hard to get to without touching anything else. Just be aware that not all 8-pin components are FETs, so you have to make sure the pins really are connected together before trying to stick a probe in between them rather than on one at a time.

Do you have a bench power supply with a voltage and current control? If so, it is possible to inject a voltage on the PVBAT rail and then feel which component is shorted (it will get warm). Since it's a dead short, little voltage is required (even 1V will do), but the amps should be limited to probably no more than 1A to start with... and then increased a bit if there is a need. 12V from a regular PC power supply will also work if run through an automotive 12V DC 1A bulb to limit the current.

If you don't have a power supply to use, then the alternative is to start disconnecting parts of that rail until the short goes away. That way you can narrow down where the short is.

Fortunately, that board has solder jumpers connecting various parts of the board. Those are very easy to desolder. Just use some copper desoldering braid to make sure all the solder is removed and the connection is completely broken between the two sides of the jumper.

The solder jumpers on the PVBAT rail are:

PAD6000
PAD6110
PAD6210
PAD6310
PAD6610
PAD6710
PAD6760

I've circled a couple examples of solder jumpers in the attached picture. Those aren't on the list of ones you need to remove, but it lets you know what they look like. They aren't hard to spot.

After removing each jumper, measure from R6000 to ground in diode mode again. Once you get a higher reading, you know the short must be somewhere in the circuit you just removed the solder jumper for.
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File Type: jpg 3.jpg (193.9 KB, 18 views)
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Old 05-12-2018, 08:52 AM   #29
shelby5041
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Default Re: Toshiba laptop C855 no power dead

hi

wow you are a pretty smart guy....its all foreign to me so im sorry i prob dont have the ability to further troubleshooot

its hard to beleive someone knows that much

like i said from my research it seems a very common issue for this motherboard is simply 1 bad mosfet...once changed the issue is resolved

from what i have said and my testing dome per your advice could my issue be a bad mosfet or is mine more involved?

i really dont want to put much more time o effort into this as the laptop really isnt worth much of anything...its a good learning though but i really just want to see if this is fixable for cheap or just trash it

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Old 05-12-2018, 08:53 AM   #30
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Default Re: Toshiba laptop C855 no power dead

fyi.....lead free solder is a pain in the ass.........it does not work with solder wick using my hobby solder iron ...sorry to say
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Old 05-12-2018, 03:20 PM   #31
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Default Re: Toshiba laptop C855 no power dead

The little ceramic capacitors and the FETs are common failure points on basically any board. And there are often many components in parallel, so it's necessary to narrow down which exact one is at fault. If you do find a FET is the problem, then you will require hot air to remove and replace it properly.

It can be very helpful to melt some leaded solder onto joints first. This mixes with the lead-free stuff and lowers the melting point. In fact, adding fresh solder before removing the existing solder can help in many cases, regardless of the type of iron or solder used.
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Old 05-12-2018, 03:24 PM   #32
shelby5041
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Default Re: Toshiba laptop C855 no power dead

yes i know..i ordered some small ends for my heat gun so i can remove if needed....

and i have tried adding lead solder in past and i can not desolder lead free solder with my cheap soler iron..it just doesnt flow at all turns into a mess
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Old 05-12-2018, 03:26 PM   #33
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Default Re: Toshiba laptop C855 no power dead

i also am planning on ordering a cheap 2 in 1 didgital temp solder hot air station from eaby so i can control temp....i saw one for cheap like $45 shipped...it should help me in future a lot

but for now the ends i ordered will go on my large heat gun and should do the job...
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Old 05-12-2018, 03:27 PM   #34
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Default Re: Toshiba laptop C855 no power dead

i want to remove the 1 mosfet i think may be susp[ect and test out of circuit..it tests bad in circuit but could be false
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Old 05-12-2018, 04:45 PM   #35
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Default Re: Toshiba laptop C855 no power dead

If you are referring to Q6012, that is not the problem. Your most recent voltage measurements show a nice diode drop's worth of voltage difference between the source and drain.

But in any case, let's think about this logically.. You have a dead short between PVBAT and ground. The majority of the time there is only a single bad component, so let's assume there is indeed only one. You need to think about what single components are connected to both PVBAT and to ground (or a low resistance path to ground). Only a component connected to both could be at fault. You can replace Q6012 with a giant blob of solder and it still won't short PVBAT to ground. The problem must be elsewhere. And the procedure laid out above is the most efficient way of finding the faulty component without additional equipment.
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Old 05-12-2018, 05:16 PM   #36
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Default Re: Toshiba laptop C855 no power dead

Great information...thanks

If u dont think the issue is one of the mosfets then i will take ur word....and maybe will do some fipurther testing if i get proper tools

There are many of these on ebay all dead...so i figured it was a common fail part...
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Old 05-19-2018, 07:15 PM   #37
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Default Re: Toshiba laptop C855 no power dead

hi blue

i removed the q6012 out of circuit just for the hell of it..i used a large hot air gun with a tip conversion to 10mm square end..it worked great...i just kept heat on it with a pair of pliers on it and was pulling as heating and it came off in about 30 seconds...didnt even use flux lol......had alum wrapped around surrounding area

i tested it out of circuit

diode mode..

D-S i get no reading then reversed probes and got reading of about 550

that seems ok?

and continuity test...

D-S no continuity but then i charged up gate and then D-S had continuity...seems ok?

you said this mosfet was not the issue....but i wanted to test my new air gun tips and removing a mosfet.. and it was a success ..in the past i have massacred multi pin surface mount chips trying to remove without a hot air gun...this way works way better :_)
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Old 05-19-2018, 07:23 PM   #38
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Default Re: Toshiba laptop C855 no power dead

those jumper pads you are referring to are very very small..so you say that each one of those tiny blobs of solder are within that blob is connecting 2 pins together? .
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Old 05-19-2018, 09:06 PM   #39
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Default Re: Toshiba laptop C855 no power dead

Yes, that FET sounds okay.

Flux doesn't really help with the removal, as you noticed. But it is absolutely essential for the reattachment.

While I'm thinking of it... something you are going to run across sometimes are parts you want to remove that are right next to some plastic connector or something. This is where some kapton tape (that yellow/golden transparent tape you see on boards sometimes) comes in handy. Keep a roll handy and place some over the nearby object you don't want to turn into a charred mess.

And another thing... be careful of electrolytic capacitors (the metal can style ones). They don't like heat. At the very least, it shortens their lives, and in the worst case they can pop. If there are any within an inch or so of the part needing removal, I like to wrap tinfoil around them until they have a nice little ball of it that acts as a heatsink.

The things that are labeled "PADxxxx" (where xxxx is a number) and have a white box drawn around them, are solder jumpers. And yes, they are connecting two traces with a tiny gap between them. If you remove the solder (cleanly with some desoldering braid), the two sides of the gap will be disconnected from each other. When you want to reconnect the sides, just put a blob of solder across them again.
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Old 05-19-2018, 11:02 PM   #40
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Default Re: Toshiba laptop C855 no power dead

Thanks

Ill let u know if i decide to try to try to desolder the jumpers
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