All language subtitles for Board Repair Basics #10 - Short Detection with Voltage Injection

[Music]

hello interwebs welcome to

let's fix computers in this video i'm

going to show you how to perform voltage

injection in order to find a short

circuit and a laptop motherboard

so i know we've had a couple of these

recently however i've had quite a few

requests to demonstrate

how to do this um formally

over the past couple of videos i've done

on it um

so i've got several videos in my board

repair basics playlist

link in that corner over there somewhere

i don't know where

which show me using this technique in

order to find short circuits in

motherboards

however i often just kind of skimp

across the details on exactly what i'm

doing and i don't show what's going on

with my dc power supply and things like

that

so in this one we're going to cover that

as well

so enter the power supply

now i've just recently put out a video

that does a quick crash course on bench

power supplies

what you can use them for and the

different kinds that you will see

um or you know just a couple of examples

anyway so

this is my uh fairly new hamatek hm

305p uh i was previously using an

uh a an older and crapper power supply

for this

however uh it doesn't really matter as

long as you have some kind of bench

supply

and as i say refer to my previous video

for a quick crash course

on what these things do um so let's get

into it

uh what i've got in front of me now this

is a

motherboard that i literally took off of

the wall from my um

my motherboard graveyard on the wall

behind me um i don't have

any um laptops that are in for repair

which

i know have short circuits in them right

now so what i thought i'd do is i'd rig

a board up

what i've got is a random motherboard

here this is quite an old motherboard

which should also make it a bit

interesting

and i have sabotaged this with a short

circuit

so now i'm going to show you how we use

voltage injection

to find that short circuit so

let's start from the beginning the first

thing we need to do

is understand the rough layout of the

motherboard

we're going to be doing this without

schematics

and when you have schematics for a

motherboard and a board view it makes

this much easier because you can

straight away just immediately look at

the

designs for the motherboard in front of

you and see where the main power rails

are and so on and so forth

however you might not always have

schematics for what you're working on

i recently did a video that's that was

titled

diagnosing without schematics which

shows you how you can look at the board

and understand the rough layout of it

without having schematics so that's what

we're going to do right now before we do

anything at all

we need to have a basic understanding of

the power system in this motherboard

so let's start from the top if we look

on both sides of it you can see we've

got a lot of action going on on this

side

and on the back side of the motherboard

we've got some extra chip sets and we've

got where the met the ram would go we've

got a dial-up modem on this thing

and we've got some bypass capacitors and

stuff

but there's not a huge amount going on

there right now so we're going to start

up here

the first thing we need to do is

identify where is the power

entering this motherboard from so that's

going to be our dc jack which is over in

the corner here

here's the dc jack where this where the

charger would plug into the motherboard

so let's zoom into that section so

looking down here you can see that we've

got the pins on the back of the dc

jack um and in this case this particular

dc jack

we've got our ground which is just the

outside we've got

two positive pins and we have a center

pin which is a sense pin that just

simply detects what kind of charger is

connected

um now we can stick our multi the first

thing you should have already done by

bit by now is to put your multimeter on

these pins to check that there is

voltage coming in so

let's do that for posterity i'll plug in

a charger i'll set my multimeter into

voltage mode

and i'll put my black probe on a ground

point so just this screw hole

and i'll touch my positive probe to one

of the positive pins

and my multimeter currently reads 19.86

volts

so a hot 19 and a half volts which is

what we should have coming into the

laptop anyway

so we've identified the positive pins

where the power is coming into the

laptop

so where does it go from here so once

again

just like we did in the uh diagnosing

without schematics video we're going to

follow the money

so if we look here you can see that the

positive pin here

it connects around to this guy here you

can see we've got

fl1 here so l is an inductor

so we've got an inductor across this bit

and it bridges

this pin up to here and if you're

wondering

what's going on with that positive pin

how does that get there well let's look

at the other side of the board to see if

we've got any hints

on the other side of the board you can

see that the two positive pins are tied

together

on this trace here there's a big just

blob trace and it ties these two pins

together

and there there's four little dots there

you can see those four dots

those are vias so that's a pathway going

through the board

and if we turn the board over

now i don't know how much you guys can

see on the camera but if you look you

can actually see

the corner of those four dots down there

one two

and then three and the fourth one is

under the connector so you can see where

this guy

is being tied to this guy on the other

side of the board and it comes up

through here

then it goes through this filter

inductor and

into the inrush limiter so the inrush

limiter is going to be a pair of mosfets

that allows the laptop to switch the

incoming charge

the incoming charge voltage on or off at

will

so it comes into the first section it

comes into here

and then on the other side you can see

that all of these guys are all tied

together they're all on the same green

trace there

so these guys are all tied together so

it comes in it goes up through here

and then it comes back down into this

guy and then on the other side now we've

got the output of the in russian limiter

so this is the beginning of our vp

of our vn rail our main power rail

voltage in

so what we now want to do is we want to

identify

where voltage in goes in the rest of the

laptop

so let's zoom out again so when we look

at the rest of the laptop

again we'll take the lessons that we

learned from my previous videos and

we're looking for

secondary power supplies and how do we

spot a secondary power supply

it's an inductor with a pair of mosfets

or some kind of power stage next to it

so let's look here we've got inductors

here these big square boys these are all

inductors uh they're really easy to spot

because they're usually the biggest

components on a

on a modern circuit board so all of

these guys are inductors

and if we have a look next to each one

inductor mosfet packages

inductor mosfet packages inductor

mosfets inductor mosfets and dr mosfets

we've seen this in other videos before

each one of these is a buck regulator

it's a secondary power supply that's

stepping high voltage down to low

voltage

by use of mosfets and an inductor

and behind each one we've got a load of

capacitance as well for smoothing and

that kind of thing

so here are all of our secondary power

supplies

the other guy that we've got to notice

is round over here here is cpu v core so

here's where the cpu would go on this

on this motherboard because this is an

old one it actually has a socketed cpu

which is missing and next to the cpu as

you can see with a big

huge power trace going into it is cpu v

core

so this is a two phase power supply on

this on this laptop

because as you can see we've got two uh

buck regulators

going into one input so these guys will

be sequenced in

alternation so they're pulsing in

they're alternately pulsing in order to

provide a smoother flow of power

into the cpu because it's more sensitive

so and in addition to that if we have a

look on the other side of the board

we've got more power stuff going on on

the other side so we'll come back to

that in just a moment

so how does our main power rail get

from all the way down here to all the

way over here

and to cpu vehicle well on other laptops

i've been able to point out a big you

know big visible trace going across the

board

and it's been really easy you can see

the trace just going blair across the

board with your own eyes

however this board as you can see is a

mess

quite frankly um this is a confusing

mess i mean

as is the old days he's got a full silk

screen on it

listing every single component on the

board but that actually makes it almost

even harder to read at face value

but the thing is we don't really need to

be able to see the main power trace

because we know where it's going to go

the main power trace

that starts down here for v in it has to

be connected to these secondary power

supplies

so we can't see it but we know that it's

going to be there so

i'm going to put my multimeter into

continuity mode so beat mode

so when i touch the probes together it

beeps and we'll put

one down here at the start of the in

and if i start probing these secondary

power supplies on the input side so

this is the this is the output side of

it the inductor

the input side of these mosfets

as you can see those are connected

those are connected that's connected

the input side of all these power

supplies

is connected to vm so we don't need to

be able to see the v

in rail because we know where it's going

to go because we know where it has to go

and likewise if we go over to cpu vcore

we should find that it's over

here as well now this guy's a little bit

more tricky and this guy might catch you

out

let's zoom in again now we've got to

watch out because at first

first glance you might go well easy

you've got inductors and mosfets right

well this one's a bit tricky because if

you have a look here here's a big old

screw hole now screw holes will always

be ground

and you might notice well hold on a sec

why are these mosfets

connected directly to ground likewise

we've got unpopulated capacitors here

there's the positive pin so that's going

to be the negative

so this is all ground so why is it the

ground is connected directly into those

mosfets

well the answer to that is that this

particular power system

again because it's more complicated it's

got high and low side mosfets

and these are the low side mosfets so

they connect the power supply back to

ground

however if we flip the board over

now these guys these are going to be our

high side mosfets

so these ones however are connected to

vn

and we can prove that so again if i

stick one of my probes onto vn

now this is going to be a little bit

tricky because

uh vn is on the other side of the board

so i'm just going to hold the board

upright

so i'll hold the board up right and just

hold that there

and so i've got my black probe on vn

and now i can probe those and as you can

see

the input of all those mosfets is

connected to vn

awesome so we've identified

where vn is and how it connects up to

the secondary power supplies

so we now have a very basic

understanding on how this motherboard is

laid out

good start so now we need to do some

very rough diagnostics to work out what

our rough problem is

so earlier on we checked if there was

voltage coming into the laptop

um so the first thing that i want to do

is i want to check

if the main power rail for the laptop or

if any of the sub rails

are shorted to ground so we'll start off

again in beat mode black probe on any

ground

so i'm on a screw hole here and i'm

going to check these inductors to see if

any of them are shorted to ground

that one isn't that one isn't

that one's not

that one's not what about our actual vin

rail

so we found our v in rail it's available

on the top of any of these input ones is

that actually shorted to ground

yes it is so our main power rail in the

laptop is shorted to ground

well that's a pretty poor start okay

so we've identified a short circuit on

the main power rail of the laptop

now we know why this motherboard doesn't

work

so what do we need to do we have a short

circuit

somewhere in the laptop that is shorting

the main power rail to ground

how on earth do we find it we need to

inject voltage

what we're going to do is we're going to

apply power into the laptop

to make sure that short circuit stays

dead short

and it will begin to heat up and by

looking for the heat source

we can find our short circuit you might

say

okay well why don't you just plug in the

charger will it not heat up then

well it won't because the problem is is

that the laptop's power management chip

will detect the short circuit and switch

off the inrush limiter

that's what the inrush limiter is for

it's there so when the laptop detects a

fault

it turns off the incoming power to

prevent damage to the laptop

so this is why we need to use a bench

power supply to inject power

manually into the board so we can

control the flow of power

and burn out that short circuit so

what we need to do now is we need to

figure out where we can inject power

so obviously we want to inject power

into the vn the main power rail

so this board as we've already

discovered is

quite a clustered mess so what i'm going

to do is i'm going to solder

two jumper wires i've got a red one and

a black one doesn't matter what color

they are

just some wire is all we need and i'm

going to solder them

onto the start of the in down here

and i'm going to put the black one onto

any ground point

so that way we are now have a loop we'll

have a loop going through the laptop

through the short circuit wherever that

is and back to ground again

so what we'll do is we'll solder those

on now ideally if you can solder them

fairly close to each other

it makes it easier to pick up the board

and move it around while we're looking

for the short circuit

and you'll see why that's useful in a

minute so i'm going to warm up my

soldering iron and i'm going to solder

the wires in place

let's get to it

okay our jumper wires are now hooked up

i've just put a bit of masking tape

around the positive crocodile clip here

just so these guys don't knock together

when i'm moving the board around

so obviously it's up to you guys whether

you feel that's necessary or not

but i've done it just in case we don't

want these things guys start sparking

together and that kind of thing

so we've now got direct injection line

so we can inject power

onto the power rail so now it's time to

set up the bench power supply

so because we're on the main vn rail of

the power supply

this vin rail is designed to carry

charger voltage which in this case is

going to be 19 and a half volts

however we don't need that much power to

do our short detection

and in fact we want to keep things

fairly low because where we're going to

be

where we're going to be forcing power

into a rail that does not currently want

to accept power

we want to only use as much as we need

no more no less

so i'm going to roll back i normally do

voltage injection at about five volts

now if you're working on a smaller power

rail like if you've determined that the

3.3 volt power rail is shorted then

obviously you're gonna need to work

lower like more like three volts or

maybe even two and a half

so understand what power rail you're

injecting onto and how much power it's

supposed to handle

although this rail can actually handle

up to 19 and a half we don't need that

much

so i'm going to dial my power supply

back to just

5 volts is all i need

so there's 5 volts and i'm going to set

my current

to i'm going to start off at

one amp now at one amp i'm not expecting

to find anything

because most components on the laptop

will probably quite happily sync an amp

of current

without any problems especially on vm

however we're going to start low just so

nothing explodes the moment we turn the

power supply on

again we use as much as we need no more

no less so

let's switch on the supply

so now as you can see the power supply

has immediately gone down to a dead

short circuit

because our voltage has now dropped to

just .34 volts

so you might go well okay well is there

any power going into it

there is the thing you've got to

remember is voltage is a potential

difference and the potential difference

from positive to negative

has gone down to nil because the two are

connected together through a short

circuit

so there's no difference between the

energy at one

to the energy at the other because there

isn't they're directly connected

together

that's why our voltage has dropped down

now our amperage however is still

sitting at one amp we've gone to control

current at one amp and as you can see

it's sinking

0.34 watts so volts times amps

so even though our voltage has gone down

to almost zero

there's still power flowing this is the

first thing that caught me out when i

first started doing voltage injection

is i was thinking but the volts have all

disappeared is there actually any power

going through it

there is because it's a dead shot so

now we have our dead short circuit set

up now we need to start looking at the

board

so the first thing we need to start

thinking about is where are we

likely to find short circuits so

the kind of things that are going to be

shorted to ground are

going to be power devices themselves

because they're connected to the power

rail

so we need to start looking at the

secondary power supplies

large capacitors that kind of thing so

well let's start off up at the inrush

limiter and i'm just going to touch that

with my finger

and i'm just going to start hunting

around

just with my tippy fingers just seeing

if there's any heat sources anywhere

this is quite difficult for me because i

actually know exactly where the heat

source is going to be so

please suspend your disbelief so i'm

checking these capacitors

i'm looking around i'm checking ics so

the little chips

but not everything is stone cold

so let's check over here any of these

guys warming up

it's just any residual heat at all

anything at all

nothing going on over here okay let's

turn the let's turn the board over

so this is why i wanted these leads nice

and separated so i can move the board

around

and i can move it freely without

snagging these cables

so there's not much going on on this

side but cpu vcore is an absolute

classic

is there anything over here

doesn't appear to be okay so as expected

at one amp we're getting absolutely

nothing

so let's jump straight up to two amps so

we're gonna go over to our power supply

and we're going to dial that up to two

amps

go right so now we've gone up to two

amps now we're sinking

what nearly one and a half watts so

we've added in a significant amount more

power

to our dead shore so once again quest

around

nothing happening

nothing happening fine okay

so there's not enough heat going on here

one and a half watts is not enough to

find anything

so i'm gonna jump straight up to three

watts of power now

and when we get up to between three and

four amps

sorry i said three what's at three amps

of power when we get up to

three to four amps this is usually where

things will start burning up

so in my experience five volts three or

four amps that's usually the amount of

power you're actually going to need

but as i mentioned i like to step into

it slowly

three amps

okay right anything going on now

okay right i'm beginning to feel some

warmth over here

it's not burning or anything like that

but the board

is not cold over here the board is just

cold it's stone cold

however over here there's some kind of

warmth going on

so we're in the roughly the right area

and there's definitely something going

on here so now i know the rough area

we're in

what i'm going to do now is i'll show

you how we can highlight the actual

source of the issue

so i'm going to get my bottle of

isopropyl alcohol

so this is just rubbing alcohol and i'm

going to spray it on the board

so let's go down to cpu v-core because

that seems to be where our heat source

is at the moment

and i'll spray a bit of alcohol over

there

now the alcohol evaporates very quickly

and at the moment it's just sitting

there not much is going on

so there's not much happening here fine

turn the board over check the other side

let's spray some alcohol here

there's definitely some heat around here

the problem is the alcohol has taken

some of the residual heat out of the

board

let's try giving it more power to see if

we can make this more apparently obvious

so back into the power supply

you'll notice that where we're pumping

in more power because of the force

against the short circuit the voltage is

increasing

we've now got one and a half volts where

we're forcing power across this

let's spray a little bit more alcohol on

there

can you see how this capacitor here is

drying up the alcohol around it

when i spray alcohol onto there

you can see it immediately dries up on

that capacitor

and in actual fact the capacitor itself

has a visible crack in it

so we've actually burnt that guy out

so this guy is actually burning hot to

the touch and when i put my finger over

there now just

very quickly i can detect the heat

source there

so that's our guy so let's turn off the

bench supply now and what we'll do is

we'll remove this guy from the board

and we'll see if our short circuit has

disappeared

and i'll use my hot air station to get

this guy off the board

so now let's check for our short circuit

so from ground

to vn there is now no longer a short

circuit and if we go into uh

if we go into resistance mode we should

find there's actually a healthy amount

of resistance on that line

there we go we're climbing up into the

killer ohms

so no problem whatsoever

and now if we switch on the power supply

again as you can see

the board is drawing no current

whatsoever the short circuit has

disappeared

so at this point now we've solved our

short circuit

and if you try and power up the board

the chances are it's going to work

now i can't demonstrate this one working

because uh

well this is all i have i don't have a

cpu for it i don't have the laptop for

it

however that is how we use voltage

injection

to hunt down and find a short circuit a

couple more tip bits that i would give

for you

there are other methods to do it what we

could have done is just whacked

we could have gone straight down to um 5

volts 5 amps and just drowned the board

in alcohol literally just spritzed the

whole board down

and just waited to see where it dried

out first

now this can work however in my

experience

um it's generally i well maybe it's just

because it's colder where i am

however i find it very difficult to find

short circuits that way

uh what i generally do is use my hands

to try and find hot spots first

and narrow down the rough area on where

we're at

and once i've roughly narrowed it down

then i'll use a bit of alcohol

to try and figure out which component it

is is it the capacitor on the left or

the capacitor on the right

you know by using the alcohol you can

usually spot which one has died

so try experimenting to see what works

for you with your board

another thing to keep in mind as well is

don't be disheartened if it's taking you

a while

typically when i'm doing voltage

injection for short circuit detection

i usually spend a good half an hour with

the bench power supply on looking for

that short circuit

so you know it does take time and as

with all board repair

don't be disheartened if the first

couple of times you try it

you can't do it it took me time to get

as far as i have

and i'm still an amateur by all accounts

compared to a lot of people on youtube

i'm an amateur at this however it took

me quite a long time to get

this to this point it was a there was a

lot of failures along the way

so again don't be disheartened if you're

having trouble but now you know the

methods

so thank you very much for watching i

hope that was interesting and i'll see

you all next time

bye for now

you