check one two all right so today we're
going to be going over a board that has
no backlight it's a Macbook airboard and
it's one that I got from this company
that sells all these refurbished
machines for people to fix and resell
and one of the things that I made the
big mistake of is I actually knew what
they were doing befor hand I I saw that
this is a company that sells
motherboards and they advertise them as
refurbished regularly and I've actually
bought them before so I kind of probed a
little bit and I was just kind of
curious so if you sell refurbished
boards and you're selling me
this lot of machines that may have bad
boards so are these the boards that are
like that that that you guys [ __ ] with
tried to fix and failed at or this just
stuff that you never touch this no we've
never touched this stuff before we would
never try to fix these boards before
sending them to you and just leave you
the pile of
crap yeah I needed stuff for the course
on short notice so this had to do so
this has no backlight and we're going to
go over a couple of things that I'm
going to measure first so I'm going to
turn it on
on try to follow along with you with a
schematic on the screen here so let me
just open up backlight
circuit so this here is the backlight
circuit all right so the first thing I
want to do is is there voltage on the
fuse so we're going to go over and check
the fuse while the machine is turning on
right now and turn it
on see what I get
and might as well move this over so you
can see the voltage as I'm measuring it
okay so on the fuse I get proper voltage
on the other side of the fuse I get
proper voltage on output I get zero if I
have output zero it could mean a couple
of different things it could mean that I
have a short to ground for some reason
it could be that power is not flowing
through this transistor after the fuse
use it could be that there's a blown via
corroded Trace so let's just continue on
and see what we get so at this chip over
here at this q77 6 which is a transistor
that's going to open based on the input
signal of LCD backlight enable uh let's
see what we
get on the beginning of it I
get a good now on LCD backlight enable
divide I
get eight bad on output I get zero so
let's let's put our thinking cap on here
is the transistor bad because the
transistor is not outputting power or is
the transistor not getting the proper
input signal so the source of this
transistor is pin 4 the source of it is
getting 8 volts then the Gate of that
transistor pin 3 is also getting 8 volts
this is a ptype mosfet meaning that when
the voltage in the gate is the same as
the voltage in the source that it will
not allow power to flow to the drain
therefore this over here is working just
fine it is doing what it is supposed to
do which is close and not let power
through the circuit now let's look over
here so how does this work so you have
these two resistors over here that are a
voltage divider between PP bus G3 hot
which is the input voltage of this
transistor you have this voltage divider
going between the voltage on the source
and then the voltage on the gate the
whole idea here is that this voltage
divider is going to lower the power
going to the gate so that it will open
remember this transistor will not open
if the power that's on the gate is
greater or equal to the power that's on
the source you have to lower it and we
have a voltage divider to do this a
voltage divider is going to go between
the high side power source and where we
want the power to go and then where we
want the power to go in ground but
unfortunately there's this little crap
in the way over here this transistor
sits in the way of the bottom of this
voltage divider and
ground so what are we going to do to to
check if this is working the way it's
supposed to so these transistors are
going to open and send this voltage
divider a path to ground but that's
going going to happen based on E these
two input signals so when you have EDP
external display port backlight enable
and backlight platform reset these
transistors will open and send that
power over to ground so let's take a
look and see what we get at those two
points three
okay and three yep now by the way if you
Google this you'll see that this is two
n Channel mosfets so these are going to
open and let power through and you have
a higher or equal voltage on the gate as
to the voltage of the source so these
are opening and allowing this to go to
ground so why is this not sending my
power the way it's supposed to so let's
check the voltage divider R 7788 is
supposed to be 301 kohms and R 7789 is
supposed to be 147 KMS so we change the
multimeter over to resistance mode now
whenever you're measuring resistance or
doing diod mode you have to take off all
the power from the machine let's see
where those two resistors are so
okay those are over here so R
7789 see what that looks
like R 7789 is supposed to
be let's
see
I get 145
KMS schematic says it's supposed to be
147 kilms close enough now let's see R
7788 schematic says 301 kilohms
and my multimeter
says
H my multimeter says 97 ohms so instead
of that resistor being 31,000 ohms it's
97 ohms do you think my voltage divider
is going to work properly probably not
so let's replace that affected area and
see if it fixes our problem so we're
going to go over to the microscope
camera make sure the microscope camera
actually
works okay let's move in over
here
turn on the
Hackle and again you think I'm I'm going
to buy those resistors from Mouser and
wait a week or two for them to come for
me to put this machine in the window
hell no I am going to take them off of a
donor board because we don't have time
for that
nonsense so the the the best source of
any of these parts is to buy a bunch of
dead boards with the holes in them off
of Alibaba and and then use those to
take Parts as
needed I'm going to replace both ends of
the voltage divider just for the hell of
it
D that is bright as
hell much
better the 3437 is a board that has a
lot of lot of heat sinking not that's
the wrong word it's a it's multi-layer
PC board in whatever way they design it
whatever materials they use this thing
just like it absorbs heat like crazy so
if you have some kind of IE station it
will just laugh at you good luck getting
anything off of this board with a cheap
ass hot air rework
station it'll make you want to
cry okay now that I have those two
resistors off I'm going to put some Zer
on the
pads
not exactly rocket science tending those
pads I'm going to pray that I don't make
that resistor flick
away I'm going to let the hot air move
it into
place there we go and the other resistor
in that voltage divider is down
here that is terribly out of
place and we just flick it into place
courtesy of hot air okay now that that
is all flicked into place now what I
want to see is when I'm done does this
actually
work
so we're going to try turning it on and
see if I have a backlight and if we
don't have a backlight I'm still going
to want to measure around and see if
anything changed so if we have any any
new new
stuff all right it's going to turn on
turn off turn on turn off turn on turn
off then turn on like all of the 2013
errs
do you come
on all right so that didn't work and
also unfortunately when I actually did
manage to follow up and fix it I forgot
to [ __ ] hit record uh so I I managed
to hit record all the way at the end for
my for me talking but I didn't show you
what I did so I'm going to go back and
show you what I did here to fix that
problem I want to show you what was
actually wrong with the board uh because
it's something that doesn't happen often
but it's something that you should know
how to fix it's something that you
should see so let me load up this
schematic here and after I load up the
schematic I'm going to show you what it
is that I did all right so this here is
the voltage divider that wasn't working
the way it was supposed to now after I
replac the resistor over here it still
measured 89 ohms instead of 301 kohms so
this to here has a path through through
here so my original assumption which was
wrong for several reasons was that this
resistor must be bad now the first
reason that this assumption was wrong is
that resistors when they blow
99.999% of the time when the resistor
blows it blows to a higher resistance
not a lower resistance so the first
thing I should have thought is what
other path could it be taking the signal
to go from path from this pin over here
to over here pins four and three of this
transistor could be shorted or this
capacitor could have blown and have been
the problem and the problem here was
this capacitor so this capacitor here
was was broken now you may ask well why
don't you measure the resistor out of
circuit measuring the resistor out of
circuit is the proper thing to do like
have you tried to get one of these
resistors on your desk so that you can
measure it out of circuit I mean really
have you ever actually tried to take a
size 2011 resistor off of the
motherboard place that [ __ ] resistor
on the desk so that you can measure it
out of circuit and do this cuz you know
you know what's going to happen to that
resistor the second that you touch it
it's going to take that little touch and
it's going to fly right away to the
sixth Dimension and you're never going
to see it again I don't I don't measure
things out of circuit because 99% I I
can't I really can and then you say what
if you use Smart tweezers I'm going to
take it off of the board I'm going to
put it on the desk I'm going to grab
with the smart tweezers it's going to I
I just know it's going to flick away and
also my smart tweezers got water damaged
by my air conditioner leaking on them
the day that I got them which is
something I should really send back but
yeah that that's that and you're going
to see later that the the board does
work that was the problem I was being an
idiot and doing the wrong thing to fix
it
and all right so this thing died while I
was recording because my stomach hit
something over here I think I don't know
what I did I hit off maybe I bumped into
it anyway let me just show you that I am
working on the same board not bsing you
this is the same
uh same board where I replaced that
capacitor
on see the I replaced the voltage
divider you can see because it still has
the flux around it and that is
let's turn the thing on so that you can
see that it has a backlight and see that
it does indeed
work plug it
in it's going to do the turn on turn off
turn on turn off turn on turn off thing
and then once it turns on I have to wait
for it to get to the question mark So
anyway the whole point here is you have
to figure out what was wrong with your
board you can't uh you know follow
something on the Internet that says I
you know this thread says check this
component and replace this thing every
single time it's going to be something
different I've been doing this for a
long time and this is the first time
that that was the actual component that
needed to be replaced that's never
happened before it's never been that
little piece but this time it was so
really you just have to understand how
the components work go through step by
step in an analytical and logical
fashion analyze compare contrast measure
and learn and if you do that you'll be
able to solve these problems if you
don't you're not and you're just going
to be wasting your time so come on here
here we go backlight beautiful another
board that mentech couldn't figure out
yeah I have I have an entire box here of
boards from them where like half of them
have the bi it's like we never touched
them we never tried to fix them and like
the BIOS chips are on backwards like
just get the [ __ ] out of here but yeah
another one that I can put in the window