need help tracing power circuit on laptop

[railroadrum]

I'm attempting to fix a laptop for a friend. The power plug from the
power supply has been broken off and lost. Rather than spend $80 for a
new supply I am trying to hard wire (via a quick disconnect) the power
wire to the jack on the computer. I'm skilled with a soldering iron
and I know I can make the necessary connections. My problem is I
don't know how to find out which pins in the jack are positive, etc.
The wire is a four conductor. Ground is easy to find by ohming to the
case but I need to know how to trace the circuit for the other wires.
Can someone please intruct me a bit on this? Many thanks.

rrr

 

[LASERandDVDfan]

I'm attempting to fix a laptop for a friend. The power plug from the

power supply has been broken off and lost. Rather than spend $80 for a
new supply I am trying to hard wire (via a quick disconnect) the power
wire to the jack on the computer.

Forgive me for being overly harsh, but tell your friend to spend the $80
dollars for a replacement supply and, in the future, take better care of it.
This is particularly true if the power adapter plug used more than two
connections. If the only thing that's wrong is the adapter, then don't screw
with the laptop and just replace the adapter. It may be expensive to buy the
adapter, but it's even more expensive to replace the whole laptop if the
modification is botched, especially if you don't have all the necessary tools
to properly perform circuit level work on complex muti-layer boards.

A modification which involves screwing with the main power feed in this manner
does yield some serious safety concerns. If the repair causes loss, injury, or
death, then it is YOU who could be liable simply because you performed a
potentially unsafe alteration of the original UL/CSA listed design of the power
feed circuit.

You can also easily damage the laptop if you don't know what you are doing.
Excessive heat from a soldering iron by itself is enough to easily cause damage
to the mainboard and its components.

Getting the mainboard out to service it can be a greatly convoluted task. You
have to get a lot of things out of the way before you can remove the board to
perform circuit level work. I should know as I had to deal with repairing a
broken circuit trace for the adapter inlet on a Toshiba laptop. The whole
thing had to be almost completely dismantled just to do that simple job.

Besides, you aren't giving us any useful information about the laptop. Who
made it and what is the model?

To reiterate, just tell your friend to stop being a cheapskate and buy the
replacement adapter, especially if there is nothing wrong with the laptop
itself. This is the only fix that will be correct and the safest. - Reinhart

 

[Railroadrum]

Thanks for you advice. I'll expand a bit. First, I am quite competent at
such projects. I am an electronics technician and work with delicate
circuits on a daily basis. That being said, I am confident I can fix this
in a matter of minutes. I won't be soldering on the circuit board but
rather on the pins in the jack. I have already dismantled the computer to
the point where I can put a meter to the circuit board. What I need to do
is trace the circuit and figure out which pins go to which wires on the
power supply. If anyone can help me with this procedure I would be
grateful.

Per the risk of playing with a power feed: I work with dc systems everyday.
I am aware that modifying a 12v source can cause damage to the equipment if
the polarity is incorrect. Personal injury at such low amperage is not a
risk at all. Obviously I want the polarity correct which is why I asking
for help! Thanks in advance.

rrr

Forgive me for being overly harsh, but tell your friend to spend the $80
dollars for a replacement supply and, in the future, take better care of it.
This is particularly true if the power adapter plug used more than two
connections. If the only thing that's wrong is the adapter, then don't screw
with the laptop and just replace the adapter. It may be expensive to buy the
adapter, but it's even more expensive to replace the whole laptop if the
modification is botched, especially if you don't have all the necessary tools
to properly perform circuit level work on complex muti-layer boards.

A modification which involves screwing with the main power feed in this manner
does yield some serious safety concerns. If the repair causes loss, injury, or
death, then it is YOU who could be liable simply because you performed a
potentially unsafe alteration of the original UL/CSA listed design of the power
feed circuit.

You can also easily damage the laptop if you don't know what you are doing.
Excessive heat from a soldering iron by itself is enough to easily cause damage
to the mainboard and its components.

Getting the mainboard out to service it can be a greatly convoluted task. You
have to get a lot of things out of the way before you can remove the board to
perform circuit level work. I should know as I had to deal with repairing a
broken circuit trace for the adapter inlet on a Toshiba laptop. The whole
thing had to be almost completely dismantled just to do that simple job.

Besides, you aren't giving us any useful information about the laptop. Who
made it and what is the model?

To reiterate, just tell your friend to stop being a cheapskate and buy the
replacement adapter, especially if there is nothing wrong with the laptop
itself. This is the only fix that will be correct and the safest. -

Reinhart

 

[Sierra]

Is this the seperate power adapter coming along with the laptop that plugs
into AC?
What model is the laptop?

 

[Sierra]

The inner conductor may be positive or negative (most use a positive inner
conductor). If you get this one wrong, it will most likely do permanent
damage to the laptop.

Be extremely cautious!!!! Check and confirm before you start heating up the
soldering iron

 

[Railroadrum]

The plug is not a normal round two conductor. If that were the case I would
go to Radio Shack and buy a replacement. As I stated it is a four
conductor. There are four pins in the jack on the back of the laptop. The
computer is a Prostar. I'm not sure of the model. Really it doesn't matter
because what I need to know is how I can use a meter to trace the circuit to
determine which pins go to which wires. The power wire has red, white, and
black wires and a shield. I just need to know if there is a method of
tracing the circuitry to determine which pin is which. Thanks again.

 

[H. Dziardziel]

I'm attempting to fix a laptop for a friend. The power plug from the
power supply has been broken off and lost. Rather than spend $80 for a
new supply I am trying to hard wire (via a quick disconnect) the power
wire to the jack on the computer. I'm skilled with a soldering iron
and I know I can make the necessary connections. My problem is I
don't know how to find out which pins in the jack are positive, etc.
The wire is a four conductor. Ground is easy to find by ohming to the
case but I need to know how to trace the circuit for the other wires.
Can someone please intruct me a bit on this? Many thanks.

rrr

Some quick thoughts and all done at your risk of course.
If this uses a NiCad pack:
There will likely be a wire each for fast charging, trickle
charging or thermistor sensing, and the mainboard proper. The
on-off switch would probably give a clue to the mainboard pin.
Does the charger have LEDs? If so try loading each wire with a
safe trickle charge level (<0v1C) load and see what lights.

Direct pack contacts to jack pins connection?

Apply power pin by pin starting with the lowest voltage through
current limiting resistors. The power supply should also have
short circuit protection. See what the LEDS do and pack contacts
voltages.

The main board capacitors near the jack will give a clue to the
mainboard voltage and polarity.

Assuming the charger is rated for one amp+, load each wire to one
amp, a seat of the pants average laptop load when not charging,
packs and see what the voltage (and change) is, and LEDs etc.
Smart packs may shut off however if a good battery is not sensed.

The pack supplies less than the charger but needs more than the
pack nominal voltage for charging so the lowest unloaded voltage
should be the mainboard supply.

Good luck and again all at your own risk.

 

[Jeffrey Dutky]

I'm attempting to fix a laptop for a friend. The power plug from
the power supply has been broken off and lost. Rather than spend
$80 for a new supply I am trying to hard wire (via a quick
disconnect) the power wire to the jack on the computer. I'm
skilled with a soldering iron and I know I can make the necessary
connections. My problem is I don't know how to find out which
pins in the jack are positive, etc. The wire is a four conductor.
Ground is easy to find by ohming to the case but I need to know
how to trace the circuit for the other wires. Can someone please
intruct me a bit on this? Many thanks.

Tracing the pin polarities on the laptop motherboard is probably the
last thing you want to try to do. First you should try to get specs
for the charger and power plug off the web. Next, you can probably
order a replacement plug from someplace like DigiKey and solder it
onto the damaged power cord from the original power supply. Third,
you may be able to pick up a replacement power supply on the cheap
from Ebay or at a local hamfest.

Finally, if you really can't get this solved any other way, you just
need to identify the power supply components on the laptop motherboard.
There are only a few companies that make most of the analog/powersupply
chips in common use (Dallas/Maxim, National Semi., etc). If you can
identify the power regulator portion of the laptop PCB, you can probably
make a good guess as to the regulator's topology and associated parts.
With that, you should be able to trace directly back to the socket.

Assuming that there is nothing more than simple linear regulation on
the laptop PCB (all the heavy lifting, 120VAC -> ??VDC, having been
done by the external power adapter) it shouldn't be too hard to find
the power regulator chips and look them up on-line. If all there is
is linear regulation, the Vin lines to the regulators should be, more
or less, directly wired to the line power socket pins. Now, just find
the linear regulator chips on the laptop PCB, look up the chips' data
sheets on-line, and find continuity from the Vin pin on the regulator
to the power pins in the socket.

If the laptop uses a switching regulator on the PCB, this could be a
bit more involved. Again, you need to identify the power regulation
portion of the PCB, look up the power regulator chip on-line, and then
figure out how it was used on the PCB (what topology was used). Next,
you should be able to guess at what are the rest of the power regulator
components on the PCB. Finally, you should be able to discern where the
input voltage enters the power regulatioin circuit and trace continuity
back to the power pins in the socket.

- Jeff Dutky