On 15 Apr 2004 17:37:50 -0700,
(railroadrum) wrote:

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.