[mdlug] Laptop Hardware issues

[Robert Meier]

Ray,

>>>> The problem: Upon power up a more or less normal POST seems to start,
>>>> but the display never lights, and after about 12 seconds it powers off
>>>> for about 5 seconds, then starts the cycle over.
>> --- Robert Meier wrote:
>>> I suspect a bad power supply.
>> As someone who has repaired hundreds of switching power supplies, I
>> agree with Bob--the power supply is likely defective or marginal,
>> probably a bad electrolytic capacitor.  ...

> By power supply do you mean the brick that I plug in the wall?

The power supply in question is the component that the "brick"
plugs into on the laptop side.  It is likely to be an aluminum box
with the socket to plug the "brick" into and several ribbon/zip
cables with only 2, 3, or 4 conduits each.  It is also likely
to be a section of the motherboard, with the socket soldered
to the motherboard, and larger than average traces in that area.
In either manifestation its purpose is to isolate the computer
(high-gain) circuitry from fluctuations due to varying load
or line (brick) noise.

A typical "switching" power supply operates like a water tower
and pump.  A capacitor (or water tower) discharges at a fixed
voltage (or head pressure).  Every 1-10us (or each hour)
a pump is turned on and remains on until the capacitor (or tower)
is full.  The fixed interval simplifies tuning the capacitor
circuit further reducing noise delivered to the computer circuitry.

Either a coil or diodes are used as "pump".  Both wear with use
(and abuse).  Eventually enough noise and load sensitivity occur
to excite the computer circuitry chaotically, and a reboot is the
likely result.

Such behavior should not last long, as the noise and sensitivity
increase the rate of wear, such that failure onset is often
measured in seconds.

The computer circuitry only needs clean steady voltage,
so attaching the motherboard feed lines to a "bench supply"
will demonstrate whether or not the laptop power supply
is indeed the problem.

Replacing the coil, diodes, or capacitor is usually an easy
solder job, as they normally need only be "big enough" not
"tuned".

Good luck,
-- 
Bob