OG wrote:
> Quad Core Desktop Windows 7 PC been running for several days.
> Today plug and unplugged USB drives to make a copy to USB Drive.
>
> Started the copy and the screen went blank and the room filled with
> smoke. Smelled like burning resistor (I think). I have smelled that
> smell before.
>
> Turned off the PC power.
> Turned on the fans to get the smoke out.
> Smoke detectors did not alert.
>
> What are typical failures to look for?
>
> Please give suggestions.
> First I need to figure out how to get the cover off.
>
Power supply.
*******
There is one chipset with a problem, then your source
would be motherboard. The Intel ICH5/ICH5R could suffer
latchup in the USB I/O area. Causing a short across the
rails powering th USB pads. In an unlucky case, the bond
wires on the pad with the latchup, remain intact, and
a burned spot appears on the lid of the Southbridge.
Since the Southbridge in that case, has no heatsink,
it's easy to spot when someone tells you where to look.
(What a damaged ICH5/ICH5R looks like...)
http://i.onfinite.com/TFG42bkgd.jpg
But that hasn't happened since ICH5 era - no PCH seems
to have suffered a similar fate. So Intel quietly figured
it out. Back when those were happening, there might
have been around 30 USENET posts from victims... I still
own an ICH5, which remains intact. And I treat it
like royalty :-)
*******
If you were to take it to Geek Squad, they might well
immediately pull the ATX supply, and connect up a
spare supply. This is fine, if the spare is a decent
name brand supply with full protections (so it can
survive whatever the motherboard throws at it). But
there might be a flaw which can damage a spare
supply (like some rail to rail short maybe).
A question arises occasionally "should I try to use
the defective computer again and again to collect
more (smoky) symptoms?" . No, don't do that. One
poster tried that while debugging his system,
and the old flaky supply blew in spectacular fashion,
damaging the motherboard. Meaning the repair went from
from costing $50, to costing $200.
You can at least start with the "nose" test, and
track down where the smoke came from. And PSU
is likely to be the source.
Visual inspection only occasionally digs up a root
cause for you. But visual inspection is cheap. And
visual inspection is safe, *as long as you stay
away from high voltage stuff*. The inside of the
ATX supply, below the lid with the four screws,
has a couple caps that are (potentially) dangerous.
They hold enough energy, you *do not* use the screwdriver
discharge method on them - it would deafen you if
you tried that. Just stay away from them.
There is a reference schematic for a simple ATX power
supply design here. R2 and R3 make C5 and C6 "safe". But
if R2 or R3 fail open circuit, then the natural draining
process provided by R2 and R3, might not actually be
protecting you from high voltage. It is C5 and C6 terminals
on the PCB, you do not want to touch... Only use
a proper resistive shorting technique to make them
safe (i.e. connect your own R2 and R3). It's better
just to "look but don't touch" while looking inside
an ATX supply... And don't forget to unplug the computer!!!
http://www.pavouk.org/hw/en_atxps.html
When you visually inspect the inside of an ATX
supply, this is what you'd be looking for.
I had an Antec, with low service hours, that
looked exactly like that. The PC had been sitting
in storage, on the main floor of my house (i.e. dry
and warm), and the Antec caps started to leak while
it was just sitting there. And I got a puff of gray
smoke when I fired it up again. The capacitors
on the motherboard, can also have domed lids and
orange dried deposits on top. And that's mainly
what a visual inspection covers. Only about 10%
of potential faults are covered by a visual inspection,
so it's not a "heroic method" by any stretch of
the imagination. But visual inspection is "free" :-)
https://en.wikipedia.org/wiki/Capacitor_plague
Of my three failed ATX supplies here, only one
of them looks exactly like the Wikipedia article.
Paul