"Tim" wrote in message
...
In article , plumber1
@________.com says...
A while back I bought a used 500mhz computer on Ebay. (that was my
big mistake). When I got it, it worked well, except for the floppy
drive. I contacted the seller immediately and he sent me another
floppy drive to replace the bad one. I replaced the drive and turned
it on. It worked fine but since I wanted to add another harddrive (as
soon as I got one), I left the cover off the case. That turned out to
be a good thing, because I was sitting at my desk on the web when I
noticed a hot burnt odor. Then I saw smoke. I looked at the computer
and saw smoke coming off the motherboard. There are these donut
looking things on the board. They look like a white lifesaver candy
wound with 20 or 30 loops of a fairly thick enamelled wire (actually
not all that thick, probably a 20 gauge). Oddly enough, the computer
continued to work the whole time, and I know that because I saved
whatever I was typing before I shut off the power. I found all the
enamel burned off the wire on that coil and it was black. I was glad
the cover was off the case so I could see what was happening.

I carefully looked for anything that might create a short and found
nothing. I got my fire estinguisher (just in case), and turned the
computer back on. It booted up, and worked fine. I turned it on and
off several more times, and it still worked, and I never saw more
smoke. The seller would not replace it because it was 2 ot 3 weeks
later by then.

What could have caused this? What are those coild called and what is
their purpose. It seems there are 2 or 3 of them on all motherboards.

Needless to say, I do not use this computer. I dont trust it. I just
took out all the drives, ram, and cards and put them on another
motherboard. I know I can use that case again, but question the power
supply. I definately will not use the motherboard again, and worry
about the CPU.

Anyone have any idea what caused this? I'm just curious more than
anything else. I figure the MB is best going into the trash, and I
have a faster CPU now too.

Thanks

Plumber

Those are probably inductors used to limit noise on the DC power lines.
If they overheated, that usually indicates a fault on the motherboard.
It takes a great deal of energy to burn those lines, so I would look for
a damaged regulator or shorted capacitor on the board. The problem may
be a short on one of the inside layers of the board as well, so you may
have to remove the board and try to look through it. If one of the
layers is a ground plane, you will not be able to see through the board.

- Tim -

Actually the most of the inductors described are not noise filters at all,
but part of a switching power supply regulator that converts either 5 or 3.3
volt supplies to lower voltages such as 1.5 volts for the CPU. They often
operate at high currents (perhaps as much as 10 - 20 amperes). There are
also some boards that have regulators to generate higher voltages for other
components on the board such as built in audio or RS232 ports. I have never
seen one get hot enough to cause smoke without also something else wrong. My
guess is that particular regulator was associated with something other than
the main CPU/memory system or else the machine would not run. Do all of the
I/O ports, built in peripherals, and add-on cards operate?

David