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- Total Failure
April 23, 2006, 8:56 pm
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Re: Total Failure
Some motherboards use a dual diode. A common cathode device. It is
black and has three legs on it, and looks like a SOT-23 packaged
transistor. It is pretty tiny. Usually you would find it near the
CMOS battery, but with the wonders of modern layout tools, a careless
designer could stick the damn thing just about anywhere. If you are
lucky, it will be burned beyond recognition, and then you'll know
which component it was.
In a previous post, I put some URLs for replacement parts.
The deal is, a number of motherboards were designed in the
past, such that +5VSB must be removed from the board, before
you go messing with the CMOS jumper. And that is why I recommend
to people to unplug the computer before following the procedure
in their manual, for clearing the CMOS.
One guy I helped, replaced his three legged transistor with a
couple 1N4148 type switching diodes, and that seemed to work
for him. Selecting an "ordinary" switching diode, means that
the battery won't be able to run down quite as much, before
you'd have to replace the battery. Schottky diodes have a
lower forward bias value, and would make the battery effectively
last a few months longer. But for most people, they'll have
tossed the motherboard by the time the battery needs to be
changed, so it really doesn't matter what diodes you replace
Page 52 of this document, shows an Intel circuit for the CMOS
battery. The 1K ohm resistor should be shifted to the left, in
order to allow a dual diode to be used. The cathodes of the
diodes are in common (that is the "bar" on the right hand side
of the diode). Now, some motherboards place their clear CMOS
jumper on the VCCRTC signal. When the CMOS jumper grounds the
VCCRTC, and the VCCSUS on the left of the diagram is still
powered (derived from +5VSB), a large current flows through
the top diode, burning the hell out of it.
With the source of CMOS power removed because the dual diode
is burned, there is no way to predict what the Southbridge
will do. Failure to post is one possible solution I guess.
So, get out the magnifying glass and start looking. If one guy
can manage to find it and jury rig a solution with a couple
1N4148s, then you can too :-)
Re: Total Failure
turning the board one with the jumper in the clear position, can on some
motherboards, totaly fry the bios/cmos chip, or circuits around them. i dont
think its as simple as putting it back in original position. :-)
you could be right though.
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