"James Johnson" said:
once again, problems with this donor board from the friend. if you turn off
all power to the p.c. through the switch on the multiplugin extention cord,
on the next boot up, it goes straight to the bios and all settings are gone,
well i changed out the bios battery with a brand new one, and as long as you
don't turn off that switch, it keeps memory, otherwise, it goes straight to
the bios. i noticed with the side of the case off, that there is a green
light on the motherboard, even though the fans aren't spinning when it is
"off", so i am guessing this is keeping the settings? what could cause this
bad bios chip?
this board is an asus a7v266 E/AA with the bios version 1011
can i purchase a new chip? i know the battery is good, would it be better to
scrap this piece and go with what i have in mine, an a7v8x?
thanx for any information that you can share.
James Johnson
Settings are stored in two places. There is a CMOS memory in the
Southbridge, and it is powered by standby power sources. That is
the volatile memory you are having problems with right now. The
BIOS chip itself can be used to hold information, and during POST,
the BIOS can actually write to portions of the BIOS chip. That
probably isn't the problem, so the BIOS flash chip is not at fault.
In terms of backup power sources for the CMOS memory in the
Southbridge, the circuit looks like this. This is a conceptual
drawing only, not a real schematic...
+5VSB ---+-------------------------|>---+------+--> CMOS
| dual diode | | RAM
glowing +---|>---+ X (Southbridge)
green LED |
| X <--CLRTC
+3.0V (CR2032 ----resistor--+ | jumper
battery) GND
When the PSU switch is "ON", the +5VSB is running. That is one
source of power for the CMOS settings. If the PSU switch is "OFF",
and/or the cord is pulled from the wall, the coin cell battery
takes over. The "dual diode" pulls current from whichever source
has more to offer.
Now, if the previous owner of the board did a "clear CMOS" operation,
without unplugging the computer, the dual diode can get damaged.
The path from the CMOS battery has a resistor in it, so the
current level can never rise too high from that path. But the path
from +5VSB is unprotected, and shorting the CLRTC jumper can draw
enough current through the dual diode to burn it. Once the
dual diode is damaged, sometimes it is hard to get the settings
to stay there.
So, there are several possibilities. The coin cell battery
holder is defective, and isn't making good contact with the
battery. Or, there is a partial short, and the battery is being
drained within a couple of days, and then when +5VSB is turned off,
the settings are lost. The third possibility is the dual diode.
Judging by this description, the instructions in the manual are
_wrong_. There should be _no jumper_ on CLRTC while the computer
is powered or there is a battery in the battery socket. This
description talks about a "glowing filament", and that could
be some component, like the dual diode, burning up.
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&[email protected]
On several Asus motherboards I've examined, the dual diode has
a label on it "K45" (visible with magnifying glass, but won't
be readable if burned). It is three legged and looks like a small
transistor. My previous posting has some info on sources for a
replacement dual diode. Another poster I helped, replaced
his dual diode with a pair of 1N4148 diodes, which while
having less than ideal characteristics, will work too. You will
need a soldering iron, solder, solder wick (copper braid for solder
removal) to do the repair - if you can buy the part, maybe a
TV repair place can do the work for you. Look right around the
battery socket for the three legged device. That is logically the
best place for Asus to put it.
http://groups.google.com/[email protected]
HTH,
Paul
