That is the voltage mod for the Southbridge. Based on the contents of the
thread indicated above, the original design was probably based on assuming
the 3.3V supply was at 3.45 volts, which in many cases it is not. The
design is wrong, in the sense that the voltage reference for the CM3708
should have come from some other more regulated source, rather than
from the power supply directly.
The problem with the mod, as shown in that thread, is it has no "failsafe"
feature. I learned from experiences experimenting as a kid, with
adjustable potentiometers, that the temptation to keep turning them
is overwhelming. If that pot is adjusted all the way to zero ohms, the
Champion regulator will be severely overvolting the Southbridge. Some
people ruined their boards doing that. You should use these two
components in place of just the "adjustable resistor" shown in the
figure:
----------------- ----------------------
A----| fixed resistor |--------| adjustable resistor |----+---B
----------------- ---------------------- |
^ |
| |
+---------------+
The adjustable in the mod can still be 2K ohms if you want. The
"fixed resistor" is something you compute to give the highest
allowed voltage, whatever that happens to be. Then, if by accident
you turn the potentiometer all the way to the left (giving zero
ohms), the Southbridge won't be fried.
+3.3V +3.3V (from the end pad of R541 resistor)
| |
| |
R628 Fixed_Resistor
| |
| |
| |
| Adjustable_Resistor <--+
| | |
| +--------------------+
| |
+-----+----+------------------------------- CM3708
| | Pin 7
| |
R630 Capacitor
| |
| |
GND GND
To figure out what the original resistor values are, see the picture
in this thread:
http://www.nforcershq.com/forum/viewtopic.php?t=16678
R628 is marked 18A. R630 is marked 12A. Using the following document to
decode the values gives R628 is 150 ohms and R630 is 130 ohms:
http://www.bourns.com/pdf/chpreztr.pdf
Using these values and assuming the 3.3V rail is at 3.45V
(Asus designer assumes what he thinks is the worst case), gives...
R630 130
Vout = ---- x 3.45V = ---- x 3.45V = 1.60
R628+R630 150+130
When resistors are placed in parallel (i.e. the mod network in parallel
with R628), the resistance is:
Ra*Rb
Rparallel = -----
Ra+Rb
To get say 1.8 volts, changing R628 from 150 ohms to 120 ohms would
do the trick. If a roughly 620 ohm resistor is placed in parallel
with the 150 ohm resistor, (using the Rparallel equation) this gives
120.8 ohms (close enough).
So, if I wanted to limit the circuit to no more than 1.8 volts, when
my power supply is delivering 3.45 volts, then I would place a 620
ohm resistor in series with the potentiometer
----------------- -----------------------
A----| 620 ohms |--------| 2K ohm potentiometer |----+---B
----------------- ----------------------- |
^ |
| |
+---------------+
Then, if the pot gets dialed to zero ohms by accident, no more than
1.8 volts gets applied. To get a more accurate value, you should
measure the 3.3V rail on the motherboard near where R628 and R630
are located, then work out what the resistor should be.
Note - I am unaware of any information being available as to what
voltage this is supposed to be. Nvidia doesn't give out datasheets
to non-paying customers, so I'm not likely to run into the info being
posted on the net. The circuit should only need a tiny tweak, and
I'm assuming 0.2volts is enough boost.
Also, when soldering the 620 ohm 5% resistor plus the potentiometer,
use #28 or #30 gauge wire, like wire-wrap wire from Radio Shack.
The reason for this is, the wire is weak, and will bend without
ripping components off the motherboard. Soldering the components
directly to the board can result in the mechanical load of the
component ripping the copper traces off the motherboard, which is
a BAD thing.
So, to do the mod, you need:
Soldering iron, solder, strippers, #30 insulated wire
Voltmeter, 620 ohm 5% resistor (1/4 or 1/8 watt are fine), 2K pot
and a lot of patience.
