Lower VCORE on A7V8X?

Discussion in 'Asus' started by LarsJ, Apr 28, 2004.

  1. LarsJ

    LarsJ Guest


    I have an ASUS A7V8X motherboard and I have recently bought a CPU Athlon
    XP-2400 Mobile 35 W.

    I want to run a cool and quiet PC (not overclocking) and therefore I want to
    lower VCORE as much as possible.

    This CPU needs a VCORE on only 1.35 V. When I test this CPU on my A7V8X I
    realized unfortunately that the lowest Vcore I can set is 1.525V. Is there
    an easy way to lower VCORE further? I have upgraded BIOS to the latest 1013.

    I saw in this newsgroup some months ago how you can increase VCORE with a
    resistor between pin 9 on L6917BD and ground. Can I just do the opposite and
    connect a resistor between pin 9 and Vcc (12V)?

    I want to have a possibility in BIOS to change between 1.2V and 1.4V.

    I wonder now if anyone has tested this approach and if so, the value on the
    resistor you have chosen. I would be very grateful if anyone has some
    thoughts about this subject.


    Lars J
    LarsJ, Apr 28, 2004
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  2. LarsJ

    Paul Guest

    I see a couple of possibilities. The solution requiring the least
    knowledge, is to pry the five VID signals on the Vcore regulator
    chip, then control them with a five position dipswitch connected
    to GND. Prying the pins up is necessary to isolate any GPIO signals
    that the BIOS is using to drive those pins. (Use solder wick to
    remove the solder, and carefully pry the pin up a fraction of an
    inch with a dental pick, while heating with the soldering iron
    to free it. Too much prying will snap the pin off, or rip the
    copper foil, so be careful). If the BIOS has an option to modify
    the VID voltage, then there have to be some GPIO pins connected
    to the VID lines, to force them, and you don't want to damage
    those signals if possible.

    If you want to take the analog modder route, then a resistor
    connected to a regulated rail voltage higher than the output
    voltage should work. The +12V on a power supply will wander
    around a good deal (+/- 5%), while some voltages regulated
    by the motherboard will be much more stable. Your wiring
    must be kept short, to keep the wire from becoming an antenna.
    And, somehow, you'll need to figure out just how much resistance
    is required. (Search some of the private forums, to see if
    someone has already figured this out.)

    The thing that makes me nervous about the analog mod route,
    is adding the resistor could be affecting the stability of
    the control loop. If you can find instructions on a private
    forum of someone who has succeeded at undervolting this
    board, that is probably better than me guessing at whether
    it is going to work properly or not :)

    If you plan on using a potentiometer to dial the resistance,
    remember that dialling the device to zero ohms could
    destroy something on the motherboard. There should always
    be a limiting resistor in series, to provide a measure of

    So, the first part of your mission, is to find a locally
    regulated voltage on the board, like the +2.5 used for the
    DIMM or a local +3.3V if there is one. (By using 3.3V,
    the resistor value used for the voltage boost should be
    in the same ballpark as the resistor value needed to cause
    the output voltage to drop, so that is another advantage
    of a lower voltage like that. The resistor value would be
    a lot higher, if you are connecting to +12V or the like.
    Like (12V-Vout)/Vout times higher. Evaluating that
    equation for (3.3V-1.65V)/1.65V gives a factor of 1X
    compared to the "resistor value to GND" mod.)

    For that amount of messing around, I think the digital
    approach is less trouble. Practice your soldering skills
    on something else first. I have a lot of soldering experience
    with more robust boards, and I found the Asus board I worked
    on, to use thinner copper foil than I am used to. Delamination
    of the foil is pretty easy to do. And those tiny pins
    cannot be bent through too large an arc, before they
    snap off. Enough clearance to slide some mylar film under
    the pin as an insulator, is all that is needed.

    An Uber-style BIOS would be another way to get there. If
    someone has freed up some settings with MODBIN or the like,
    then a modified BIOS might open up more options. The A7N8X
    BIOS has been hacked, but I don't know if the A7V8X has been
    blessed this way or not.

    Paul, Apr 29, 2004
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  3. LarsJ

    LarsJ Guest


    I want to thank you for your answer and your valuable advices.

    I think I am going to do the digital mod which feels safest.

    Now I am going to practice to solder on an old, not functioning motherboard
    to increase my skill on SMD components. I hope I can report in this thread
    about a successful mod in the coming week.


    Lars J
    LarsJ, May 1, 2004
  4. LarsJ

    LarsJ Guest


    I have now modded and tested the motherboard and I am very satisfied with
    the result. I now have VCORE =1.25V
    and the CPU runs very cold at 1.6GHz. The computer is also very stable and I
    have not have a single blue-screen or hanging.

    The modding was rather difficult because of the small sizes of components
    and wires but with patience and steady hands I succeded.

    /Lars J
    LarsJ, May 20, 2004
  5. LarsJ

    Paul Guest

    Did you have any trouble getting the pins free of the board ?
    And, did you use switches, or just hardwire the five VID pins
    to GND or open, as appropriate ?

    Also, does this allow using less fan on the CPU ?

    Paul, May 20, 2004
  6. Wow, Id love to do this with my A7v8x-x but it does sound tricky.
    Creeping Stone, May 23, 2004
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