What you have there is a voltage divider. I'm going to be a pain in the rear and make you do your own work, because that way you'll be able to do it in the future
If you aren't familiar with voltage dividers, start here:
http://en.wikipedia.org/wiki/Voltage_divider
The long and short of it is that you have a voltage that comes in, via a resistor, you have the read point in the middle, and then you have a resistor connecting to ground.
The output voltage is calculated by this equation(R1 is from the voltage source to read, R2 is read to ground):
vOut = R2 / (R1+R2) * vIn.
Given a source voltage of 2v and two 1k resistors, you have: vOut = 1000 / (1000+1000) * 2. vOut = 1.
Now that we've done the voltage divider bit, here's how the card is wired!
There's a resistor connecting the FB pin to vGPU, that's the voltage source.
There's a resistor connecting the FB pin to GND.
vOut goes into the FB pin, depending on the controller the voltage it is looking for will be different, let's pretend it's 0.7v for this example. It probably isn't on that card. Let's use 2v for vGPU (or vRAM, or vWhatever).
We know the GND side is 0.99kOhms, let's call it 1k for giggles.
As we know three variables, we can compute the high side (voltage source) resistor. It's 1.86kOhms. (I recommend
cheating on the calculation).
If the FB pin sees a voltage lower than 0.7v, the controller raises the voltage applied to vCore, until the FB pin sees 0.7v. If it sees more than 0.7v it lowers the output voltage to vCore. It does this on a continual basis, once per PWM clock cycle. (Essentially)
Continuing to the voltmod part, what we do on a voltmod is lower the resistance to ground, changing the voltage divider. If we change it so that in theory the FB pin sees 10% less voltage, it will raise vCore 10% to compensate.
In the above example that means we want to change the low side resistor so that the FB pin sees 0.63v. Going through the calculations that means we need to have 0.86kOhms (860ohms) on the low side.
We have two choices, we can either remove the 1k low side resistor and solder on an 860ohm resistor, or we can add a resistor that drops the resistance to 860ohms. To do this we add a second resistor in parallel, I strongly recommend
cheating on the calculation.
According to the above cheating link, a 6.2kOhm resistor in parallel with the 1kOhm resistor will result in 861Ohms. Now we have our 10% higher voltage!
We don't actually need to know what voltage the FB pin is looking for, we just need to raise it, whatever it is, X amount. For the calculators to work happily we need to know the high side resistor however, so measure from the FB pin to vGPU (or whatever). That'll give you the high side resistance, and you're off to the races!
So, give it a shot and let me know what you come up with (and what the high side resistance is) and I'll double-check it for you and off you go!
(With a VR of course you can vary the resistor you're putting in parallel with the low side, making it rather easier to find a 6.2kOhm resistor. In this example you could get a 6.1k by taking a 5k and two 2.2ks, put the 2.2ks in parallel with each other, and in series with the 5k. There are always options if you don't mind it being ugly)
