XFX 4830 vmod?

[celemine1Gig]

OK, so the value we're looking at is 2.56k unmodded.

That means the best would probably be to use 2 100k potentiometers connected in series (i.e. 200k resistance but with much better precision), connected between pin#13 and pin#4 or any directly connected solder points/pads.

The rule to work out the resistance is the simple resistance in parallel rule.

You want to get a resulting resistance, when using the formula, that means not too much change in comparison to the original unmodded resistance(assuming the parallel resistance is set to its max!).

That means in detail:

R1=unmodded resistance = 2.56k
R2=parallel modding resistance(s)/potentiometer(s)= X (what you are looking for)
The rule of parallel resistances says that the resulting resistance will always be lower than the lowest of the two original resistances (the ones you connected in parallel; R1 unmodded on the card and your parallel conected R2). --> if you don't want the final resistance to be much different from R1, R2 has to be much bigger than R1. BUT, don't choose R2 too big, as potentiometers are usually very non-linear in the lower resistance range(i.e. sudden resistance decrease and following voltage bump in the last 10-20% of the turns, if you are not careful and chose R2 too big).

--> using the formula (R1 * R2) / (R1 + R2) = Rmodded with R2 = 200k --> Rmodded is ~2.53k which should be OK, as it's only 1.2% lower than the unmodded resistance. 100k would be a bit too low for my liking, but it depends on the voltage you are looking for effectively. If you always want to use voltages higher than default then a bit of overvolting, even with the poti(s) at max won't hurt anyway.

You said that 2.56k (unmodded) means 1.2V and 2.1k gave you 1.45V.
2.1k / 2.56k = 0.82 --> voltage increase should be about 21% (1/x)
1.45V / 1.2V = ~1.21 --> looks about right, doesn't it.

With the formula you could also compute the needed value for a fixed resistor needed for a certain voltage.

All simple EE basics.


Oh and BTW, the card looks like it could need some capacitor mods. Looks like XFX intentionally left some solder pads unused, to save money and maybe even to cripple the card clocking performance. Could be worth it to try some additional high frequency ceramic and some polymer caps in the right locations.