Convert your 7900GT into a 256Mb 7900GTX

[t024484]

The biggest functional difference between the GT and GTX is that the GTX switches the Vgpu between 1.2 in 2D to 1.4 Volt in 3D.
Thats why the GTX can be clocked at 650 Mhz in 3D.
EVGA has their 7900GT-SS board with increased hardware in the volt mod area, which also enables the switching from 1.2 in 2D to 1.45 in 3D.

My firm believe is, that this voltage switching extends life of your GPU, since most of the time you are working in 2D, and thus you will have the 1.2 Volts on the GPU.
This can be the only reason that I can think of why the 7900GTX as well as the EVGA7900GT-SS have this feature.

I have made the same voltage switching option to a standard 7900GT. This modification can be applied to any standard 7900GT board. You will need a BIOS that switches from VID 0 in 2D to VID 1 when in 3D mode. You can check if your BIOS does this, with the monitor function of Riva Tuner going from 2D to 3D.
You could use the EVGA 7900GT-SS BIOS for this purpose, but you can just as well change your own BIOS with Nibitor, using the advanced voltage editor.
You will also need some soldering skills.

The components that I used, are 2 dual schottky diodes, type BAT54C, and 1 N type Mosfet BSS670S2L, that's all. I bought them from Farnell.com.
Any dual schottky diode with common cathode will do , and any low power N channel Mosfet with less then 10 Ohm channel resistance when switched on is O.K.

The board in the unmodified state looks like:



And after Modification:



Red gives you +0.05 Volt
Yellow gives +0.1 Volt
Purple gives +0.2 Volt

So with the standard 5Kohm resistor giving you 1.2 Volt in 2D, you can now switch to anything between 1.2 and 1.55 Volt in 3D.

If you change to 50 Kohm, you can go anywhere from 1.5 to 1.85 Volt.

Do not bother what Nibitor is telling you what the exact 3D Voltage is, although it should be different from the 2D voltage, with the advanced editor you can give it the value you like. The thing that is important, is that in 3D the GPU switches to VID 1 from VID 0 in 2D.

With these additions, your board is functionaly 100% comparable with a 256Mb GTX.