GeForce 8800 GS/GT/GTS Voltmods

Introduction

Got an itch to push your new NVIDIA 8800 GS, GT or GTS 512MB a little by doing a voltage modification? Well, you're in luck. All you need is a soldering iron and some simple resistors.

Disclaimer:
Performing the voltmod will void any and all warranties of your video card.
You - and only you - are responsible for any damage caused indirectly (or directly) by these modifications.

Overview

Backside view of a GeForce 8800 GS/GT/GTS 512MB with an NVIDIA reference design circuit board.
Note the chips of interest (blue arrows) and vGPU and vMEM measurement points (green arrows).

GPU Voltage Mod

Locate the chip labeled "Primarion PX3544" on the backside of the card. Solder a wire on the middle pin and another one on one of the outer pins of a 500 Ω variable resistor. I personally like to use those pins that make resistance decrease when the tuning screw of the VR is turned clockwise consequently increasing voltage. Cut or insulate the 3rd, unused pin. Use the resistance measuring mode of your digital multimeter and tune the VR to maximum resistance (very important!). You may want to write down on a piece of paper which way you need to turn the tuning screw in order to decrease resistance. Solder the free ends of the wires on the pads as depicted in the instruction picture above.

Tune down the resistance slowly to increase voltage. Don't be worried if the first few rotations don't seem make any difference, this is normal. The more rotations you turn the faster vGPU increses - so tread carefully.

Always monitor the resulting vGPU with your multimeter while tuning. Red probe to the point marked with a green arrow labeled as "vGPU" (see paragraph: Overview), black probe into a ground (Molex connector, eg.). Default vGPU voltage for the GeForce 8800 GT is ~1.10 V under 3D load. 33% overvolt (1.463 V under load) should be enough for maxing out the GPU under conventional cooling methods. Note that vGPU increases spontaneously by ~0.05 V when entering 3D load.

Caution!
Higher vGPU voltages can cause the heat output and temperature of the GPU chip and the voltage converter circuit to increase dramatically. Make sure you provide adequate cooling on the whole card when overvolted and overclocked.

Memory Voltage Mod

Locate the chip labeled "6549CBZ" on the backside of the card. For this modification use a 20 kΩ variable resistor. Prepare the VR as described earlier. Solder the wires on legs #4 and #7 as per the instructive picture above.

Monitor the resulting voltage with your multimeter: Red probe to the point marked with a green arrow labeled as "vMEM" (see paragraph: Overview), black probe into a ground.

Reverse memory voltage mod

Certain memory chips such as Qimonda HYB18H512321BF perform sub-optimally at default vMEM voltage of 2.0 V (±3%). Higher overclocks can be achieved by slightly lowering the voltage fed to the chips. Reverse vMEM mod targets the very same Intersil 6549CBZ as the more conventional vMEM mod - but note that the reverse mod uses different legs on the chip (#4 and #13). For the modification use a 50 kΩ variable resistor. Every card and batch of GDDR3 RAM is unique for what comes to it's quirks - and blessings - so there is no universally applicable optimal vMEM voltage. You may be able to find a sweet spot from between 1.80 V and 2.00 V that allows a higher RAM overclock ranging from 50 MHz to even 100 MHz - or nothing at all.

GPU Over current protection Mod

High overclocks can increase the GPU's power consumption to a point that the built-in over current protection of the vGPU phase controller is triggered. OCP manifests as a sudden blackout during 3D load accompanied by a collapse of vGPU voltage to ~0 V. Reports indicate OCP activating at frequencies exceeding 850 MHz for GPU and 2 GHz for shaders respectively. Users with 2-phase GeForce 8800 GS and GT are more likely to encounter OCP compared to the owners of 3-phase GeForce 8800 GTS as higher number of phases means lower current flow per phase. Luckily, there is a way to push the OCP-trigger point by altering the resistance of certain capacitors lingering around Primarion PX3544 regulator chip. Note that reference design GeForce 8800 GS and GT cards have two "OCP capacitors" as these cards have a 2-phase vGPU regulation. The GeForce 8800 GTS 512MB on the other hand is a native 3-phase design and consequently has three "OCP capacitors".

Refer to the pictures above for a demonstration of the mod.

Default resistance measured across each of the surface mount capacitors is ~1.2 kΩ. In order to push further the OCP trigger-point this resistance needs to be lowered - this can be done by soldering resistors of appropriate values on top of the capacitors. Resistors rated at 4.2 kΩ should give enough headroom for most overclocks. Minimum recommended value for the added resistors is 1.2 kΩ. It is not advised to go below this value as these capacitors are also used for balancing the outputs of the vGPU phases and lower resistance only interferes with this function but yields no practical gains.

Note that OCP mod does not increase your overclock unless your card actually is limited by OCP:

• Sudden blackouts during 3D load accompanied by freezing/resetting
• Collapsing vGPU voltage