GeForce 8800 GS/GT/GTS Voltmods

[largon]

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

[infrared]

Thanks largon, very useful article indeed.

[warhammer]

Nice article Largon, what benefits would this mod give me. I can push my GPU to 780 with no probs.

[largon]

warhammer,
780MHz at stock vGPU is really nice indeed, vGPU mod would allow you to push it well past 800MHz and with the right voltage I bet 900MHz could be gaming stable - assuming your card(s) are under water as well...
Shader clocks gain from vGPU too.

[wolf]

at 1.3vcore+ i hit OCP, so im keeping it under that atm, max benchable freeq's are 840/2052 core/shader. at around 1.3v, atm i run my old oc (760/1836) jsut with 1.2v as it was not completely stable before and loves some extra juice.

weird thing is i get the very low volume high pitch whine/buzz that other 8800GT owners have talked about, it did this the first day i got it with no o/c's and doesnt seem to effect system stability....should i remove all mods and RMA?

[warhammer]

Thank you for the explenation largon.

[largon]

Originally Posted by wolf

at 1.3vcore+ i hit OCP, so im keeping it under that atm, max benchable freeq's are 840/2052 core/shader. at around 1.3v, atm i run my old oc (760/1836) jsut with 1.2v as it was not completely stable before and loves some extra juice.

To be sure it's OCP that's limiting you measure the vGPU after the screen goes blank. When OCP activates vGPU goes to ~0v.

weird thing is i get the very low volume high pitch whine/buzz that other 8800GT owners have talked about (...)

The buzzing is nothing to be worried about. It's the vGPU coils that start resonating at high frequencies.

[wolf]

so no need to RMA at all? no sweat? cos i am a little worried....

[largon]

No worries, the coils that cause the buzzing are just simple coils of copper wire, they are pretty much indestructible so resonating will not hurt the card or the components. The buzz can be irritating though. :\

Pretty much all high-powered video cards do it anyways...

[wolf]

cheers man you are an absolute legend, for the article, the advice .... jsut cheers man

[candle_86]

Originally Posted by largon

No worries, the coils that cause the buzzing are just simple coils of copper wire, they are pretty much indestructible so resonating will not hurt the card or the components. The buzz can be irritating though. :\

Pretty much all high-powered video cards do it anyways...

my GS has yet to buzz

[infrared]

Get a really quiet system, and turn the fan down on the card and i can pretty much guarantee that you'll hear a faint high pitch squeel when you open the 3d veiw in atitool. It's just that some cards are slightly louder than others.

[albinowino]

First I must say this article is awesome and makes it all much simpler than reading through the massive XSF thread, thanks for taking the time to write it. I am wondering about the reverse VMem page however. From what I gather, this mod decreases voltage as resistance is lowered so to have the pot set up for CCW turns lowering the voltage, the pot needs to be wired differently. Do I have it right?

[largon]

albinowino,
That's right. Anyways, I edited the pics for vMEM and Reverse_vMEM according to voltage change and tuning direction.

[albinowino]

Excellent! BTW I like the renditions of the wires and pots. So much easier on the eyes than dots I'll be trying the VMem pencil mod to see if my GT responds to more VMem. Stock is 1.88v strangely.

[Titus]

Great thanks Largon

Nice thread ! I own a 8800 GS and i touch the limits wiyh it suddenly : i think i can push it more but not without a V-mod. I will take my soldering iron and follow your directives after a bios flashing test @ max actual stable frequencies.

[gOJDO]

Very good Largon!

But what about the Pencil/BIOS mods?

/me is making a custom water block but has no balls to do a hard mod on his GTS.

[largon]

Pencil mod is not possible, and IMO the only BIOS mod worth mentioning would be a GDDR3 timing-tweak if you got the dreaded Qimonda onboard.

[Titus]

Originally Posted by largon

... and IMO the only BIOS mod worth mentioning would be a GDDR3 timing-tweak if you got the dreaded Qimonda onboard.

If someone has an idea for a GDDR3 timing-tweak for Qimonda chips for my XFX 8800 GS bios modded

[nflesher87]

any recommendations on where best to purchase a soldering iron and resistors?
I have a DMM I can borrow from the physics department here at school

[phyx1u5]

Hi there,
Just a query with the reverse volt mod, do you set the resistance of the resistor to minimal then turn up the resistance? or start at 50k ohms and turn down the resistance?
Also is a 1k ohm VR ok for the vgpu mod? Im not planning to exceed 1.35volts loaded
Cheers

[largon]

Always set the VR to maximum resistance. And yes, 1kΩ is perfectly good for vGPU mod.

[Laurijan]

Thanks lagron for this thread and the one at xtremesystems..

[largon]

Cheers!

[Dar_T]

Hey largon, so I have my cards at 1.38v but every time I try to join a game in Cod4 it will use the over current protection on me. I was sort of unclear as to how to fix it in your OP. Since I have 8800 GTS and 3 points do I use two resistors? Which type of resistors (ohms).

Thanks