7800GT Volt-mod

hardnrg - 2012-10-19 08:15:18 in Video Cards
Category: Video Cards
Reviewed by: hardnrg   
Reviewed on: October 10, 2006

Guide by: Steve Goossens
Original Date: October 10, 2006


Do not attempt this mod unless you are confident in your ability to solder very thin gauge wire to surface mounted components. There is a VERY HIGH RISK that this modification can end in the card not functioning due to components being damaged from excess heat, solder bridges, and adjusting the variable resistors too quickly. It almost goes without saying that this mod will void your warranty, so you should only do this mod if you are willing to accept that you might kill your graphics card. Do not attempt this modification without ample cooling. You may get away with using an air-cooled heatsink but I would recommend you consider a gpu waterblock and copper ramsinks (or indeed a waterblock that also covers the ram).


Right, so most of you reading this guide will already know about volt-modding and may well have done it before on previous graphics cards. If you don't know, volt-modding involves soldering variable resistors to your graphics card to increase the voltages to the gpu and the graphics memory. The 7800GT has stock voltages of 1.4V gpu and 2.08V ram. The gpu voltage can be increased to 1.5V by editing its bios and the ram can also be increased with graphite - i.e. "pencil-modding" - so if the thought of soldering to your beloved graphics card(s) worries you, then look into bios editing using NiBiToR and the pencil-mod. The 7800GT really benefits from more voltage, so for maximum overclocking a hard-mod is required.

What you will need

Hardcore soldering:

The success of this mod really relies on your skills and confidence with a soldering iron. I use a 50W variable iron with a tip tapered much like a pencil.












Tip: Use a fine-tip soldering iron bit


Even for someone with soldering experience, this is possibly the most difficult soldering task, so if you are unsure about your ability then go practice on an old network card or something else with small surface-mounted components.

Reference photos:




These three pictures show the locations of the solder points. The first is an overall view of all the points, the second is a close-up of the gpu points, and finally the third is a close-up of the memory points.

Placing the components:

You have two variable resistors and a molex connector to place on the card.











The essential ingredients for this mod


It makes the most sense to place these components along the top edge of the card to give the best access when the card is installed.


Place the components in the most convenient location on the card


I chose to place the resistors all the way over on the right as there is a blank area of PCB here to make it easy to attach them. Similarly, I placed the molex on another blank area of PCB but over to the left because I have to avoid the tubing for my watercooling. So you can see that it would be a good idea to decide on the location of these components with the card still installed to avoid any problems with nearby fans or tubing etc.

Preparing the resistors:

First you need to turn the resistors all the way to the left (anti-clockwise / counter-clockwise). The reasoning behind this is that you want to set the resistor up so that turning it clockwise increases the voltage, and turning it anti-clockwise (counter-clockwise) decreases it. This follows the logic of screws, taps (faucets), and volume controls so the added voltage controls act in the same way.













Use a small screwdriver to set the variable resistor


With the multi-turn resistors they don't stop turning once they get to one end so you will have to confirm that they are indeed at the end by testing the resistance between the centre leg and each outer leg. So after turning the 50Kohm resistor to the left many times, check the resistance for ~0 ohms on one pair of legs, and ~50Kohms on the other.

Testing the position of the 50Kohm resistor


If the resistances aren't around 0 and 50K then keep turning until they are. Then do the same for the 10Kohm resistor.

Testing the position of the 10Kohm resistor


Once you are sure that both resistors are set to their leftmost position you want to keep the legs with the maximum resistance. So the pair of legs with ~50Kohms for the 50K resistor and ~10Kohms for the 10K resistor. Cut the other leg off after you have double-checked which legs to keep.

Next you need to prepare some lengths of the thin-gauge cable long enough to reach from the resistor location on the card to the respective solder points. Strip the wire, tin it, and cut some heatshrink and place it over the wire before soldering to the resistor so that you can slide it up and shrink it after soldering.

"Helping hands" soldering tools really come in handy here (pun not intended, honest!)


After soldering each resistor and shrinking the heatshrink you should end up with something like this:

10K and 50K variable resistors prepared


I have skipped over the preparation of the molex connector as you only need to crimp two of the pins onto a pair of cables. If you can't do this then just give up now. Place the pins in the molex so that the left corresponds to the memory and the right corresponds to the gpu reading point. That way it matches the resistor location so that you can take readings from and make adjustments to the left pin and resistor (memory) or the right pin and resistor (gpu).

Soldering to the card:

And now the hard part... First you need to prepare the solder points. Both measuring points and the 10K ground point benefit from a little scraping with a knife as there is almost certainly a thin layer of PCB lacquer. The other solder points (i.e. the surface-mounted resistors) you can just leave as they are.














A magnifying glass can be used to confirm the position of the wires prior to soldering.


I found a magnifying glass was useful to line up the cables in the right position, but the change in magnification meant that it was easier just to solder with the naked eye. I do have very good eyesight though and many people would probably find this a lot easier if they used a magnifying lens of some description.

Just keep the reference pictures at hand, make sure the wires are lined up at the correct position, solder carefully and avoid excessive heat transfer to the surface-mounted resistors. Check the solder joints after for any signs of bridging to nearby components or any stray solder.

Attaching the components to the card

After soldering you should end up with the two resistors and a molex connector tethered to the card.

Soldering finally over!


Use hot glue, super-glue, or some other adhesive to fix the new components in place on the edge of the card. You should end up with something like this:

The volt-modded 7800GT.


Stand back and admire your work... But don't stand there too long as the mod is not over yet!

Testing the resistance:

Now you have to confirm that the mod was successful and that turning the resistors does have an effect on the resistance of the surface-mounted resistors that you soldered to. First check the 50K solder points (i.e. across that surface-mounted resistor). The original resistance is 620 ohms. Turn the 50K resistor clockwise until the resistance starts dropping. Tune the resistance down to 600 ohms.
















Then do the same for the gpu surface-mounted resistor (across the resistor at the solder point) and tune it down to 220ohms.


Tuning down the memory resistance.


Then do the same for the gpu surface-mounted resistor (across the resistor at the solder point) and tune it down to 220ohms.



Tuning down the gpu resistance.


If the resistance does not drop or you get values that are way off what you see here, check your solder joints and check for bridging. I had a small bridge near the memory solder point which gave me a reading of ~62ohms. Checking your resistances is vital before reinstalling the card.

Increasing the voltages:

On-the-fly voltage adjustment is both very convenient and also potentially dangerous. Reinstall the card and connect the ground probe of your digital multi-meter (DMM) to a black pin of a spare molex connector on your psu. Warning: make sure the DMM is set to read DC voltage and not resistance or current! Connect the red probe to the memory voltage reading pin on the molex that you modded onto the card. Boot up the computer (after a possible short and silent prayer) and let the computer idle at the desktop. You should get a reading around 2.15V from the preparation before. The memory only has one setting unlike the gpu so you can adjust the voltage without having to run any 3d applications or games.



Make sure you are adjusting the correct resistor and taking the corresponding voltage reading. Turn the resistor clockwise very slowly until you start to see the voltage increase. From that point you can then increase or decrease the voltage but I would recommend you continue to turn the resistors slowly to avoid increasing the voltage to a point where it causes damage, and likewise, to avoid decreasing the voltage to a point where the graphics card crashes when heavily overclocked.

For the gpu YOU WILL HAVE TO RUN A 3D APPLICATION TO ADJUST THE VOLTAGE otherwise you will be adding to the lower 2d voltage and when the card enters 3d mode the voltage will jump to a setting that will almost certainly damage your card.

If you are running an SLI configuration then you will have to run an application or game that uses an SLI mode in order to get the second card to use its 3d voltage.


I would highly recommend some form of temperature monitoring when running the card with the extra voltages. RivaTuner has temperature monitoring and logging for the gpu core. The memory is not as much as a problem, just make sure it is sufficiently heatsinked and it has a sufficient supply of fresh air and they should be fine.


Now you can overclock your graphics card much in the same way as your cpu and ram! The extra voltage means you can increase the gpu and memory speeds beyond those that you achieved on stock voltage meaning higher performance that could potentially rival or even surpass that of an overclocked 7800GTX!

Before the mod I could only run speeds of 508 MHz core, 1.180 GHz (DDR) memory using the 1.5V GPU bios volt-mod. This was in SLI configuration so the clocks were slightly lower than a single card (520 / 1.200). After the mod I could successfully run the cards in SLI at 540 MHz core (+40 MHz delta) and 1.300GHz DDR memory when using 1.71v gpu and 2.35v memory.

Here are some comparisons of the benchmarks scores of my 7800GT SLI setup with the maximum overclocks before and after the volt-mod:







A comparison of the maximum scores achieved before and after the volt-mod

The performance gain may look small, but remember this is the increase over the maximum overclock before the volt-mod. Now the scores are high enough to compete with 7800GTX SLI setups!