How To: Voltage Mod Your Sapphire HD3850
03.03.08 - 09:43pm
There comes a time when your need for speed will outstrip your computers ability to supply it and drastic measures are called for. With graphics cards you are often limited in both cooling and voltages with the stock implements given to you by the manufacturer so you must take matters into your own hands. Here I will be detailing the process of modifying and utilizing the voltages possible with the HD3850 and HD3870 video cards. These cards feature the RV670 core, a derivative of the R600 core, and with this heritage comes an excessive thirst for voltages and clocks.
Sharpen Those Pencils
The basest of all modifications is the venerable pencil modification. The graphite in pencil “lead” is rather conductive and makes the perfect material for making partial bridges between components or across resistors. Pencil mods are easily reversible but very coarse in their adjustments and fraught with issues of magically disappearing with the swipe of a sweaty finger. To start you’ll need a relatively soft(HB to 2B, #2 to #1 hardness) pencil and a steady hand. With the HD3850 vGPU mod you are simply adjusting the resistance between the feedback pin on the voltage regulator IC and ground. By default the resistance is set to 5120 ohms, lowering this resistance increases the vGPU while raising this resistance decreases vGPU(who would want to do that?). In the image labeled vGPU Mod, you will want to take your pencil and scrape it between the points that straddle the blue point. The bottom point is connected to the ground plane while the top point is connected to the feedback pin. Carefully pencil in a trace between the points and the resistance should drop. Compare it with the rudimentary chart below for a general idea of where the voltage should range.
Soldering Minuscule Points
If you fancy more professional modifications with the ability to adjust the voltages mid-bench, this mod is for you. For this you’ll need a 50K Ohm or 100K Ohm variable resistor, two pieces of thin 24 gauge or thinner wire, a soldering iron and solder, and a steady hand. Just like with the pencil mod above, your goal will be decreasing the resistance between the feedback pin and ground but you will be able to accurately tune the resistance with your resistor. In the image above you will want to attach a wire to either of the points straddling the green dot. Luckily for you that gives a large area to splatter with solder, either of those points will do, I literally bridged them with solder and then stuck the wire on the bridge. Your second wire will go to the ground point of your preference, I used one of the screws on my custom mosfet heatsinks as they are attached to the ground plane. Then you will connect one leg of your resistor to the ground plane, one to the feedback pin, and begin tweaking your voltages. I suggest starting around 50K Ohms and working your way down slowly. A quality multi-meter will be crucial here, I cannot stress how important accurate measurements are. Remember to not measure resistances with the card powered on, you can toast it.
Results
The HD3870 and HD3850 share nearly identical PCBs and cores with the HD3870 being slightly higher binned. This equates to the HD3850 being the runt of the litter and taking a healthy bit of voltage to scale. Thankfully these chips run cooler than the HD2900XT, on my Accelero S1’s they barely hit 50C with a single 12″ deskfan slowly blowing over them and 20C ambients. That being said, they still scale incredibly well with well over 1GHz possible on air. I was able to bench at 1014MHz core, 945MHz memory with 1.55vgpu but such speed wasn’t necessary for daily usage and such voltages will easily kill these cores. The daily settings are 1.425vgpu and 904MHz core, 925MHz memory which is a rather satisfactory overclock. Below is a chart with corresponding feedback resistances and voltages to help you with your tuning. Goodluck and be careful when soldering, I am not responsible for any damage you do to your card while following these directions. If you toast your card, do not RMA it, you burnt it up yourself. One last thing, no, I will not vmod your cards for you, too much risk in that but thanks for thinking about it.
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