THANKS TO A reader, we learned that the modified video BIOS for Radeon HD 3870 has been posted on a website in Finland and later, on TechPowerUp!.
This latest ATI baby has a clock-lock at 862MHz for the GPU, 85MHz faster than default frequency. The RV670 GPU has multiple power phases internally, and PowerPlay is activated all the time. When you overclock the GPU, the PLL VCO divider goes bananas, which will overvolt the card just enough to cause a lockdown.
One solution to the problem is a rather elegant video BIOS upgrade. This upgrade changes the dividers in the GPU, so you're free to go crazy and unlock the full potential of the card. You could tinker with the card to clock the RV670 chip all the way to 900MHz, massive 123MHz faster than the default clock. If you want more, you have to splash out on a waterblock, but it seems that both Nvidia and AMD have manufactured extremely OC-friendly chips.
At the time of writing, the enthusiast community is waiting for appropriate BIOS flashing tool, but the link below is where the flashing utility should appear first. It is still not available through official channels, but that is just the matter of time. µ
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