6950 + system hanging

[warsmith]

Hello guys.

I am mining with 2x unlocked XFX 6950's. Lately I've been trying to find an overclock that's stable with 63% fan speed (night-mode), but all of them are pretty much unusable, and the way it crashes the system is very weird - on a "bad" overclock firstly the cards heat up to working temperature (between 65-80 depending on mV and mhz) and then stay there for a while (from 1 to 60 minutes); then, suddenly temperature drops on a single card, and the hashrate halves. If I move my mouse, the computer hangs. If not - the second GPU continues to mine for a little longer (2-3min) before a total freeze.

What surprised me the most, however, was results I got from last night:  I let both of them run @ 1100mV(stock voltage) and with 900mhz core/625mhz mem and went to bed after 20-30mins of it being stable. It ran smoothly for an hour or so (according to BTCGuild logs) and crashed an hour later. I surely thought running it on stock voltage and such a low overclock (stock max is 840mhz core) will help the stability. I am sure it crashed the same way (single gpu halting for no reason, and then, even with 99% usage dropping temperature until a full system crash).




What could be the reason for such weird behaviour? What kind of overclocks do you have on your 6950s?

[Meatball]

I have a Sapphire 6950 2 GB with the Shader unlock bios and the best I can get out of it on stock voltage is 870/625 and it puts out just over 370 MH/s.  Any higher Core clock or lower mem clock and I get crashes.  At 70% fan I run around 62C

I can probably boost up the voltage and get more, but it's on my main PC and I don't want to kill the card.

[tunatime]

i have 3 6950s unlokced and i run them at 1.2v and i can get to 930 core and if i go to 1.25 i can get to 960 to pull about 410mhs per card. and 900 is still a lot for stock vcore the 6970 runs at 880 stock core at 1.17v so up you v to at least 1.175- 1.2 range (if you temps are lower then 80c) and then try for 900

[warsmith]

i have 3 6950s unlokced and i run them at 1.2v and i can get to 930 core and if i go to 1.25 i can get to 960 to pull about 410mhs per card. and 900 is still a lot for stock vcore the 6970 runs at 880 stock core at 1.17v so up you v to at least 1.175- 1.2 range (if you temps are lower then 80c) and then try for 900

Yeah. ATM I'm running them both 1175mV 925/625 for ~800mh/s total. With a fanspeed of 70% and an open window, they run at 60'C and 64'C.
Sad thing is, I cannot find that golden ratio between heat and speed when the fan speed is a tad bit lower (60-65%) - either it has to run the same 1175mV voltage and use quite lower clocks (~900mhz) with higher temperature (79'C+), or use near-stock overclock (~845) with stock voltage, which would let me use it bellow 60% fan speed anyway. All I'm trying to figure out, why there's such a gap in fan speed performance, and why it crashes so weirdly when approaching it.