A tale of two 7900s ... you'll laugh, then cry

[graffen]

Thanks in advance for reading. If you have the patience to read this, you'll probably laugh at how thorough and plodding my planning/testing has been, only to see it all go down the tubes. In the end, I now seek advice: Is it my power supply?

I recently bought two (2) XFX 7900 GTs, one of which I purchased as a gift for a friend. While I had both cards in my possession for a while, I benchmarked them relentlessly, each card getting several benchmarking days in the same box. The two cards performed exactly the same in every manner, including temperature ranges in each card. My conclusion is that these two cards are thus identical in performance.

These XFX 7900s at stock are 470/1370 . Before any of the below, I extensively tested my 350W PSU for constant and consistent voltage on the +12V and +5 rails. Also, the machine has never had any power-related problems before. My PSU gives 18 amps (rated) on both rails, max combined is 28 amps (rated).

So I shipped off one card to my friend as a gift. He immediately used Coolbits on his card to o/c to 562/1612 . He didn't crawl up the clocks, either. He just went straight to 562/1612 using Coolbits recommendation. Zoom. All of his benchmarks increased 15-20% thanks to this sweet overclock.

I am decidely more cautious, so I inched mine up only a tiny bit to 474/688 ... an almost microscopic overclock. All was well after some days, so based on his experience I decided I would FIRST install a Zalman VF 900 Cu VGA cooler, and THEN follow in his footsteps on a path to a high overclock.

Well, the Zalman certainly does a great job of cooling. All my temp ranges slid colder by 16-20 degrees Celcius. Sweet! So now I was going to have a much cooler card while I started to overclock. This joy was my last moment of success -- it's all been a wash since then.

I used Coolbits to take it to 559/1600 (almost identical to my friend's o/c). Ran 3dMark05 and enjoyed about a 5% increase in performance. Hmm, well, not nearly the gain of my friend, but better performance nonetheless. Then the fun began...

I wound up 3dMark05 for another go ... result: Crash, BSOD, nv4_disp infinite looping. I recovered and reapplied drivers (I use DriverCleaner properly always) ... now for another 3dMark05 test. Result: Fizzle pop random pixelation all over the place, freezing. I cleaned with DC again, now using RivaTuner, and lowered the clock substantially to 500/730 ... result: black screen, monitor gets no signal. Keep in mind that I continue to use DC Pro so that I can start fresh as I switch between Coolbits and Rivatuner ... and by now I have worked my way downward on the clocks ... at this point today it won't even overclock to the microscopic 474/688 I had with the stock cooler a few days ago. "Test settings" for anything above 470/685 and sometimes, pixelation, sometimes black screen, but always Windows crashes.

So basically I am back to a cooler running card, but it is at stock timings, so that the Zalman cooler ends up being a useless expenditure.

Here are my questions:

(1) I did get a full run of 3dMark05 on my FIRST go at 559/1600. Why would it run once and give me a 3dMark score, and then refuse to run 3dMark05 again at any overclock?

(2) The Zalman cooler fan requires power directly from the PSU, and this has to be pulling more power than the stock fan did because the stock fan needed power only from the VGA board itself. Do you think this higher power draw with the new Zalman fan could point the way to a need for a new PSU based on the errors described above?

(3) I have the Zalman fan running directly from a 4-pin molex to 3-pin converter, so she's getting juice straight from the PSU. I have several other molex connector available in there, and I have read about load balancing and what not, but I don't understand it. Is there any point to my trying a different molex connector on a different line bundle?

Thanks again. All thoughts much appreciated.

[Spearball]

Well first off what kind of PSU you got there. To my nowing isn't 350 too low for two 7900 GPU's?

[graffen]

In answer to the question above (thanks for the reply) :
It's a Dell 350W PSU. Before any of the overclocking attempts, I extensively tested my 350W PSU for constant and consistent voltage on the +12V and +5 rails. Also, the machine has never had any power-related problems before. My PSU gives 18 amps (rated) on both rails, max combined is 28 amps (rated).
It's impossible to test amperage on the rail without being in-circuit (thus destroying PSU), however I did a thorough multimeter test of the voltage and it is good.

According to nVidia, the 7900GT draws 70W max. Also, keep in mind here this is ONE (1) 7900GT in the machine.

[flanzig1]

Even if the DC voltages are correct, could be getting more AC ripple under load. Depending on which 7900GT card you have, the mininum power is anywhere from 350 to 400W. I think with OCing the card, you have no head room left in the PSU.

[graffen]

Thanks to everyone for the replies. I've ordered an OCZ 520W Powerstream PSU and going to give it a go. This will give me 33 amps on a single 12V rail (though that doesn't mean much to me, the single rail vs. dual rail thing, I mean). It also has adjustable voltage knobs on the outside of the PSU (though I wouldn't adjust them without seeking help & understanding first). I don't know if the new OCZ PSU will result in my being able to overclock -- but I figure it's worth a shot. Price $99.99 with rebate. I always knew I would need to spend $100 for a quality PSU replacement.

[glc]

Overclocking with a power supply at or near published minimums, no matter how high the quality, is not recommended.

[graffen]

glc wrote: "Overclocking with a power supply at or near published minimums, no matter how high the quality, is not recommended."

That's also a good point. I am using a 350W and that is the min. recommended by XFX for the card. It'll be great if the 520W OCZ can help out in the headroom dept.

[graffen]

This is the original poster just following up on the resolution of the original problem. That problem was the discovery that I could not o/c my 7900GT by even one megahertz. I suspected the PSU was the problem. Thanks especially to flanzig1 & glc for helping me. I replaced the Dell 350W PSU with a 520W OCZ Powerstream power supply, and the problem went away. I now have the card o/c'd to 562/810 (from 450/685) and this provides about a 5% improvement in benchmarks and game fps averages. Ironically, even at this overclocked state, temps went down by 3 degrees in all ranges of idle and stress. I guess this has to do with the card fan receiving more stable power? The only disadvantage to the 520W OCZ is that it makes my CPU fan spin at astronomically faster rates when the CPU is very stressed. This isn't too often though -- the worst is during the CPU tests of 3dMark.

[rspassey]

Originally Posted by graffen This is the original poster just following up on the resolution of the original problem. That problem was the discovery that I could not o/c my 7900GT by even one megahertz. I suspected the PSU was the problem. Thanks especially to flanzig1 & glc for helping me. I replaced the Dell 350W PSU with a 520W OCZ Powerstream power supply, and the problem went away. I now have the card o/c'd to 562/810 (from 450/685) and this provides about a 5% improvement in benchmarks and game fps averages. Ironically, even at this overclocked state, temps went down by 3 degrees in all ranges of idle and stress. I guess this has to do with the card fan receiving more stable power? The only disadvantage to the 520W OCZ is that it makes my CPU fan spin at astronomically faster rates when the CPU is very stressed. This isn't too often though -- the worst is during the CPU tests of 3dMark.

\

Glad to see your problem got sorted out.

The CPU fan is probably gauged so that it runs according to the stress of the CPU - this keeps it quiet when you aren't doing anythind difficult. If it didn't pick up speed when the CPU is going 100%, then the temps would rise. Again, glad to see you've got it sorted out. There weren't any problems replacing the Dell PSU?

[Foosa]

That was a good read.

Congrats on getting it overclocked and working right.

[Chris_Rootes_Wizard]

Just read it all and i thought i'd say well done

[jfk]

Just ANOTHER example of what happens when your PSU (EVEN if "meeting" specs) is inadequate. Glad you got the problem sorted out.