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Old 01-27-2010, 03:09 AM
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TiN TiN is offline
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Lightbulb Gigabyte GTX275 non-ref mods, subzero pretest

I was overclocking GTX 275 with video live broadcast on 26 January, and here is report of that event.
Almost all pictures are clickable/enlargable.

Left side of top vcard

I had left 10L on LN2, and not to spoil overclocking spirits decided to try on Gigabyte GTX 275 overclock GPU. This vcard based on non-reference design, and got to my hands already damaged by non-careful modders, but functional good.

It's rev.2 card from Gigayte, so used Intersil ISL6327 6-phase voltage regulator for GPU and uPI6205 for memory. It's 1.7GB card, with 448b bus, as usual 275. I had some crap LN2 pot for GPU to try. Before start I checked voltages.

Lurking ISL6327 datasheet, we know that output voltage for GPU set by 8-bit VID signals, which hardwired to GND or VCC(+3.3 level) to get logic one or zero state. Stock VID[7..0] is 01000110, which sets out to 1.175V. In need of decent clocking on G200 B1 we need 1.55-1.65 V on GPU. Stock volts of drams are 1.99.

So I planned next mods for this vcard.

Vgpu to 1.58V
Add easy indication LEDs to see current state of GPU VRM
Vmem to 2.2+V with trimpot
Protection override
Extra bulk caps for power storage, and extra cercaps for decoupling.

So lets start reading teh datasheet to find out how to regulate Vout for GPU.

From DS we get, that OVP (overvoltage protection, which shutdown whole power) on controller trip set to VID+150mv. Most simple, foolproof method - is to get VID changed to more. So done this, VID6 was set to zero, which now render new VID[7..0] code 00000110, which from table in DS mean 1.57500V.

I've also added a bit offset to this VID, still to fit margin of OVP. OFS pin 10 on ISL6327 sets offset voltage (add extra/or subtract) to VID voltage. To add volts needed to put resistor to VCC (+3.3v). And calculate by easy formulae:

Rofs = ( 1.6 * Rref ) / Vextra

to subtract need to add resistor to ground and use:

Rofs = ( 0.4 * Rref) / Vsubtract.

Rref on this GTX 275 is equal to 1K.

Rofs of 30k was added as in schematics below. REF resistor is already present.

So this added volts a bit to 1.581V

Adding whistles aka LED's for status indication.

I wanted to make some more confort during bench, to see if DC-DC got latches by OCP/OVP. And ISL6327 have some outputs for saying it's status to outer world. To not overload outputs I've added NPN SMD SOT23-3 transistors with series resistors and LEDs wired to VCC.

R7 and R8 are for limit led current. R10 and R9 limit base current sink from ISL for NPN Transistors.

Sorry for mess on PCB Rofs was put under blutack on photo here.

After all the stuff I've sucessfully got 1.581V on GPU die. Note the green led lighted, that one hoocked to VR_RDY pin (which mean voltage regulator ready and okay).

Mem voltage mod

As for memory - it's really dumb easy. Just FB (feedback) sense pin trim to ground. It's tied to GND by onboard resistor R817. Stock resistance = 380ohm. So i soldered 10k trimmer set to 3.3k + series 480ohm 0603 SMD resistor (for protection) to GND. That trimmed FB resistance to near 330ohm.

This gave us ramp to nice 2.22V on Hynix DRAMs.

Protection override

Reserved to next try. During this test just test if OCP/OVP occur with VID+ a bit offset.

Extra bulk caps for power storage, and extra cercaps for decoupling.

Some was already there

I've added big polymer caps for energy storage.
Used two Fujitsu 4v 820uF, Nichicon's 820uF 2.5V, OSCON SEPC 820uF 2.5V and 16V OSCON and Chemicon.
For drams power DC-DC used extra 220uF tantalum and Rubycon MFZ 2700uF 6.3V.

After insulation it look much ghetto. No cooling for NVIO, it stay nicely cool with frozen PCB.

After initial checks test rig was assembled

EVGA E760 modified
3x2G Goodrams 2000
Gigabyte GTX275 overclock rev2.0
DIY copper pot
Enermax ERV1250EGT
XP32bit + 195 forcewares
UEI DT200 for temp reads on GPU and CPU pots.

Rig during progress...

As a final from 10L of LN2 and first GT200 oc i got 8th place in GTX275 category with 352276 Aquamarks.
CPU clocked to 5047MHz 1.52v -80C, GPU near 850MHz/1400 memory, with 1.58V/2.22V and -25C.

Had runs with 950MHz GPU, but wacky score/non-stable system. At 1G it don't pass AM3. So need to mod more later.

Soon after this result run out of LN2.
Next time with OCP mod for every phase + current measurement system for GPU (to get exact current values, what oced G200 consume).

Will oc this card furter this or next weekends with 50L on LN2.
Stay tuned.

Last edited by TiN; 01-28-2010 at 12:02 AM.
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Old 01-27-2010, 05:48 AM
Shammy Shammy is offline
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attaboy adding 0402 components!
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Old 01-27-2010, 05:55 AM
FatAlbert FatAlbert is offline
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it was so cool to see ya benching

waiting for revenge on this card :P
no cash Venom Lover <3

I don't write posts because I have nothing to say when you are extreme I am a noob without good hardware to extreme oc and only with dreams
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Old 01-27-2010, 09:14 AM
MaJ0r MaJ0r is offline
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I hope everything will be well with VGA )) And it won't die
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Old 01-27-2010, 09:39 AM
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Karmakazi Karmakazi is offline
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excellent guide and writeup TiN!

Can you explain why you chose the specific capacitors you used? Are some better for a certain application than another?

Originally Posted by glenboy View Post
super shammy ,you're a mad scientist, i always like looking at your bench setup it always looks like you trying to resurrect frankenstein
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Old 01-27-2010, 01:07 PM
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k|ngp|n k|ngp|n is offline
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Sick Tin. Wait till you get a new gpu pot on that monster you made
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Old 01-27-2010, 05:12 PM
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Dualist Dualist is offline
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Tin I love your work mate I would love for you to mod the heck out of my 5870.!
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Old 01-30-2010, 02:28 PM
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Putittogether Putittogether is offline
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It is clear to me after reading TiN's and Shammy's mod post's, I need to spend a little time on Wiki...
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Old 02-05-2010, 10:20 AM
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TiN TiN is offline
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This card will be tested tomorrow all day long with LN2 + 3540 CPU LN2 too.
Will be LIVE since GMT+2 at 10.00 AM
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engineering, gtx275, ln2 oc, modifications

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