Warning: 8800GTX volt mods

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cap232

n00b
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May 18, 2007
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After reading up what i could about volt modding the 8800 and being discouraged by some people concerned about my cards health! i couldnt resist the temptation to tweak...

The GPU volt mod guidance provided by an number of online sources suggests soldering a 500ohm VR across the resistor adjacent to Primarion PX3540 PWM IC. Or strike it with a 2b pencil to gradually lower resistance. Well i think some mods have been made to the board or this info is plain wrong. Firstly striking with a pencil wont work as the resistor has a coating which prevents graphite form the pencil sticking to it so it doesnt affect resistance one iota. Secondly, since the SM resistor in question is 470ohms if you solder a 500ohm VR in parallel to it (to ground) as suggested you will halve the resistance (1/(1/500+1/470)=242ohms) and massively increase voltage fed to the GPU (up 2.6v! when std voltage is 1.3v). Instead you need to solder in parallel a 20kohm VR, which gives you a more reasonable max resistance of 460ohms (1/(1/20000+1/470)), boosting voltage only slightly to start.

I wanted to share this info so anyone considering doing the same could learn what i have learned the hard way. If you do as i did and follow the instructions hitting your GPU with 2.6v isnt fatal if you switch off fast!!
 
cap232 said:
Secondly, since the SM resistor in question is 470ohms if you solder a 500ohm VR in parallel to it (to ground) as suggested you will halve the resistance (1/(1/500+1/470)=242ohms) and massively increase voltage fed to the GPU (up 2.6v! when std voltage is 1.3v). Instead you need to solder in parallel a 20kohm VR, which gives you a more reasonable max resistance of 460ohms (1/(1/20000+1/470)), boosting voltage only slightly to start.
Click to expand...

Thats assuming the voltage scaling is linear to the resistance.
 
No assumption needed, empiracly that appears to be the case, at least to 50% of starting resistance at any rate. No doubt it isnt perfectly linear as regulating power linearly is nigh on impossible given a variable load. I notice that at 370 ohms i get 1.6v to the GPU (a tad high but ok with good water cooling), under load it drops to 1.45 to 1.5v, the latter seems to be the comfort zone for max water cooled OC's out of the chip. Any more than that and some extreme cooling would be required. The regulators in PC kit are definitely built to a price, and a low one at that.
 
i dont think i will need to volt mod my gtx for awhile... but this is very good to know.

i was looking at them too and i saw what they said to do and just thought "wtf, that voltage is gunna be wayyyyy higher." Its good to know that someone out there is proactive enough to actually do the math and actually find out how much more.

thanks bro, ill keep it in mind when i have to stop playing games as high quality as possible and still getting 60fps
 
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