There appears to be a lot of people out there, myself included, with G70 based 7800gs cards which start to suffer from "black screen lockups"; more usually subsequent to any overclocking after voltmodding the core volts regulator?
Following this up, and after much testing, tweaking, and a bit of reverse engineering, have now identified the "black screen lockups" appear to be related to certain incorrect pcb circuit tracks ---> therefore design/manufacturing fault?
One example is that an important voltage ref line of the 6568 chip appears to be connected to a "current/droop sensing" output of the regulated supply, rather than the real output?
There may also be some less than optimum components fitted. However, no real way to determine this without access to nvidia original circuit design?
So the affected cards essentially have a noisier and less stable core regulator. Fortunately, for the manufacturers, the "black screen lockups" problem usually only happens when overall regulator loading exceeds a specific level. On my particular G70 card, it periodically locked up when any combination of core volts and core/memory speeds achieved 3dmark05 of 7500+, or 3dmark06 of 3900+.
Also found that artifact severity and lockups were greately influenced by how mobo AGP communications bus was set. Using ATI tool for overclocking, found that setting AGP above 66Hz and 1.5v dramatically reduced the volts/overclock level at which artifacts and/or lockups would start.
And although using NGO_8456 driver "appeared" to fix the problem for some, any influence on my card was only because that particular driver was limiting overall performance and thus reducing the loading on the core regulator supply. With my card, the NGO_8456 typically caused a drop of around 150 points in both 3dmark05 and 06 scores!
Also, raising the value of the overcurrent OCSET resistor can help in a small way. However it also is not a cure, since completely removing the resistor does not stop card lockups.
The real "cure" has been determined to be the cutting and rerouting of certain pcb tracks. Unfortunately, because of the pcb's complex multilayered design, that particular option is not currently considered the way to go. However, all is not lost, since simply connecting an easy to fit wire link will more or less achieve all that is necessary for stability (see red wire link at attachment below). Although by doing this, and not cutting tracks, it has to be accepted that the built-in "droop" and "current sense" functions will not operate correctly ---> however considered a small price to pay for achieving stable core volts and better overclocks.
With my G70 card modded as above, there are no more lockups. Currently at 540/1600, typically achieving 7650+ 3dmark05 scores; and 4050+ 3dmark06 scores. And an excellent knock-on effect of the more stable core volts supply has been an ability to more easily achieve 1600+ memory overclocks. None of which was previously possible!
And I expect to achieve more. However, because of still using card's original heatsink, must be carefull and thoroughly test so as not to overtemp anything !
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