1) Sorry I don’t know exactly which resistors determine which nibbles but I’m sure that I did the same thing that was proposed in the image as highlighted by the red rectangles. Removed the 25K on the right top. Added a 20K below the removed one. And changed the forth column top resistor from 5K to 15K. Could you please show in the image what is your suggestion??
My first suggestion is that you understand what the mod does and why it works before taking a soldering iron to an expensive piece of equipment.
The top pair of resistors controls the 3rd nibble of the device ID. Nibble is half a byte. So if your card's PCI device ID is, e.g. 0x1180 (for a GTX680), the 3rd nibble is 8, and the 4th nibble s 0. The resistors controlling the 3rd nibble are at the upper pair in the picture, and for the 4th nibble are the lower ones.
The values for the 4th nibble are fully configurable in both hardware or software (see first page for details of resistor values for the hard-mod). The 3rd nibble is partially controllable in software (the 5th bit is controllable via a soft-strap in the BIOS, the other 3 are not). Thus, to convert to a Tesla K10 no hardware modification is required - it can be done in software. See my post on page 41 of this thread for an explanation of soft-modding on UEFI BIOS cards.
For modifying to a K5000 or K2 you have to hard-mod at least the 3rd nibble resistor pair.
2) Also could you please tell me more about soft-mod?? Is that some sort of firmware update for the graphic Card? What do you mean by stabilize with soft-mod??
Yes. See page 41.
What I have found is that you can remove the 3rd nibble resistor without replacing it. This is a
bad idea, but it does work for most people, at least on the GTX680. The result is that the value of the 3rd nibble will flap between the highest settable value and one blow it (in the case of GK104 based cards that means 0xA or 0xB, randomly after reboots). This is only a difference in the least significant bit of the 3rd nibble, so you can "stabilize" it by setting that bit in the soft strap.
If you are not getting even a POST, then something else has been damaged/modified on the card, e.g. if you shorted something by too much solder, static shock damage, or similar. Or you just didn't push the card into the slot properly. Even without any of the hard-strap resistors the card should still POST, but it's device ID may be unstable (for a lot of people, myself included, no resistors in any of the 4 locations results in reasonably stable 0x11BF (K2) device ID on the GTX680. Note that leaving pins disconnected on an analogue circuit is a bad idea as it can cause inpredictable behaviour, so don't do this unless you have no choice.