My Powercolor 4870 1GB "pcs" has the same controller (NCP 5388) and I just modded my card successfully. Its actually just a few posts above yours!
I measured 16kOhms between pin19 and ground and soldered a 200kOhm potentiometer between that pin and ground. This worked for me and I can now adjust the GPU voltage.
Look for the resistance to ground from the 19'th pin on the image below:
Good luck!
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