thanks a lot for this post, it definitely helped me to bring new life to my PK250 (spanish name for the PK350, I think).
I had a 0,14cd/m2 black level before the mod, and now it is a much more convincing 0,04-0,05 cd/m2.
I followed more or less the instructions of havro, though I didn't notice any changes with the modifications of some of the pots. For my set, Set_Up is the main responsible of black level changes, in fact I couldn't tell any of the other pots made a diference in this regard. VA and Vy seem to give more room to Set_Up going clockwise.
I didn't notice that "pop" or dullness in the image produced by VZB, it didn't introduce any artifacts either, so I left it almost totally clockwise.
Set_dn is a pain in the ass, I moved it and the festival of pixel missfires began, I think i finally returned it to its original position.
I found very useful the AVS HD disc for doing these tests:
[I cannot post URLs, it seems, but I guess you know where to find it
the main screen with the "AVSHD" logo was perfect to see problems appear. In the lower left section (over a white background), Vy too much counterclockwise makes magenta pixels appear (set_dn also helps), and in the dark grey box that contains the submenus, red pixels begin to sparkle all over it (mainly in the bottom of the box) if you set Set_up too much clockwise. Also the grey field of the last submenu (the icon of the submenu) flickers if Set_up is too high.
The ISM method "color erasing" (don't know the exact name of the option in the english menus) also was very useful to detect pixel missfires.
Another problem that Set_up too much clockwise produces is "black bars bleeding", I mean, in a film with black bars, if a near to black image is next to the black bar, the absolute black of the bar partially "invades" the image. I noticed this in "The incredible Hulk", 1 exact hour of film I think, Norton's nape overlaps the upper black bar and this effect appears.
In a first tweak I got a 0.018cd/m2 measure, and that was amazing, but then I began to notice all these problems, finally I left it in a safer 0.045 cd/m2 level. Not so great, but much better than my original black level.
It is possible that with a better understanding of what each pot does I could get better results, but I don't really know what to do with Set_dn and VS, for example, or even VA ... clockwise or counterclockwise? (not much of a difference for me, really).
Anyway, great post, great contributions, and I will follow reading this to see if some "standard method" comes out!
Thanks all ! (and sorry for the english if it is a bit confuse or something)