A mod for HP WX4150 and WX3200 cards
(content and post by @franetic)
Here I'd like to report another successful POST of a modded HP WX4150 card in an iMac12,2 (27' 2011 iMac). I now have two modded HP WX4150 cards happily running in two 27' 2011 machines, reported temperatures are sensible (ie. they are no different from those reported by a machine with a Dell WX4150 built in). I also ran some GPU benchmarks - I did not notice any anomalies after 30 minutes of benchmarking.
I (actually some of us) did investigate this further. With an additional small mod, the HP WX4150 POSTs and works properly in the whole iMac11,1 - iMac12,2 range! See additional mod below.
If your soldering skills are above basic and you have the tools required, please do try this. (Note: the additional mod requires fine soldering skills.) Here's how I did it:
- a (at least) 50W soldering iron with precision tip and temperature control (80W+ is better, 50W should be enough for leaded solder)
- flux (pen)
- solder wick (to remove excess solder)
- tweezers
- magnifying glass (I did not use it, but I should.)
- a (non-POSTing) HP WX4150 card
- two 0R 1206 resistors (Buy more. They are really cheap and you will probably destroy one...)
- 0.5mm (or thinner) solder wire
Two jumpers (the 0R 1206 resistors) are needed between the pads in the orange rectangles under the red arrows 
. Those pads had some excess solder on my cards. I removed it with a solder wick to facilitate soldering later. Note that those pads are a part of a (relatively) large copper area, which means that a fair amount of heat will be required to melt the solder. Be very careful not to remove tiny capacitors and other components which are in close proximity. Cover cleared pads with a generous amount of flux. Place the resistor and align it with tweezers. Take your time. Use tweezers (in your left hand) to press down on the middle of the resistor (if it moves, align it again), then pick a small amount of solder with the soldering iron (in your right hand) and solder one pad of the resistor. Remember the 'large copper area'? A certain amount of heat will be required again. (This is really important to avoid cold joints. Believe me...) Again: Be careful not to short the nearby components. If you do, remove the shorts with the solder wick. Repeat for the second pad and second resistor. (Add flux as needed, you can remove excess flux when finished with a toothbrush and isopropyl alcohol.) The result could look alike this: 
This additional mod requires fine soldering skills (or rather a visit to a cell phone repair shop). It is, however, mandatory to make the card POST and function properly in 21.5' iMacs (iMac11,2 and iMac12,1). On the bottom layer, lower left corner, close to the IC with DUFC top marking, there are two 0402-sized 0R resistors (jumpers) - (again) in orange rectangles. These need to be removed. See last next two photos:
