Hope it helps someone, may even work for other HD5xxx cards.
My Radeon 5850 took the dive!
Two decoupling capacitors at the back of the board were literally glowing red and were burnt out,
por que AMD! The card was showing up in windows, and drivers loaded correctly. Could not run cgminer, as the system would just become unresponsive with dead screen.
Anyways after removing the heatsink, I realised that two phases of the VRM were dead and not just the decoupling capacitors, but two 0.01Ohm Chip resistors feeding the +ve rail of the VRM phases were dead as well.
So here are the parts that I replaced / fixed (Access to parts with 'I', need the heatsink to be removed)
Ia) MR524 0.01Ohm replaced with 2 thin strands of copper wire
Ib) R527 replaced with 2 thin strands of copper wire
Oc) C510, C520, replaced with chip capacitors (should tolerate 12V or more)
Od) C527, C529, replaced with chip capacitors (should tolerate 12V or more)
You can easily find donor devices for the chip capacitors such as DSL modems, Cooling fans, dead PSU, etc. Make sure it's rated for 12V.
Here is link to inside pic
Here is link to outside pic
https://i.imgur.com/k7MRJh.jpgI did a hasty job (was planning to ebay the card), and didn't have liquid flux, so the soldering job was a bit shoddy, but it can be more elegant.