Lenovo H50 power supply/video card

I am trying to upgrade (marginally) a Lenovo H50 by changing the video card. Not for gaming on this mini rig, just trying to get a better video watching system. I know that you need to change the power supply first. I bought a new 400w and a 14 pin converter but when I start the computer, the fan spins for a half second and it stops. I bought a different 14 pin converter and tried a different known good power supply and the same thing happens. Any ideas as to what else I can do? Thanks in advance.
 
Confirm PSU is good - will it run a normal 24pin ATX system board?
Confirm old system is good - did you move a jumper or RAM or anything?
If both good - examine and compare your adapters - are they exactly the same pin outs? - try adapters again.

When making changes, it is important to keep the number of variables low. All too often I have tried to make 3 or more changes at once and it means trouble shooting is exponentially harder (I.e., I hope you did not try to replace video card, adapter and PSU all at once as introducing video card may require changes in BIOS and in drivers ).
 
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