DEAD MAC PRO 1.1 LOGIC BOARD AFTER INSTALLING A PCI EXPRESS VIDEO CARD

Two months ago I was given away a Mac Pro 1,1 (A1186), in perfect working condition. Some days later, I had the bright idea of testing a working PCI express video card I removed from a PC  on my Mac, but it didn’t turn on. So I removed the video card, reinstalled the old one, and again, it didn’t turn on. I removed the 3v battery to reset the PRAM, replaced it with a new battery, performed a SMC reset, and tried everything I read on the forums, with no luck. I checked the logic board status led’s and I noticed they all remained off. Then, suspecting that the culprit was the PSU, I ordered a second-hand one on eBay (Delta DPS-980AB A), I installed it yesterday but my Mac still doesn’t turn on. No lights, no clicking sound (relay), no spinning fans. I checked all the connectors again. I tested it by unplugging  connector # 4 and shorting ‘PS_ON’ plug and ground either a paper clip. A click is produced, the PSU switches on, all the fans start spinning, all the LED’s on the RAM raiser light up, but the logic board LED’s don’t light up, the switch-on button doesn’t respond and as soon as I remove the paper clip, the PSU switches off.  Conclusion: the “new” PSU works fine, so does the old PSU (I tested the voltages on both of them), and my logic board is toasted, I’m afraid. Is there any “miraculous” procedure to revive my Mac, or I have to buy a second-hand logic board?