Need help troubleshooting

Ok so my Macbook Pro 15" 2.2 Core 2 Duo 2007 Model recently got the dreaded black screen of death. Boots up, no screen. Yet I can hear it running and enter my password when I can tell it's at the login screen and hit the volume buttons and can hear them going up and down. I even tried an external monitor and still nothing showed up. I know it's not the display on my Mac because I replaced it and installed it myself a few months ago and it was working great. I recently purchased a new logic board from an ebay store, got it, and installed it. Again same thing, black screen, computer working, and external monitor not working. I'm going to be returning it and getting a replacement. Hopefully it works. Just wondering what could be the problem besides the logic board? I mean these models are known to have a bad Nvidia chip and it's barely 2 years old. What do you guys think or suggest? Thank you so much for any help.

What I think is that inevitably a certain small percentage of Macs are going to fail during their useful lives. You just happen to have one. It's too bad, but it's an unavoidable chance that we all take whenever we buy any computer. We should just be thankful that our Macs are among the most reliable machines out there.