The internal HD had been slow for a while, so I thought I would add an SSD.
The first time I opened it, I was afraid I broke it, because the vertical sync cable didn't look like it was connected to anything and was in fact missing its connector. Also, the video cable popped right out of the motherboard without me first using the tab to unlock it, which made me think I might have broken that cable.
When I put everything back together, I didn't fully heed the advice about making sure cables were taped down before reseating the motherboard, so I took out the motherboard perhaps 2 more times than I needed to.
When it was all put back together, I powered it up and heard the familiar chime, but then there was no video and the fans spun up to full speed. I thought I had killed the computer.
But, just for grins, I decided to re-try attaching the LCD display to the motherboard. To save time, I tried to start it without any RAM in the computer, but that's... a non-starter. No chime, just a tone.
I was totally scared by now, but I put in the RAM, and I got video. And the SSD was there. I initialized it and now I am copying the HD to it so I can just boot off of the SSD.
1. Don't give up.
2. The vertical sync cable appears to be optional. It did not look like it was attached to anything when I first opened the Mac, and I didn't attach it to anything when I re-assembled everything.
3. The repair guide says which cable goes to which connector of the WiFi antenna, but mine had the cables in a different orientation. So I wrote that down and re-connected it that way instead of going with what the guide said.
4. When you are taping the cables down before re-seating the motherboard, it can be helpful to go backward through the repair guide to check that every cable is where it needs to be. That way, even if they are taped, they will also reach where they need to go before you screw the motherboard down.