Did you clean off the old heat sink grease (probably dry white powdery residue at this point in time) before replacing the heat sink?Thank you all for your help! I just removed the motherboard and components, cleaned everything, put everything back and the 6100 has startup!
I took a picture of the motherboard for you to see if there is anything wrong. I removed the processor Heatsink for the picture but I put it back afterward.
If not, I recommend that you do that and reapply new heat sink grease. Otherwise you're setting yourself up for random system failures after from a few minutes to an hour of use.
The bad heat sink grease problem is more common on the 7100, but because you removed the heat sink, you've disturbed whatever still-working formation of old dried out heat sink grease was in place.
Do not over-apply the new grease. The stuff is just meant to fill in the minute gaps that exist between the CPU die (shiny rectangular bump in center) and the heat sink metal. So it really only takes a tiny dab. Too much and the stuff will run when it gets hot and get on the CPU pins. Since it's electrically conductive as well as thermally conductive, that would be bad. I ruined an x100 clone logic board learning this lesson.