X61 dead/comatose, leds blink, no beeps

[hackbunny]

Thinkpad X61, stored disassembled for a couple years due to a broken fan. Recently I wanted to revive it, so I got a new fan and thermal paste to reinstall the heatsink. The parts apparently held up well, just a broken corner on the top case and a slightly bent pin in the battery connector which I straightened easily. I put everything back together, plugged the power in and... nothing. The leds blink and then nothing. Sometimes it keeps doing that in a loop. I tried the "power button CPR", I tried holding the power button for 30, 60 seconds. I tried removing the RAM banks, swapping them, putting them in one at a time. I tried removing the hard disk. I tried both with and without a battery. What I didn't do was blast the components with canned air or clean them with rubbing alcohol, I probably should have but I don't have either

The maintenance manual says it's either a loose connection or a dead system board. I think I can rule out the loose connection, as I had someone else disassemble it and put it back together. An X61 system board can be had for under $50, which for a ~$3000 laptop isn't a lot, so...

Theories on what it could be? I suspect either something shorted out, or I damaged something during reassembly (pushed down too hard when screwing the fansink back in maybe? no clue). Can I revive it somehow? Is a new system board worth it? It is a ~$3000 laptop (not counting the personal significance), but a 5 year old one too. On the other hand, the part that could save it is much cheaper that I thought

What can I do next?

Thanks