There was a spare internal-sized screw loose. I don't see any possible place it could have come from; maybe it got left inside by the factory, and the owner just never bothered to remove it? Mysteeerious.
Construction seems solid. It was easy to get apart (once I found the correct-sized hex wrenches -- 2 different sizes, 10 screws) and put back together.
I shall probably attempt to power it up tomorrow, unless my life decides to interfere.
I started to connect it (the power supply has an indicator light, yay!), but then heard something else rattling around inside.
Went and fetched the hex wrenches again, got it opened up -- and found a loose nut under the main board. (Took a bit of doing to get it out.)
@Harena spotted what was loose: there's a transistor with a heat-sink (you can see the heat-sink on the right in the interior photo), and the screw+nut were supposed to attach them.
Reattached now. Good thing that got fixed before trying to use it. The transistor looks pretty replaceable, but I'd still hate to have to mess with taking it even further apart to re-solder that (not to mention identifying that as the problem and finding the right replacement part).
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