So close, yet so far away...
So, thanks to Marc's helpful advice yesterday, I took another look at my power supply, and lo and behold, there was a second four-pin power cord that I hadn't seen!
Allow me to explain. In my previous post, I mentioned that I had been doing something "wrong" with the power cable. You see, the case we picked out (an Antec Sonata) has a 24-pin power cable of which 4 pins can detach for older systems. At first, I misread the manual and thought this was for newer systems, so I detached it and plugged it into the handy 4-pin plug next to the CPU. Doh. Having corrected that (without the extra 4 pins in the main plug, not even the fans turned on), I was under the further mistaken impression that the 4-pin plug on the motherboard was the backwards-compatibility plug. I did actually check for an extra 4-pin PSU connector, but I must have overlooked the one that existed.
In my defense, here's what the manual states about the power connector:
This motherboard provides two power connectors to connect ATX12V power supplier.
NOTE: This 24-pin power connector "ATXPWR1" is compliant to the former 20-pin type. Pay attention to the orientation when attaching the power cord. (Pin-11, 12, 23, and 24 should be left un-connected).
Note: It is recommended to connect to a power with 350W, 20A +5VDC capacity at least for heavily loaded system, and a 2A +5VSB capacity for supporting wake-up features.
Not exactly crystal-clear on what needs to be connected under what circumstances. The case manual is at least correct if you follow along carefully, and they even found someone who can speak the language. Unfortunately, this section is phrased in a way that's highly prone to misunderstandings, particularly if you have an incorrect idea lodged in your head to start with:
Connect the 24-pin Main Power Connector and the 4-pin +12V connector to your motherboard as needed. If your motherboard uses a 20-pin connector, detach the 4-pin attachment on the 24-pin power connector.
You have to read that quite closely to realize that there are two 4-pin connectors involved!
Anyway, so I can actually get the machine to post and boot. Unfortunately, it's highly unstable once it does boot. It runs for up to several minutes, but then exhibits various sorts of erratic behavior: freezing, spontaneous reboots, random error messages, that sort of thing. I've made sure all the cords are plugged in tightly and reseated a couple components (particularly the memory); about the only thing I haven't checked is the CPU+heatsink mess, which would be a real pain to disassemble and re-assemble.
Still, it's nice to make some progress. Maybe tomorrow I'll discover some other cable I forgot to plug in that will make everything work (hey, I can dream).