The 840av does, however, have a known tendency to do what yours has done. Remedies vary: stick the logic board in the dishwasher, reseat all components, keep it running for 40 mins or so and then try re-booting. But if it has a bad scsi drive, it wouldn't be fixed by any of that, and replacing the drive would be an easy thing to try first of all.

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I didn't know that. When I first got the 840AV and turned it on, I had this same issue, except hitting Command-Control-Power Key would make it restart properly. After that first time, I didn't have the problem again until I had a power outage, and it was solved just as quickly. I thought this had something to do with the system battery being dead (which I've since replaced), so I didn't think anything of it. Now I'm permanently stuck in that situation with the difference being that a force-restart doesn't work. The 840AV had been in storage in the previous owner's basement for some time (possibly years), so maybe the components were just weakened by being down there and not being used or something.