Wow, I'm very impressed everyone, thanks for all your responses. Okay, I think we're onto something---the little black rod I made reference to above has a pizza slice-shaped wedge at the end which is evidently responsible for controlling light through that photointerruptor. I taped the paper guide on the other end of it down and the printer went into warmup mode for the first time all day! When I look at the fuser unit in place though the little wedge does not drop naturally back to its lowest position. Should it be sprung or is it relying on gravity to hold it down? It's as if it isn't heavy enough on its own to stay in the position that gets the printer going.
And good call on the fuser---I'll keep my eye out for one. For the few seconds that the printer went into warmup the groaning was rather immense, it certainly seems to be coming from the main body of the printer. Is the transfer belt much of a job?
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