I have an SE that seems to boot up, but that has a dead screen. I also have a spare SE/30 with a good screen (though a checkerboard), and thus that has a dodgy logic board.

SEs were upgradeable to SE/30s. This may be a stupid question, but can an SE/30 be downgraded to an SE?

This would give me one working machine out of two boat anchors. I would try the swap without asking, except for fear of frying something.