I'd like to add a little something to this. I've been playing around with it more and I removed the memory modules and ROM chips and put them back in. Now the symptoms have changed a bit. Now the screen stays totally blank, no matter where I set the brightness control on the front. However, I do hear the chime from the logic board, but only through the headphone jack and not through the main speaker. So now I'm thinking the logic board is okay.