Hi all, quick question. I've managed to last this long without needing to replace a projector bulb, but that time has finally come. One of the projectors seems fine to begin with, then the image goes very dark. I'm assuming it's the bulb, as I've done my best to clean the filters. It's not a classroom, so it's not in use all day every day, but equally I'm assuming it's not been changed before. Is this a simple enough job? I've looked for a manual online, but to no avail. Probably too old!