Thanks both of you. No, I didn't notice the source looking particularly dirty when I changed the filament, but then again, it's pretty hard to see inside the source box. I will try to clean it (first time doing so, it's a new instrument - well, refurbished - and I'm new at this). Before I shut it down though, I thought I'd see what the H2 config was looking like, so I switched to it and focused it the other day, but one major thing I noticed was that the m2 background was quite high (~3V). I'm not sure what might be causing this - do you think it has something to do with the source cleanliness?