Here's how I clean the cartridge slot. Not sure if it's the safest way, but I've never had probs.
I buy those small felt disc milk filters (you can get them from most tractor supply/fleet places) for filtering maple syrup. I take one of these & fold it many times over a butter knife. I then dip the felt covered butter knife into rubbing alcohol half way then press it into the cart slot & use a gentle in-&-out motion (reserve your perverse comments please), remove it & move down a little further on the slot & repeat this process. After no more gunk shows up on the filter, I flip the felt so the dry half is covering the butter knife blade. I then use the same motions & procedures to remove any excess rubbing alcohol. I just let it air dry overnight.
Peace