DH just got home from work and helped me figure out how to take the interior panel off so that the tension discs were accessible. I got the end cap off, but the little interior panel was a mystery! Anyway, once I could get to the tension discs, we both shined a flashlight into that area. We didn't see any lint or stray threads, but we swiped between the discs with a thin flat paintbrush. Then I noticed there's a pre-tension thingy that closes when the pressure foot lever is lowered. I swiped around and between it, too. A tiny (as in 1mm or less) piece of lint came out.
Then we put the panel and end cap back on, and I tried embroidering a satin-stitched border.
It worked fine, with the tension set at the usual 4.
Perhaps we swept out a bit of thread or lint that we didn't see, or perhaps that tiny 1mm piece of lint in the pre-tension thingy was causing the problem, but hopefully it's fixed now, and it better STAY fixed! LOL!
Thanks again for the responses.
P.S. If anyone else has this same machine and needs to know in the future, the interior panel comes off by
gently pulling forward on the area that's marked with a 3, and an upward-pointing "U" arrow. There are 2 or 3 little plastic clips that will unsnap, allowing the whole panel to be removed, and giving you access to the tension discs.
(Don't pull hard! You don't want to break those plastic clips.)