Welcome to Quilting Board. It warms my heart to hear when inherited machine are kept and put back to original use.
I think that perhaps this knob (for drop feed) does have a tendency to get stuck. I know there are other machines that tend to have stuck drop feeds. Maybe this video will help
https://www.youtube.com/watch?v=w0W7EBKvzIY at about the 6 minute mark.
I was able to find a couple of free manuals but aren't very good. Surprisingly the one from Bernina's website looks to be the same as on manualslib.com site.
https://www.bernina.com/Bernina/medi...125-Manual.pdf and
https://www.manualslib.com/manual/38...rnina-125.html There looks to be a better one on sale on ebay.
https://www.ebay.com/itm/32515167915...ntid=101721283 Unfortunately, for you, it is in the US. If you can find one to buy, it would be wise to have them send an image of one or more of the pages as there are some people that try to sell pdf or printed ones that are the same as those available for free.
Have fun with your machine.