Can't do a drop test on a 401, as it is a drop in bobbin.
bkay, did you put a new bobbin in when you changed thread? I'm wondering if you got the bobbin thread under the tension spring?
As for the upper tension you might want to check out
http://sewing-machines.blogspot.com/...djustment.html or
https://www.youtube.com/watch?v=UttHpnQy0xk
Janey - Neat people never make the exciting discoveries I do.