there are lots of things that can affect tension- what thread you are using, (weight/type) using the same in the top & bobbin? what needle you are using (the right size/type for the thread being used) when was your machine / bobbin area cleaned last? I would start with a good cleaning, then a new needle (making sure it is the correct needle for the thread) re-thread the machine from the spool all the way- making sure the thread is through the tension disc's correctly- wind a new bobbin- with the same thread (or at least the same weight thread) then loosen that tension back up to your starting point & try some practice stitching- adjusting just a little bit at a time (1/4 turn or less) and sew a few inches. keep adjusting *slowly* and checking until you have the tension 'balanced'. (you said the machine is 50 years old...when was it cleaned & oiled last?- could be a factor)