can you thread the machine and then manually pull the top thread and the bottom thread by hand? Cathy (Mizkaki) showed me that you can get the tensions pretty close by doing this manually and feeling the force required to draw each.
They should be relatively the same.
Also I believe that these era machines have a tension release when the presser foot is up which can be an issue when doing that test.
DISCLAIMER: (I am much better at fixing machines than fixing sewing issues...)