Tension can be a fuzz to get right, and it's all about cleaning, getting the adjustment right. Don't give up on this, and don't assume it's done until you have done a bit of test sewing and fine tuning along the way.
General way about it; set top tension dial in an inbetween position; between 4 and 5. Thread the machine up with a basic sew all weight thread, a good quality mercerized cotton or Gütermann sew-all polyester are up to the standard. You know the thread is not causing the problem. Do the drop test on the bobbin case; then take two layers of shirt weight cotton fabric or quilting type cotton and fine tune until the stitch is balanced.
Other things to check that can mess up:
Match needle size and thread weight well, too small needle for the thread can cause trouble. Good quality needles are a must, most brands are luckily. Check for needle in the correct way.
Take the top tensioner appart and clean them. It can be full of lint, a piece of thread caught between the disks. Make sure the dialed knob turns freely from about 0 -9 with out hault. Check the spring on the bobbin case too.
Take the throat plate off, thoroughly clean the feed dogs; scrape all grooves and corners clean with a tooth pick. In theory it shouldn't affect tension, but if there's a lot of lent in the bobbin - feed dog are it will.
Make sure the bobbin case is clean, as mentioned behind the tension spring as well. Make sure the thread is properly behind the spring.
This is a very trouble free model this way, and I'm sure it's one of the basic things that needs to be sorted out.