if you can remove the bobbin case you can remove the 2 very small screws that hold the tension plate.clean underneath and refit tension plate.when you pull on cotton after replacing bobbin youmust be able to feel some tension on cotton.also check top tension,after threading machine normally and with cotton through needle pull cottonto side and check needle must bent slightly before cotton start to move.pull a lint free material between top tension plates to remove dirt between them.
johan
February 2012