Elna - top thread breaking?

I have inherited my mum's Elna Supermatic sewing machine, which was working fine when she used it last. But the top thread keeps breaking. I have changed the needle, thread, cleaned and oiled it, tried various top tensions.
The thread seems to break near the bobbin lock. When turning the fly-wheel by hand, it goes down into the machine easily, but feels tight to turn up again.
I don't know what to do to get it to work.
Please can anyone help.
Thanks