Recently my bernina 830 made a buzzing hissing noise and smoke came out the back of the machine, so i quickly unplugged it. As it had recently been serviced i thought it can't be too serious and now when i do some sewing i unplug as soon as i am finished. Sometimes smoke and a bad burning smell comes out the machine but it hasn't affected operation yet.... Any advice what is wrong with this machine and how to fix it? thank you :)

Sewing Machines - Bernina