The Italian made Necchi isn't the most common name in sewing machines. However, it is one of the most reliable. Being an imported item may make them hard to come by, but, thanks to the Internet, parts and manuals are just a few key strokes away. With newer models, the parts, including the bobbin, have become more standardized and can be bought at any sewing supply store. If the bobbin is beyond repair and you can no longer adjust the tension, you can buy a brand new one for about $17.00 as of January 2011.
Raise the needle to its highest point. Bobbins cannot be removed when the needle is down.
Press the switch to the left of the hinged door below the needle to open the bobbin compartment.
Pull the hinge latch on the bobbin to release it from the compartment.
Use the jeweler's screwdriver to turn the tension screw on the top of the bobbin. Turn it right to tighten and left to loosen. The thread should pull steadily and evenly without hesitation or being completely loose.
The tension screw can work itself loose and fall out entirely. If the screw is completely gone, you can order a new one online or buy one directly from your local sewing supply store.