How to thread a necchi 3102. I am just trying to learn. the bobbin is fine. I need to see a diagram

Try this web site:

http://www.sewusa.com/Sewing_Machine_Threading_Diagrams.htm

May not have your exact machine but one may be similar enough so you can thread your machine properly.

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try this link for a manual and the information should be in it Necchi 3101 FA 3102 FB Instruction Manual

It may not have seized there may be a very small piece of thread jammed in the raceway area, you need to take the bobbin case and the raceway out to check

try www.sewusa.com there are threading diagrams for some Necchi models. even if yours isnt there the bobbin should be similar.

try sewusa.com they have free threading diagrams

Ok which is it? A Singer or a Necchi?

If it is a Singer or a machine like the one picture here.... slide the grey metal plate toward you. You'll see the "drop in bobbin" Look for a little lever or button that will lift the bobbin up so you can get it out.

Ensure the bobbin thread is mounted on correctly and using your right hand turn the gear that makes the needle go up and down until it catches the thread.

There are a load of threading diagrams HERE, although your model is not listed, one may be similar enough to get you out of trouble.
As to a manual, a google search should give you several options for pdf downloads

From the reel pin take the thread to the left and over the top thread guide above the takeup lever, make sure the takeup lever is up and and make sure the presser foot is up, take the thread straight down and round the right hand side of the tension unit and back up the left hand side to the takeup lever, thread the takeup lever right to left and take the thread down any guides to the needle, if the bobbin case goes in the front thread the needle front to back, if the bobbin case goes in the side thread the needle left to right.

Check the thread coming out from the bobbins maybe on reverse side,also check the bobbins tension adjust it there is a small adjusting screw.Check also the needle alignment lastly check the upper tension from the tension regulator knob,tension adjustment is depend on the fabrics that you sew.