What size bobbins does the necchi machine take?

http://www.fixya.com/support/t9472526-necchi_alco_a709_3c_06906_made_in https://www.ebay.com/p/Vintage-Necchi-alco-A709-3c-Sewing-Machine-With-Case-Made-in-Japan/2197773494

Is the bobbin wheel actually turning? If so, the bobbin winder is still activated. Have you tried turning the inner wheel on the handwheel to de-activate the bobbin (see video)?

Amber Necchi Machine Basics

If that doesn't help, you may need to take the machine for service.

Manual:
https://www.manualslib.com/manual/940234/Necchi-534-Fb.html

...

NO. IF IT IS TOO TALL IT WILL CAUSE A PROBLEM. GO TO THE LOCAL SEWING MACHINE SALES AND ASK IF THEY HAVE THE RIGHT ONE. ALSO, YOU CAN TAKE OUT THE BOBBIN CASE (HOLDS THE BOBBIN) AND TAKE IT WITH YOU. MAKE SURE IT DOESN'T EXCEED THE HEIGHT OF THE CASE AND IT SHOULD SPIN FREELY.

You can use either 16 or 16J. If your threads are loopy on the bottom side of fabric, the problem is either your needle thread tension is too loose or you have a snag in your needle plate where you have broken a needle. Check needle plate and clear snag if any. Then tighten up your needle thread tension until both needle and bobbin threads are in middle of fabric.

Please put on the bobbin correct size. And check the tension . Then use new threads.

check with sewingpartsonline.com for a manual & the bobbins for the machine.
necchi's are a good reliable machines, I have 2 myself
one sewer to another

I also own 2 necchi sewing machines, my mechanical is a 4595,,
lower thread tension
to test the bobbin tension, remove the bobbin case & bobbin & hold it by suspending it byb the thread, **** it once or twice, if the tension is correct, the thread will unwind by about an inch or two.
if the tension is too tight, it does not unwind at all, if the tension is too loose, it will drop too much.
to adjust the tension,, left is loosen & to the right tighten

thread breaks
rethread the machine making sure to get all thread guides
top thread tension usually 3 or 4
replace needle- make sure the needle is inserted correctly
some needles don't work with some thread, here's a tip

Cut 6-8 inch piece of thread of the spool you are going to use for your project
Take the needle you are going to use for the project
& insert the thread thru the eye of the needle
& lift one end of the thread to a 45 degree angle
& if the needle is the right size for the thread it will slide down the thread
But if the needle hangs on the thread you need one size larger needle for the thread

Hi - I have a 510 Necchi sewing machine - how do a fill the bobbin?

Call 1-800-322-6980 for Service and Parts.

Almost all Necchi use a class 15 bobbin & bobbin case, a new case should cost around 7 to 10 dollars!

Dwayne