Have taken the tension knob off my necchi 535fa sewing machine. how do I reassemble it?

thread is not between tension plates but behind one

Hm, I'm not able to locate a manual but may be able to offer some suggestions.

Remove the top thread from the machine.
AVOID OLD OR BARGAIN BIN THREAD!
Use the same thread in top and bobbin.
Be sure to use a brand new needle--needs to be compatible with the thread and fabric of the project..
ALWAYS RAISE the presser foot and rethread from the beginning.
Set the top tension to the midway point.
Verify the bobbin is loaded and threaded correctly and it turns the correct direction.
Pull the bobbin thread up through the fabric before taking the first stitch. It also helps to hold both thread tails in your left hand when taking the first stitches.
Test your machine and tweak the top tension until the top and bobbin threads meet in the middle of the fabric. (Increased top tension will pull the bobbin thread up. Decreased top tension will allow the bobbin thread to pull the top thread down.)

How to Thread Necchi Sewing Machine

How to Thread Necchi 535FA 535 534FB 534 Sewing Machine and Bobbin Case (not your model, but the threading and basic instructions will be similar or the same as your machine)

How to thread Necchi model 535 FA sewing machine now with captions

If it doesn't have thread stuck somewhere or a broken gear, then it is probably still an oiling issue.

Wheel on right side that controls the needle up and down is very tight...

Oops! All sewing machine diagrams/manuals are available, online. Oddly. From the late 1800's, on.

Try this web site:

http://www.sewingpartsonline.com/needle-threader-singer-503277.aspx

Manual at link below:

Instruction Manual Necchi 535FA

here are 2 links for a manual or you can try youtube for a video
http://sewusa.com/Sewing_Machine_Manuals/Necchi_Manuals/537FA_536FB_Instruction_Manual.htm


http://www.sewingpartsonline.com/instruction-manual-necchi-535fa.aspx

here are 2 links for a manual for the machine, you can choose where to buy it from,, the information you need is in it.
http://sewusa.com/Sewing_Machine_Manuals/Necchi_Manuals/537FA_536FB_Instruction_Manual.htm

http://www.sewingpartsonline.com/necchi-535fa-sewing-machine-parts.aspx

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

If the belts are not tight enough they will slip, they should all be fairly tight but not like a piano string if you know what I mean.
As you do not describe what the machine is doing wrong I can not really say any more at present, except make sure there is no oil on the belts and the machine has no tight spots when you turn it over by hand.