Where can I get a free copy of a manual for a Husqvarna 400 Computer sewing machine?

Question edited for clarity.

Here is your manual. See Page 12 and Page 18 for manually setting tension. It is done electronically via the display.

If you press CTRL+F (find) you can type 'quilting' or 'tension' and it lists all mentions.
https://www.husqvarnaviking.com/SiteFiles/ClientSite/62/62e4d636-fd8a-4b22-a2a2-5f4a02e9af00.pdf

I haven't seen a free copy of a service manual, but have seen free copies of operating manuals. I assume from your question that you do not want the operating manual, and want a free service manual.

Service manual for sale for $10:

http://sewingnz.blogspot.com/2010/09/husqvarna-viking-service-manual.html

Service manual for sale:

Since you implied you do not want to purchase:

http://www.husqvarnasewingmanuals.com/husqvarna_service_manuals/husqvarna_service_manuals.htm

Contact a local Husqvarna Viking dealer. They should be able to download an italian version of the manual.

You should be able to download one on the Husqvarna web site

http://www.sewingpartsonline.com/instruction-manual-viking-400-500.aspx

http://www.sewingpartsonline.com/instruction-manual-viking-400-500-download.aspx

Viking Husqvarna Sewing Machine Manuals PDF and CD click there for a manual

For a $5 download for the 400 go to http://gotoes.org/sales/VikingSewingMachineManuals/ViewInfo.pl?Viking=1&Sewing_Machine_Model=HUSQVARNA-400-500. OR

If you contact http://sewingstyle.com/viking-400.html they can send you a photocopied manual for 21.99 if you need a paper copy.

Let me know how you make out. Good luck and if I can be any more help let me know!

You can get a free copy here:

http://s2.vsmgroup.com/VSM/Husqvarna%20Viking/HVManual.nsf/1fd5659e6f37141041256697003e64c0/8569fb0aa78ab071852575ed00723d7a?OpenDocument

Hello, here is the website:
Viking Husqvarna Sewing Machine Manuals PDF and CD