Adjust bobbin tension on Singer Model 2005

Question

Despite having the upper tension set as high as it goes, I still get looping underneath. How do I loosen the bobbin tension? There is nothing in the owner's manual. I'm assuming Singer doesn't want end users fiddling with it, but something has to be done. This machine has had 3 upper tension assemblies which work for a little while and then the problem re-occurs. I got so frustrated, I bought another brand and use this machine as backup when my ''good'' machine in unavailable. It's a shame because the 2005 makes excellent buttonholes when it is working right.