There are two types of upper tension mechanisms on Novas, the older all-silver one and the button-style with the colored dot in the center. The Miras all have this later style. On most machines with the button-style tensioner, the color of the dot matches the machine color.
To take these apart for cleaning, start by taking the mechanism out of the machine casting. You'll need to raise the presser foot for access to the set-screw that loosens the barrel.
When you take the barrel out, be sure not to lose the pin that goes through the center of the barrel; if it's not gunked up, it should slide out of the back.
Next, take the button cap off by simply unscrewing it.
Note that there's a small pin inside (at about 8:30 in the photo below) that holds everything in. You'll need to push that out so it's flush with the inside of the barrel.
Sometimes you can push it with a screwdriver; more frequently, you'll need more force. Pliers used at an angle work pretty well.
Once you have that pin pushed out of the way, the top piece of the inside assembly twists right out, revealing the guts of the mechanism.
Note the order you take the pieces out. First you'll remove the two springs... and when you do that, take note of the small piece of black metal (at top inside the barrel in picture below) that is the indicator "finger" when it's put together. See that notch at the 9:00 position in the top piece, photo above? That's the slot this indicator finger will slide into when you put it all back together. Remember that, there'll be a test.
Below are all the pieces that you'll take out, in the order they were removed (left to right, starting with the back row).
I like kerosene for cleaning parts like this. Just dump them all in a bath and use a toothbrush to scrub clean. Wipe dry with a paper towel.
I clean out the barrel with kerosene, too, using a Q-tip... and also pick at any grunge with a toothpick or dental pick to get it clean inside. And I polish any rust or burrs from all parts--with particular attention to the center pin and tension disks--using polishing paper.
I polish up the bright bits that are going to show (e.g., outer ring of the button cap and outside of tensioner barrel) with metal polish using a Q-tip, wax the center button of the cap, and smear a thin coating of oil on all the metal insides to keep rust at bay... but not on the insides of the two tension disks, of course, they need to stay smooth and dry.
Time to put it all back together... Start with the two tension disks, faces together... next the black piece with the bar across the center hole; that bar goes between the two sides of the center shaft... next the small black "cup"... next put the indicator finger in place, so the finger points to the number on the outside (a serger tweezer works great for positioning this, and use gravity to keep it against the side)... next position the two springs, the smaller nested inside the larger... and finally, that top piece twists in. Remember your test answer above? Be sure the indicator finger is in the slot in that top piece.
And finally, put a thin coat of oil (even better, TriFlow oil) on the center pin and insert it from the bottom. You tension assembly is ready to go back into the casting.
You need to raise the presser foot to get to the set-screw, which will press against that center pin to release pressure on the disks... so you'll have to push the barrel in just a bit to seat it properly. Put it in with the presser foot down first to see where it should end up, then raise the presser foot to get at the set-screw, holding the barrel in position.
Easy as that!