Installation of the simplex SX410
These are the alloy versions of the 'Sachs Huret' steel efforts of that period and I am familiar with the tension problems that can happen if you take them apart and put them back together again.

You undid the lower Allen key bolt, didn't you?

You need to take the little bolt on the side of the cage off, then undo the lower Allen bolt, swing the cage around approximately 180 degrees, tighten up the Allen bolt making sure the spring goes in, then pull the cage round approximately 180 degree, then pop the lower cage bolt back in.

Got it?

It can be done, don't go out an buy a new one, persevere.