First off, I hate calling you Foxfold. You must have a regular name so somewhere along the line, post it, please. Put it in your signature at the bottom as you can see mine. Now, with that said, as to your tension problem, your saw is very similar to a Hegner if I am not mistaken so you should only have to set your tension once at the back of the saw. If I recall on my Hegner, I would tighten the knob in the rear, with the blade in place, to a point where all the slack was out of it. Meaning the knob/bolt in the rear. Once that is done, you pull the lever at the front of the saw nearest to you. That is what actually is applying the tension to the blade. You can make fine adjustments at the rear with the knob but once that's done, you shouldn't have to mess with it again until you change blades and possibly not even then. I'm going by memory on that. I'm going to give you a link to download/read the manual for the Hegner saw on Advanced Machinery website. I take it you don't have the manual for yours. If you do have it, it should tell you how to apply tension in the correct steps. So, here is the link..... https://cdn.shopify.com/s/files/1/1188/6562/files/Hegner_Scroll_Saw_manual.pdf?3248921969229599006
And here is a breakdown of the parts if you are interested......... https://cdn.shopify.com/s/files/1/1188/6562/files/Hegner_Multimax_18_DIAGRAM.pdf?3248921969229599006
There are other models of Hegners besides the one I gave you the links for so here is the link to Advanced Machinery. You can look around there and see what saw matches yours. https://advanced-machinery.myshopify.com/pages/hegner-home
I hope this helps a bit.