I have three vintage sewing machines that have the same issue: the top thread tension is tight, and changing the dial doesn't seem to do anything. I have already verified that I've threaded the machines correctly (and rethreaded them several dozen times), changed the fiber content and thickness of the thread and fabric, used different bobbin manufacturers, used vintage bobbins, superficially cleaned out the mechanisms (no disassembly), and nothing ever changes. The machines are a Brother Tingler 031, a Singer 503a Rocketeer, and a Singer class 66 treadle. I had the Tingler seen by a repairman, but the test swatch he provided to "prove" that he repaired the machine still shows evidence of the top thread being too tight.I'm not sure what I should be doing at this point! I also do not have experience disassembling any of the mechanisms, so if that's one of the suggestions I'd need more explicit direction.
ETA: I've also changed needles several times on each of these machines.

https://preview.redd.it/exz9nbyvzgca1.jpg?width=3263&format=pjpg&auto=webp&s=51316fec92ad78327734dd8c294258c53317c19b