So, I took apart the upper tension assembly on the Kingston Japanese clone machine.
It's a little difficult to see in the photo, but the check spring was broken; it should have a little wire loop sticking out. It's interesting seeing how the tension units are designed. Really all they are is a bunch of plates, springs, pins, and washers. It can be tricky to get all the pieces back on in the correct orientation, especially if you have been handling them to clean the parts, so take lots of pictures as you go. And try to get a service/adjuster's manual that shows what orientation the parts are supposed to go in. Just because it was that way when you got the machine doesn't mean that is the correct orientation for the parts. Who knows what kind of moron owned the machine before you got it and what they did to it.
It took me a couple of tries to get the tension assembly back together correctly. The first time I had a pin in backwards (it popped out of the machine while I was disassembling the tension unit & I didn't see what direction it was originally in). I've taken apart the tension assemblies on 3 machines now, & I'm getting better at figuring it out.
Finally got it all put back together again! New check spring on and I'm good to go.
On a quilty note, I've been sewing a rainbow of string blocks on the vintage machines (plus the Megasaurus, whhich is only about 12 years old). Here they are up on the design wall. BAM!