does anyone have a design for , or sell a testing jig for the steppers only?
You want to test the steppers? You can measure them with an ohmmeter to check them. But if you want to run them, a control board is the best bet.
My problem is not quite with the stepper. It’s with the A4988-style driver chip. Those things die for unknown reasons in my machines.
I was working on building a test rig, but then we had to move to a new building. (I’m a dual credit high school / college teacher.) Everything is boxed up and waiting for them to finish up construction, which depends on building permits.
I could imagine something to power it up, maybe even make it easy to read the voltage setting and spin a single motor. I bet if you had a big fan on it or something, you could even test that it would skip steps at certain speeds. I haven’t seen anything like that (but I’m not an expert).