TB6600's: weird behavior

A friend bought some stuff from banggood and got a couple of tb6600’s that he gave me once he realized they were actually the lower-current TB67S109AFTG. I have some small projects that could use a free stepper driver so I hooked one up, and I do not understand what it is doing. I have direction tied +5V, enable unconnected (or grounded, doesn’t seem to matter) and I’m putting a 5V square wave into step/pulse. What I’m seeing is that every other positive transition, the stepper locks, the way you’d expect when current is flowing through the coils, but on the other half of the positive transitions, the stepper is free to rotate. The stepper is good and I’ve tested the connections from the driver to the motor. This looks to me like one of the phase drivers is blown out.
What’s odd is that it happens with all three drivers he got.
Maybe they put in a whole different chip, like a single motor driver.
I’ll go get some trinamics anyway, but I’m curious if anyone else has seen this.

Maybe ghost shift chips? That would make sense for not tested and having a common failure mode.

Hi John, this happens because the thermal protection kicks in. Adjust the little pots to a lower current and it fixes it. There is a bit of fuzzy logic in these chips so a slight over current gets them locked for a while which is very annoying. So tuning is a bit hard. Best is to start with a low opt selection and slightly increase the current until the stepper motors run smooth. If you increase them slightly again you will see that they lock up and you need to let them cool down. However they are great drivers and up to 4 Amp per coil.