Ah, that makes sense! I haven't tried changing the firmware because I thought I had it narrowed down to a problem with the hardware. But if the Repetier firmware is sending signals that are out-of-spec for these chips, it would make sense that one chip might still be able to handle them but another wouldn't. Unfortunately the cables for my oscilloscope disappeared the last time I moved house and I haven't replaced them, so I can't see directly if that's truly what is happening, and I haven't delved into the source code either.) But that would certainly explain it.
Also, as a sanity check I stuck the drivers on a breadboard and drove them with signals from a Arduino with a very simple sketch on it, so I knew exactly what signals should be going to the driver and their timing. Under that circumstance, being driven by a ~2.5 kHz square wave, both the 8824 and 8825 drivers worked exactly as they were supposed to. (Phew!) More evidence that the drivers are fine, but aren't getting the signals they need from the firmware. I'll try a different firmware, or see if I can change the signal timing in Repetier, tomorrow.
EDIT:
Thanks, bobc, for that suggestion! The configuration.h file for the Repetier firmware has a define STEPPER_HIGH_DELAY, which I changed from zero to one to make the step pulses sent to the drivers last microsecond longer. That fixed my problem! Now all my axes move exactly as far as they're supposed to, and the motors sound much happier when they move too. Cripes, I don't know how long it would have taken me on my own to think to look at the signal timing.
Also, as a sanity check I stuck the drivers on a breadboard and drove them with signals from a Arduino with a very simple sketch on it, so I knew exactly what signals should be going to the driver and their timing. Under that circumstance, being driven by a ~2.5 kHz square wave, both the 8824 and 8825 drivers worked exactly as they were supposed to. (Phew!) More evidence that the drivers are fine, but aren't getting the signals they need from the firmware. I'll try a different firmware, or see if I can change the signal timing in Repetier, tomorrow.
EDIT:
Thanks, bobc, for that suggestion! The configuration.h file for the Repetier firmware has a define STEPPER_HIGH_DELAY, which I changed from zero to one to make the step pulses sent to the drivers last microsecond longer. That fixed my problem! Now all my axes move exactly as far as they're supposed to, and the motors sound much happier when they move too. Cripes, I don't know how long it would have taken me on my own to think to look at the signal timing.
