Before getting a real stepper-driver for the big torque stepper, i just try the same circuit and same program, with another stepper :
a smaller one i get from an old scanner : 4H4018x0503, i do not have the complete specification just what is written on it : 6.8V 1.0A (200 steps).
It works better but i still got some troubles.
The stepper works at higher speed (150 rpm) with USB power only, but the circuit still do not work with only external 6V power (the LED only blinks at 1 Hz, do not understand where it came from).
I got another behavior if i plug an external 6V power + USB (if i remove USB see above) :
At rest, the stepper and the L298 get hot (but not very hot, but hotter than with only USB).
At rest, the stepper seems to have a very little shaking : if i touch the stepper i feel a very small vibration, if i touch the rotate axis also and if i press on it, the small vibration get a little higher :the stepper try to stay at its position i think, but i do not understand the brake command.
I try to activate brake when there is "running" order but it doesn't change anything.
At rest the stepper alway try to keep it position when powered by an external 6V + USB
digitalWrite(brakeA, LOW);
digitalWrite(brakeB, LOW);