Am testing an L297/298 driver board that I constructed, pretty standard chopper design. It works fine with two different examples of small unipolar stepper motors. A larger stepper drives ok whilst my test clock is pulsing it but as soon as the clock stops the L298 output goes into an illegal state with all 4 outputs on (as indicated by 4 leds tied to each output)! Clock back on and off she goes no problem. Have tried a larger capacitor across the supply to no effect. Have tried another example of the same 34 frame motor and get same problem.

Output from the L297 at idle seems to be correct with no more than two inputs to the L298 being driven.

Tried another L298 chip, same problem.

Any ideas? Suggestions for diagnostics? Help would be appreciated.

Cheers