Stepper motor - was working great - strange problem now

I'm using the bipolar circuit on http://arduino.cc/en/Tutorial/MotorKnob - same hardware, and a separate 12v battery for motor supply. The motor (from an old Epson Stylus, 1.8 degrees) was working very well - massive torque.

Then I fiddled with my sketch (I'm using the AccelStepper library) - but the strange thing is that the motor is not rotating properly now even after I commented out all my changes. And there is no torque at all. Both coils still give me 8 ohms as before. And I have connected a PC speaker to be able to "hear" the pulses - all four pins to the motor are pulsing.

So I assume that the motor is still ok, the chip/Uno are still ok - and it must be wiring order that I've messed up? But I've tried several wiring combos with no change

You have something mis-wired. If you were turning on ANY phase of the motor the motor would hold that position. since the motor has no torque it is not getting any power, thus it has no torque.

Got a voltmeter?

I've since discovered that the SN754410 chip gets very hot (I'm using a 12v gel battery) - and it has thermal protection.

In the process of fiddling, I've just blown my second uno - these things really don't like any back-voltage. This site really should have better tutorials - but then again I guess Italians struggle with something as obvious as ships don't like rocks. lol

garthn: I've since discovered that the SN754410 chip gets very hot (I'm using a 12v gel battery) - and it has thermal protection.

In the process of fiddling, I've just blown my second uno - these things really don't like any back-voltage. This site really should have better tutorials - but then again I guess Italians struggle with something as obvious as ships don't like rocks. lol

Maybe your next device should be a Ruggeduino? http://ruggedcircuits.com/html/ruggeduino.html Or build circuits on breadboards from bare AVRs -- if you blow that one, it's only a $4 replacement.