It is not likely you would get more than one defective decoder. That means you need to look at these other issues. Make sure you have the correct power supply for the Zephyr.
When you turn on your Zephyr, does it run for a short time? Or did it go up in smoke as soon as the power came on? Also, do other DCC locos work OK? Does the NW2 run OK on DC?
You can measure the current consumption of the NW2 motor by hooking it up to a standard DC train transformer (without the decoder installed) and use a DC ameter in one lead while holding it in a stalled condition at full power. The rating of the decoder must be at least that much. If the motor is faulty, it may be drawing too much current, thereby causing the decoder to fail.
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