Kind of an expensive way to sort out your phone - Nokia rapid chargers are notorious failures (they usually blow a diode in the bridge) and can easily be repaired or replaced at very low cost. I've done 4 of them now, 3 the same type.

Get a DC multimeter on a 0 - 20 V scale and check the output voltage is no lower than about 20% below the rating value on the casing. Alternatively, post the number of the charger and I should be able to tell whether it's a regular fail: the batch number would also help as well.