Simple alternator test for externally regulated alternators--monitor the alternator output voltage while taking a wire from the positive terminal of the battery to the field connection on the alternator while the engine is running. This essentially bypasses the regulator
, and calls for full output from the alternator. If the output voltage doesn't jump up at least half a volt, the alternator is dead, and in your case I would suspect the diodes are fried.
The zap-stop is supposed to limit the alternator voltage in the event that it is run with the batteries disconnected. However, it really only works with internally regulated alternators, which sense the alternator output voltage (making the voltage spike only a short-term transient).
With some externally regulated alternators, the regulator voltage sensing wire is connected to the battery, not the alternator. If SOMEONE turns the battery selector switch to off (it only takes half a second), the external regulator does not notice that the alternator voltage is spiking, and continues to excite the alternator to full output. First, this will melt the the zap-stop, then all that energy the alternator is putting out with nowhere to go will raise the output voltage until the diodes are fried.