>You have alerted me to the need to isolate the computer ground
>though when in the future the circuit connects to a PC.

Well you should certainly be running the whole mains side of an isolation 
transformer while you do your development, before you let the magic smoke 
out of yourself.

>Looks like I'll need to isolate everything anyway.

If the circuit still lets the smoke out when you have no instrumentation 
connected, my pick would be that the frame of the transformer is connected 
to the mains earth lead, and you have connected the secondary ground to the 
same point, but not the negative end of the 10u 500V capacitor. Once you 
connect that point to mains ground the magic smoke will certainly be 
released from the diode bridge. 

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