VT100 video board repair.

[Tony Duell]

>  Just as a rule of thumb, the diode needs to be rated
> at least 2 times the output voltage for an AC input.

Assuming there's a capactor on the output side of the diode (e.g. the 
smoothing capacitor of a power supply), that will charge to the peak 
voltage of the AC input. On the next half cycle, the cathode of the diode 
will be at that voltage above ground, while the anode will be at -ve the 
same voltage. Therefore you get twice the AC peak voltage across the diode.

And note that it's the peak and not RMS voltage you have to consider.

-tony