The only time I've ever seen a speedo fry was when the three-wire edge connector attached at the back of the speedo head was inadvertently flipped over. I think that sends 12 volts directly to the signal input. Normally, the plug is "polaroized" so it can fit one way only. This is done with a tiny plastic insert gizmo that blocks improper orientation. However, if the plug is disassembled, the plastic gizmo can fall out -- and then the plug will fit the speedo right way up or wrong way down (zap). I've seen it happen once on a 740 and once on a 240.

Keep in mind that if you had a wiring problem around the engine or alternator, other things in the car would go awry. I doubt very much that a screwed up alternator (or wiring to it) would or could hurt the speedo alone.
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Don Foster (near Cape Cod, MA)