positive earth tach

I'm finally starting work on my 67 Sprite and need to know about the tach before
it goes back together
Car came w/ 1500 engine and alternator.  Engine was locked so I don't know if
tach works
I'm putting in a 1275 and want to keep the alternator.  What do I do to the tach
or is most likely cooked by DPO
Can I test it easily?

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///  (If they are dupes, this trailer may also catch them.)