Doc here,

imp:
You might try, If you feel inclined to do so, Pull the tach apart and carefully remove the calibration card, look for burned traces or burned components, If you find any, replace or repair as needed.
You have nothing to loose, it's already broke, and you are going to replace it anyway, maybe you can save it and keep the Tach you like..anyway, you can't break it more..
Doc

imp: