Hi
i have a jab lsa with 1400 plus hours on it for the last month the tacho has been intermittent. today i removed it and very carefully removed the black outer ring that holds it together. i could not see a crack in the solder joints as my eyes are not as good as they used to be , so i re-soldered all of the large one, reassembled and it works. the face comes off if you work the folded over ring all the way around gently
hope this helps Keith