I am trying to open the archive of checking tachs with AC lights but get an error message Suggestions http://www.pilotsofamerica.com/forum/archive/index.php/t-42166.html I
Welcome to PoA, did you mean https://www.pilotsofamerica.com/community/threads/fluorescent-tach-strobe.42166/ ?
There's an app for that... https://www.tunelab-world.com/tach https://play.google.com/store/apps/details?id=com.realtimespecialties.enginerpm&hl=en_US&gl=US
Your mechanic probably has one of these or something similar. Borrow it and find out what your tach isn't telling you. https://www.aircraftspruce.com/catalog/topages/trutach.php
It actually works pretty well. You have to know approximately what the rpm is and tell it, along with the number of cylinders, then it locks onto the appropriate part of the waveform it hears.
In some planes, 50-100 rpm difference can be the difference between green zone and red zone. Get something you can check. The optical unit when pointed at a fluorescent light should read 3600. Mine reads exactly that.
Or find a plane with a certified airplane with a primary digital tach and compare the result of the app with the panel tach reading. My experience with a similar Android app (Headspeed tachometer: https://play.google.com/store/apps/details?id=at.idsoftware.headspeedtach&hl=en_US&gl=US) is that it exactly matches the tach readout of my CGR-30P.
