My standard Smiths tachometer stopped working after fitting an electronic ignition distributor on a 1969 MGB. It worked OK before this and I’ve tried fitting an earlier tacho with the loop wire at the back in the positive feed to the coil, to no avail.
The original voltage-sensing tachometer is controlled by the coil and the pulse is not sufficiently strong to trigger the currentsensing unit with the electronic ignition distributor. The earlier unit may operate, but due to the low signal used by the distributor, the loop wire should be moved from the SW (+) side of the coil to the CB (–) side. You should also ensure that the earth contacts are clean. Of course, this is assuming the tachometer is working correctly as they do have a built-in capacitor that’s prone to failure.