My tach stopped working, so after checking all the connections and finding them good, I took it apart.
The circuit in mine is quite different from that described in this useful link from Mark Olson: http://www.classictiger.com/techtips/motach.html
For example, mine only has one transistor, and the toroidal transformer has two windings apart from the turns of the wire from the coil.
I suspected the 2uF electrolytic capacitor (these are prone to failure when very old), circled in yellow in the following photo:
Sure enough, it was a direct short when measured on the multimeter. I snipped it out of circuit and soldered in a 3.3uF tantalum (the value is almost certainly not critical)
in its place.
Now my tach works fine again :-) I wish all the jobs on the Jag were this simple.
PS Gordon Tompkins, who has a Series 1 XJ6, has a similar tachometer, and contacted me about its repair. Here is a photo of Gordon’s unit: notice the different orientation of the 2.2uF capacitor: