Tachometer Repair

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:

About JB

Back off ... I'm a physicist.
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2 Responses to Tachometer Repair

  1. Barry Miller says:

    Is there the vague possibility that you have a wiring diagram for this .. I have just replaced the capacitor but havent yet tried it in the car

    • JB says:

      Hi Barry,

      I don’t but check the link in the post – he may have one. I have since sold the car, so no way to make a schematic now :-(

      If you have a signal generator, you can test the tach on the bench …


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