View single post by Mark Rosenbaum
 Posted: 12-30-2005 01:32 am
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
The British car club sites have quite a bit of information posted on Smiths tachs, and much of it is pertinent to the JH.  Here's part of a post made back in August 1994 by Mark W. Olson, who apparently was one of the first LBC owners to 'reverse engineer' the Smiths tach:

"[...]  For those of you interested in the tach itself, the Smiths tachs of that  vintage have an interesting design. The ignition wire loop acts as the  primary winding of a transformer. On the inside of the tach is a coil that picks up the current pulses from the ignition wire. Each ignition pulse triggers a one-shot made with two germanium transistors. The output of the one-shot drives an ammeter. The more frequent the pulses, the higher the duty cycle of the one-shot output, the higher the reading on the ammeter. The tach is calibrated by varying the one-shot's pulse width via a pot. They tend to calibrate well at low and high RPMs, but they tend to read a couple of hundred RPM high when the needle is nearer to vertical.

"The design was obviously originally done for a positive ground car, as the circuit is referenced to the 12V and it floats off the ground through a resistor. [...]

"The most common failure mode I've found so far is failure of the main timing capacitor, 
which can lead to erratic behavior or total failure. Mechanical failure of the ammeter is the second most common failure. [....]"

If the above makes sense to you, then you can probably fix the tach.  Other options are to find someone who can fix it, or who can replace the electronics with something more modern; here, the internet is your friend.

Finally, try eBay: most Smiths RVI-type tachs will work without changes, and Smiths RVC-type tachs will work with only minor wiring alterations.  Often you can even exchange tach faceplates so the replacement will look original.  In any event you should re-calibrate the new tach by comparing its reading to a known accurate tach or tach-dwell meter.

Good luck.