25 or 30 years ago, I built a tach from instructions in Popular Science or
Mechanics. I suspect the design is pretty much universal. As I recall,
one picks up signals from the input to the points (coil primary). These
trigger an integrated circuit that puts out a square wave each time it
sees an input voltage above 0.5 volts or so (think slang term for such an
IC is a "one shot" but memory could be failing me here). The rpm reading
occurs when the meter responds to a signal that is either X volts (during
the IC pulse) or zero. The meter essentially reads the average of this
output, because it cannot respond to the pulses. As rpm increases, the
fraction of time that voltage is X volts increases, and the meter reading
increases. To change a 4 cyl tach to a 6 cyl tach, you need to adjust the
duration of the pulses, making each shorter so six pulses of the IC have
the same total length as 4 previously had.
Your address suggests proximity to some EE types. They should be able to
find the appropriate circuit to jigger. There may even be an adjustable
pot in the circuit for calibration, which may give you a clue.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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