<VV> Spyder Tach and XR700 ignition

[tony..]

OK...  the Corvair factory tach is an analog device.   This means it 
will by default have a means of calibrating it, which in this instance 
(both early and late tachs) it's a wire-wound potentiometer, or a 
"control" mounted inside the tach that adjusts how the meter movement in 
the tach actually reads off the rpm that the engine is actually 
turning.   In some aftermarket tachs that adjustment isn't an adjustable 
variable resistor or potentiometer, it's a fixed resistor or two, 
attached to a switch that allows selection of inputs, such as for 
matching the tach to 4, 6, and 8 cylinder engines.   Not terribly 
accurate, but they usually are consistent which works about as well as 
anything else.


Corvair tachs operate by sampling the negative side of the ignition coil 
"signal" which is a square wave and sending it through an amplifier 
circuit after passing it through a rectifier diode to turn this square 
wave signal into positive pulses which drives a transistor that inverts 
the pulses and sends them through a clamp circuit that stabilizes the 
actual voltage level of the pulses so the tach doesn't change its 
reading as charging system and battery voltages vary slightly during 
regular vehicle operation.   The clamp circuit is a pair of zener diodes 
connected cathode-to-cathode which makes sure the pulses are always at 
the same voltage level, but differing only in pulse width which is sent 
to yet another circuit (RC) that turns the amplified and 
voltage-regulated pulses into varying width DC pulses that feed the 
meter movement.  Calibration is done by simply adjusting the cal. 
potentiometer which is accessible from the back of the tachometer.

  The hole is covered from the factory with a piece of paper tape so to calibrate/adjust the tach you need to poke a hole through the tape to adjust the calibration potentiometer (small screwdriver), and Yes there is more than enough range to re-calibrate the tach which is almost always necessary if you make some sort of major change to the ignition, either by changing the coil or adding electronic ignition to the car or switching back to points/condensor/factory ignition when the electronics fail following an EMP attack from North Korea because nobody's hardware is ever the same as anyone else's, always slight differences that will require adjustments for an accurate readout on the tach.  If the calibration adjustment isn't able to make the meter indicate correctly the tachometer circuit likely requires repairs, also fairly easy to do with a respectable knowledge of basic electronics.

The Corvair/GM (same tach electronics used in other same year GM products) tach isn't rocket science and in fact it's quite simple once you get a good look at the circuitry, unlike modern tachs which tend to turn a simple thing into a Rube Goldberg device that goes out of its way to take a short cut.  ;)  (you should see what Chrysler did in the mid-60s with their factory tachometers)


tony..


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