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I have the schematic for the TR7/8 tachometer. I have used it to re-calibrate
several TR7 tachs for TR8 use and to modify the tach for use with some types of
capacitive discharge ignition systems. The circuit is of 1960's/1970's
"Hybrid"
construction. It consists of carbon film deposition resistors on a ceramic
substrate onto which "non-integrated" components (e.g., capacitors and the TI
IC) are soldered. The TI chip is probably a so called "house numbered" part.
It is very common for chip manufacturers to provide a standard chip or a
variant
of one with labeled with a (large) customer's specified number. My experience
has been that getting them to tell you about it is impossible.
Anyway, the tach circuit operates like this: The input signal from the primary
coil is sent through a second order "pi" filter to remove spurious noise and
prevent false triggering. The filter signal is then fed to the IC which
contains a trigger (probably Schmidt) circuit. The output of this trigger
controls a charge pump which is a circuit that uses the charge storage
properties of a capacitor to convert a frequency to a voltage or current. The
relationship is i[cap] = C dv/dt. A resistor (Rcharge) and capacitor (Ccharge)
set the charge current (and therefore the calibration) of the tach. In
production, the value of Ccharge is the same for all 4/6/8 cyl. designs. The
resistor is laser trimmed (I know it's hard to believe they would have
something
that high-tech, but it's true) to the correct value for the application (TR7,
TR8, etc.). This also adjusts the calibration to account for variations in
meter movement sensitivity. The output of the charge pump is then integrated
(filtered) with another capacitor and fed through a buffer amplifier as a
current to drive the meter movement. The chip also contains an zener diode to
regulate against supply voltage variations. The blue capacitor you can see
across the IC and a 150 ohm film resistor form the rest of the zener circuit.
Sooo, after all this longwinded explanation, two points:
1. The correct method to adjust the calibration of the tach is to adjust
Rcharge as is done at the factory. Not with a laser of course, but by
substituting a potentiometer in place of the laser trimmed film resistor. This
will preserve the operating linearity of the circuit. If you're going from a 4
cylinder to an 8 cylinder, you can just place a pot in parallel with the film
resistor. To do this, connect a 100K pot (ten turn is best) between pin 7 and
pin 4 of the IC. Either calibrate against another tach or use a signal
generator to calibrate (250Hz = 3750 RPM for an 8 cyl).
2. The cause of the startup problem is just friction in the meter movement.
The blue cap has nothing to do with it (it is fully charged in 7.5 ms which is
less time than it takes for you to turn you key from "acc" to "start"). Until
the drive current through the meter is large enough or the vibration level is
high enough to kick it loose, nothing happens. This is why revving the engine
(raising drive current) or hitting the dash (vibration) kicks it loose. This
is
also why some do it more than others. I had a meter on the test bench and with
no vibration and a simulated low speed signal, the tach didn't move. Up the
sig
gen frequency or tap it and viola, movement.
A chip that operates very similarly to the one in the tach is the National
LM2917 frequency to voltage converter. It's not identical, but the concepts
are
the same.
I hope this useful.
Scott
'80 TR8
'73 Stag (For Sale)
"I drive therefore I am"
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