I found the following info. on the Listquest server from 1997:
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)
At 06:42 PM 3/26/99 -0600, you wrote:
>Randall, that is exactly what I believed to be true about how tachs
>actually convert the "signal"going to them into linear movement!! I
>believed that is was voltage, not "frequency" that actually drove the
>needle of the tach. I will see if I can get the schematic out of one of
>the service manuals, and try to get it to you.
>
>Many Thanks!
>
Mark Tinker
Systems Engineer, U.S. Channels
Cisco Systems
Dallas, TX USA
Main: 972 364-8791
Fax: 972 364-8733
Pager: 800 365-4578
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