Yes if you want to match resistors, you can make a voltage divider with
just one additional resistor, but depending on the value in the car, and
the available values (tolerances) at RatShack, you may have to swap in
and out a few resistors. With a pot, you have flexibilty and can find
the perfect half-way point with just a shop tach. and no multimeter.
And one solder-job and you are finished. Both methods work equaly well
in the end, though. The fixed resistor may not drift in value with age
as much as the pot will, but a 20-turn pot would be fairly impervious to
that effect.
Just my $0.02
James J.
Paul MacDonald wrote:
I have a modified TR7 tach in my TR8. I took the original TR8 gauge out when I replaced the faces with white faced ones that I made.
The tachometer was modified by a friend with instructions that I saved from the TR8 mailing list, and a schematic of the wiring in a TR7 tachometer.
The modification done to the tachometer was simply the addition of a resistor across an existing resistor, there was no need to have a potentiometer. Two or three different resistors were tried and tested, with the best fit being soldered onto the tach.
The testing, as best as I can describe it, was the running the tach from a signal generator, and reading the output on an oscilloscope..... I watched.
Here is a copy of the instructions:
Paul MacDonald
PS If anyone would like a picture of the back of the tach showing the modification, I had two made up at the same time, so I have one that I can take a picture of.
----------------------------------
Fred, I recalibrated my TR7 Tach for Rover V8 by soldering a 100k
potentiometer onto the back of the Tachometer in the input signal circuit.
I think 100k should cover the V6 as I was able to compensate from 4 cyl to
V8 with the one potentiometer. The V6 is somewhere in between.
Then I used a regular handheld shop tach to show actual RPM while adjusting
the 100k pot to get the same reading on the modified TR7 tach. One thing
I did notice was that the TR7 tach (mine in any event) does not seem to
give a linear readout. I picked a 2000rpm spread over which it seemed to
be relatively accurate (3000-5000rpm if I remember correctly) and made sure
that the TR7 tach was relatively accurate in this range. Perhaps fiddling
with the tension springs on the clock movement would help fix this, but I
wasn't interested enough.
Oh...you'll have to remove the tach from the dash pod to do this, and I
think that there have been some e-mails here the last week on how to best
do this.....Careful with the printed circuit....
If you choose to go this route, let me know how things work out for you.
regards
Mark Elbers
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