Kendall Robinson writes:
Is it possible to have that loop of white wire on the back of
the tach (the one that goes from the ignition switch to the
coil) backwards? The tach in my positive ground B isn't
functioning and I'm trying to figure out why. I replaced the
wiring harness recently, that's what makes me think this could
be the problem.
It sounds like your 67 MGB tach is real similar to the Smith's tach in
my '66 Sunbeam Tiger. I have considerable experience with the Smiths
tachs of that vintage.
I have been told that if you put the loop in backwards, the tach will
not work properly. I threw a scope on my Tiger and found the ignition
voltage pulses to be pretty much square waves (with lots of noise).
Since the tach is inductively coupled, the polarity of the loop
**could** affect it's function if the current waveform is not symmetric,
something you can't easily see with a scope. I have not tried reversing
the loop polarity, but it should be the easiest thing to try, so go for
it. Please let us know what you find.
If that doesn't fix it, I'd be happy to check your tach out for you on
my bench. I've turned one of my PCs into a bench tester/calibrator for
these types of tachs. Let me know.
For those of you interested in the tach itself, the Smiths tachs of that
vintage have an interesting design. The ignition wire loop acts as the
primary winding of a transformer. On the inside of the tach is a coil
that picks up the current pulses from the ignition wire. Each ignition
pulse triggers a one-shot made with two germanium transistors. The
output of the one-shot drives an ammeter. The more frequent the pulses,
the higher the duty cycle of the one-shot output, the higher the reading
on the ammeter. The tach is calibrated by varying the one-shot's pulse
width via a pot. They tend to calibrate well at low and high RPMs, but
they tend to read a couple of hundred RPM high when the needle is nearer
to vertical.
The design was obviously originally done for a positive ground car, as
the circuit is referenced to the 12V and it floats off the ground
through a resistor. The circuit is identical in the tachs for positive
ground Series 4 Alpines, but the 12V and ground connections are reversed
inside the tach.
The most common failure mode I've found so far is failure of the main
timing capacitor, which can lead to erratic behavior or total failure.
Mechanical failure of the ammeter is the second most common failure.
I've repaired and/or calibrated a bunch of Smith's tachs so far (from
Tigers, Alpines and even from a Cobra.)
This is my first post to the SOL list so let me know if I've bored you
to tears or committed any no-nos.
Mark
_______________________________________________________
Mark W. Olson
molson@gomez.intel.com
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