Greg,
It sounds like your thermistor is gone. To get real down and dirty, what I
expect you will find is that the pulse width of the one shot in your tach
gets wider than the ignition pulse width at about 5000 RPM. When the tach
sees the trailing edge of the ignition pulse, it terminates the one shot
pulse. This results in a very funky one shot waveform, with a lower duty
cycle than it should have. With less average current flowing through the
meter, it is deflected less, therefore it registers a lower RPM. The RPM
reading will go down or up depending on how funky that one shot signal
looks. (Funky is a technical term.)
I suspect the problems are related. If the shop that worked on your tach
didn't notice that the thermistor was shot, then they wouldn't have noticed
that they had to make the one shot pulse much wider when they calibrated it
in order to compensate for the higher resistance since the 55 ohm resistor
wasn't reduced by the 100 ohm (nominal room temperature) thermistor. When
they made the one shot pulse so long, they got into trouble at the high end
of the deflection. Also, without the thermistor in the circuit, it is no
longer temperature compensated. Hence the variations.
I'd recommend replacing the thermistor with a new one. I have a supply of
50 ohm thermistors that should work well for you. Send me your snail mail
address and I'll send you one. Of course the tach will need to be
calibrated again after this work.
Mark
-----Original Message-----
From: Greg Locke [mailto:glocke(at)ihug.co.nz]
Sent: Saturday, January 08, 2000 10:53 AM
To: Olson, Mark W; alpines(at)autox.team.net
Subject: Re: Tach Lubrication
Hi tachman
I have numerous weird experiences with the rootes smiths tachometers also. A
goodie on one of my humber sceptres is that once the revcounter goes above
5000rpm, it then starts going backwards. Quite amusing. This really points
to electrical problems, such as Jarrid outlines. Even after having them
overhauled by an expert, these oddities still persist. The main one I'd
really like to overcome is variation in readings with ambient temperature.
On a hot day, the tach will read up to 20% high. Again, this is the same on
all my rootesmobiles and means my ear is really a safer judge of engine
critical point.
Greg
----- Original Message -----
From: Olson, Mark W <mark.w.olson(at)intel.com>
To: <tigers(at)autox.team.net>; <alpines(at)autox.team.net>
Sent: Friday, 7 January 2000 09:26
Subject: Tach Lubrication
> Sunbeamers,
>
> Over the holidays, I worked on about a dozen Smiths tachometers, in
addition
> to the many tachs I've worked on prior to that. I have noticed that a lot
> of them were unable to register above 4 to 5 grand. I surmised that the
> movements were getting dirty and or worn, so I went to my local jeweler to
> tell him my problem and ask him how he lubricates watches. He suggested I
> try sewing machine oil. The good news is it didn't seem to make the
> movement worse, but it sure didn't improve it either. So I gave up on
that
> idea.
>
> I solve the problem literally using brute force. I lower the resistance
of
> the thermistor/resistor pair to provide more energy to the deflection
coil.
> While this works, I'd really rather have a way of effectively lubricating
> these old tachs. The original lubrication is 30 years old.
>
> So the question is, what do the various materials and/or mechanical
experts
> on the lists recommend I try to extend the life of these old beasts?
>
> I've already had one person suggest sperm oil, but I wouldn't even know
> where to find that, let alone want to risk bringing down the wrath of
whale
> lovers everywhere onto my head.
>
> Mark
>
> PS, this email is also a test to see if the "smut filters" at Intergraph
> will reject an email with the word "lubrication" in it. T.J, email me if
> you don't get this.
>
>
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