Hi Richard
The only bit from a four pot is one of the internal components which had the
correct ratings on it so must have been common to both. the rest of the
components are all from the original six pot tacho. Thanks for the idea I
will try it and see what happens. Do you know which bit I should adjust
first the return spring or the potentiometer? The only thing I would like
to correct is the fact that the supply does not come from the stabilised
side of the voltage regulator as these are merely a Bi-metal switch which
averages the voltage and have an off period every so often that would cause
the tacho to stop working briefly.
Cheers, Graham.
----- Original Message -----
From: Richard Gosling <rbgos@perkins-engines.com>
To: List, Spitfire <spitfires@autox.team.net>; <Graham@perkins-engines.com>;
<Stretch@perkins-engines.com>
Sent: Tuesday, February 15, 2000 3:07 PM
Subject: Re: tacho recalibrating
>
> Graham,
>
> I have a vageu recollection of reading somewhere a tip for calibrating
> electronic tachos. Since they work by counting 12V pulses off your
> coil, all you need is a source of 12V pulses at a well-defined
> frequency. This you already have at home... your battery charger! This
> takes your 240V mains supply at 50 Hz alternating current, reduces it to
> +- 12V alternating current (still at 50 Hz), and then turnes the
> negative 12V peaks upside down to make them positive peaks. Result -
> 100 pulses per second, peak a little above 12V. For battery charging
> purposes there is no need to smooth the supply (as you may get with some
> transformers), so this is your output.
>
> Since you get 2 pulses per rev off a 4-pot engine, 100 Hz represents 50
> cycles per second, or 3000 rpm. For a 6-pot, 100 Hz gives you 2000 rpm.
>
> I haven't needed to fiddle with my tacho, so I haven't investigated the
> wiring on the Spitfire, but the general supply needed as far as I can
> remember is a regulated voltage from your voltage stabiliser, the signal
> supply which can vary in voltage (after all it will vary depending on
> your alternator output 12-15V), and a common earth. Since you have a
> through feed plus live and earth, I would suggest +ve from the charger
> to +ve on the through feed, regulated voltage to the live feed, connect
> the -ve from the through feed to the earth on the case, connect this to
> both -ve on your charger and to your car's earth.
>
> By the way, will putting an electronic tacho from a 4-pot car work in a
> 6-pot car? Since they work on counting pulses from the coil, surely it
> would end up reading 50% too high? Or am I missing something here?
>
> Richard Gosling and Daffy
>
>
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