Re: The tachometer

["James J." <m1garand@speakeasy.net>]

To add to Keith's words, I'd use as high of a value as I can find (many 
mega-ohms) since you want as little current passing through it as 
possible.  These types of pots are often called "trimmer pots"  or 
"cermet pots" because the resistive element is made from a plastic 
called "cermet".  You can find them in 20 or 30-turn units.  Unlike 
conventional "volume" pots that you find in radios, etc., these are 
often long and rectangular with 3 little terminals coming out.  We're 
talking about something that looks like a 1" french-fry (or, ahem, 
'freedom-fry').  RatShack might have these, but your best bet is Allied 
or Mouser Electronics on-line.

Now I don't know if there is a procedure for callibrating these, but if 
the Tach is just a voltmeter, then all you have to do is measure the 
resistance across the entire potentiometer, and then adjust the tap 
until you read exactly 1/2 that value.  A 20-turn pot gives you alot of 
resolution to get to exactly the right value.  This will give you 
exactly 1/2 the input voltage, but if the pulse-to-volt converter is 
off, this will also be off.  If you have a DMM that has a pulse-counter 
(or a timing-gun with a tach) you can tweak the potentiometer to 
compensate for any linear errors in the p-to-v converter.

Best of luck!!!
JJJ


kmwheeler@ualr.edu wrote:

> Barrie,
>
> Someone will have the specifics but here's the basics.  The tach in a 'B
> (or TR7 for that matter -- those guys face the same problem) is based 
> on a frequency to voltage convertor.  This circuit takes the ignition pulses
> and turns them in to a voltage. The movement on the tach is actually
> a voltmeter.
>
> The circuit is based on an IC.  The IC is mounted on some
> "thick film" circuitry on a ceramic substrate.  The problem with
> the V8, as you've seen, is that now you've got twice the frequency
> (number of ignition pulses) for any given RPM and the tach reads high.
>
> The fix involves adding a variable resistor and adjusting the circuit
> so that the output voltage is corrected.  Again, someone else
> may have the info, or it may be on the web.  I would suggest
> that you get a "multi-turn" potentiometer for this, a $3 or so
> Radio Shack (if they still sell such things) item.  After setting
> the "pot" so that you get the right reading, you'll want to glypt
> it so that it doesn't rotate.  The part you'll need is a "multi-turn
> trimmer potentiometer", but I'm not sure what the value is.
>
> I'm not at home right now so I don't have access to my info.  
> If no one else comes through I'll try and find it for you.
>
> I've made it all sound a bit more complicatd than it is.  Basically
> you add the "pot", tweak it so the tach reads right, and then
> glue the adjustment in place.  If you're not comfortable putzing
> around circuits and soldering things I'm sure someone on the
> list can help.
>
> -Keith Wheeler
> Team Sanctuary
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