[T3] MPS calibration

[Bryon Garvin]

Forgive the newbie question (actually been on here 1.5 years, but definitely
still a newbie)...

I'm just trying to understand this 71 dilemma in a big-picture kind of way.
 Once your "C" PS fails and you have trouble finding good used ones, what
are you options if you want to keep FI ?  Can you somewhat painlessly switch
to a different year's setup with a new brain and sensor, and maybe other
equipment?

I really enjoy my '71 :)  as I'm sure many of you all do as well.  I'd love
to know what's "next" as these sensors begin to fail.  I've heard folks
talking about trying to make the diaphragms.  I keep hoping for success on
that front!

 - Bryon, '71 Fastback

On Mon, Jan 3, 2011 at 7:17 PM, Jim Adney <jadney at vwtype3.org> wrote:

> On 3 Jan 2011 at 19:20, Keith Park wrote:
>
> > Well, what I MEANT was that the owner cant do it, im sure Bosch had a
> > complex and dedicated machine for setting them up.  I agree with Jim, its
> > most likely mutual inductance that is being measured, but the correct
> spec's
> > for setting up something like the inner stop are not available.
>
> What I've assumed all along is that one should be able to match a
> tampered PS to a known good one by adjusting the mutual inductances
> to track together over a range that included the full load
> enrichment. I don't have numbers, and the method I use doesn't use
> numbers.
>
> I drive both primarys with a square wave, which produces rising
> currents in both primarys, which are equal because they are in
> series. Then I look at the voltage developed across each secondary
> with each one loaded by separate but matched resistors.
>
> What you see on a dual trace oscilloscope is a square wave with a
> decaying top. The rate of decay is a measure of the mutual
> inductance.
>
> I connect a hose to each PS with a Tee so that I can suck on the hose
> while I watch both traces. The Tee assures that both PSs see the same
> vacuum at the same time. It's easy to run up and down the vacuum
> scale while adjusting the tampered PS.
>
> The PSs are matched once I get them both to track together over as
> much vacuum range as I can muster. It's trivial to get them within a
> quarter turn, and not much harder to get them 4x better than that.
> That's for the '68-9 and '72-3 PSs. I really haven't spent any time
> thinking about the '70-1 PSs, because the larger problem there seems
> to be how to make replacement diaphrams.
>
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
>
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