[T3] MPS calibration
Jim Adney
jadney at vwtype3.org
Mon Jan 3 19:17:51 PST 2011
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.
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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