[T3] MPS calibration

Keith Park topnotch at nycap.rr.com
Mon Jan 3 19:55:53 PST 2011


Our 72-3 PS's have diaphragms too don't they?  Im told by Ray Greenwood that
they all do except the pre-70 units.  I cant remember what the late ones had
in them.

Keith


Top Notch Restorations
topnotch at nycap.rr.com 
http://www.a383ina68.addr.com/radiorest/main.htm
71 Squareback "Hothe"
65 Notchback "El Baja Rojo"
65 Squareback "Eggcrate"
87 golf "Winterat"
93 RX7 "Redstur"


-----Original Message-----
From: type3-vwtype3.org-bounces at lists.vwtype3.org
[mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of Jim Adney
Sent: Monday, January 03, 2011 10:18 PM
To: type3 at vwtype3.org
Subject: Re: [T3] MPS calibration

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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