[R-390] PTO Rebuild - and Core Disaccommodation and other effects

Thu Apr 7 18:20:37 EDT 2005

So now we need to take into consideration the "Half Life" of the PTO cores 
as well as the meters and mechanical filters....

Interesting..

Cecil..
----- Original Message ----- 
From: "John KA1XC" <tetrode at comcast.net>
To: "R-390 reflector" <r-390 at mailman.qth.net>
Sent: Thursday, April 07, 2005 4:56 PM
Subject: Re: [R-390] PTO Rebuild - and Core Disaccommodation and other 
effects

> Hi Miles and group,
> that's very interesting about what you have found about the PTO caps and
> corrector adjustments. No doubt the handful of old caps that are in that
> circuit contribute to many PTO problems but there are also another couple 
> of
> ageing mechanisms at work, one of which can easily mimic the symptoms you
> describe and may be the "core" of  some problems.
>
> I've got a friend who is an old time Bell Labs engineer and fellow R-390
> aficionado. Having worked there for a couple years well after it became
> Lucent I learned how long ago the facility used to be a place of expert
> manufacturing of *all* the parts used to build the telecom equipment; they
> did their own metal work, cut their own crystals, grew their own quartz,
> wound their own coils and transformers, silicon wafer processed to make
> their own transistors and ICs, and mixed and fired their own powdered iron
> and ferrite magnetic materials. They literally had train loads of raw
> materials unloading at one end of the factory while finished equipment was
> trucked out the other end; they were true masters of engineering and
> materials science. Well, that was then and this is now and most everything
> that one needs to build anything is available from a catalog but some of 
> the
> knowledge remains.
>
> Anyway, he was very familiar with the magnetics part of the operation and
> told me of  the effect of magnetic core material disaccommodation which
> simply put is a naturally occurring decrease in the permeability of all 
> core
> materials over time. The permeability change follows a log curve - it is
> rapid starting with the manufacture of the part but its rate of change
> sharply falls after a relatively short period of time but it does continue
> forever. In very critical designs the effects of disaccommodation are
> considered into the design and are translated into a typical lifespan for
> the part where it will stay in "spec". These PTO cores (as well as most
> other parts) are now 30, 40, and 50+ years old so it's safe to assume they
> are well out of their intended lifespan and can cause havoc and hair 
> pulling
> when attempting precision measurements and adjustments of such things as
> linearity corrector stacks. It is his opinion that the R-390 PTO cores are
> affected by this effect and I also concur.
>
> What the actual composition mix of the PTO core material consists of is
> anyone's guess but it's probably safe to presume it's some type of iron
> powder mix since its characteristics provide the best stability for LC
> resonant circuits, however the industry literature states that its
> disaccommodation factor is much higher than ferrite core materials and 
> thus
> has more long term drift. What this means is that as the PTO core ages its
> permeability decreases, so the PTO coil inductance decreases and the
> oscillator frequency increases. This is also the same symptom of an
> oscillator trimmer cap decreasing in value, so it makes sense that it 
> could
> be compensated for by adjusting the value of the trimmer capacitance 
> upward
> as Miles described.
>
> There is also a second and perhaps even larger factor in considering PTO
> inductor value drift and according to the literature it involves the
> mechanical stability of the coil winding as it ages over time. Coil forms
> and even the binder material used to anchor the windings to the form do go
> through small dimensional changes over long periods of time. My *guess* is
> that the coil form shrinks which would mean a tightening of the windings
> and/or a decrease in the coil diameter thus a probable increase in the
> inductance. This is *counter* to the effects of disaccommodation but may
> explain why some PTO's tune "long" in span and why turns or partial turns
> have to be removed from the series-connected endpoint adjusting coil in
> order for it to get back in range. (BTW this endpoint adjustment coil is
> also susceptible to all the various long term drifts as well.) In this 
> case
> a downward adjustment of the PTO trimmer capacitance might bring things 
> back
> in the right direction.
>
> High temperatures accelerate the effects of disaccommodation and coil
> dimensional drift, so maybe even whether a PTO was run with its ovens ON 
> for
> a period of years had an effect on the various internal parts as well.
> Hopefully these various aging drift effects (and counter effects) and
> unknowns about the operating environment may shed some light on the all 
> the
> crazy PTO symptoms seen and make them appear to be a little more science 
> and
> a little less voodoo :^)
>
> Lastly, one of the interesting things about disaccommodation which sets it
> apart from a pure aging process is that it is a resettable and repeatable
> effect. If the core material is raised above its Curie temperature (or 
> even
> severely mechanically shocked) the core will reset to its *original*
> permeability value which will then start to decrease again at a 
> logarithmic
> rate. If someone wants to win the R-390 Scientist of the Year Award a very
> interesting experiment would be to take an old PTO core and measure its
> permeability, and then bake it until it hits its Curie temperature 
> (possibly
> a few hundred deg. C) and let it cool. Then let it age undisturbed for a
> period of a few months until it reaches the slow moving part of the
> disaccommodation log curve and measure it again. What you should now have 
> is
> a PTO core which has been restored to its original factory permeability!
>
> For the technical definition of Disaccommodation Factor see  #12 of :
> http://www.elnamagnetics.com/library/catalogs/TGL/TermsDefinitions.pdf
>
>
> 73,
> John
> KA1XC
>
> ----- Original Message ----- 
> From: "Miles B. Anderson" <mbalaw at optonline.net>
> To: "R-390 Mailman" <R-390 at Mailman.qth.net>
> Sent: Wednesday, April 06, 2005 9:20 PM
> Subject: [R-390] PTO Rebuild
>
>
>> I'd like to caution against jumping to the conclusion that the corrector
>> stack needs adjustment whenever the PTO can't be made to track at the
>> intermediate points. I tried once to adjust the corrector stack on a
> Collins
>> PTO and made such a hash of it that the whole assembly wound up in my 
>> junk
>> box.
>>
>> I later discovered that the real problem is often much easier to solve.
> The
>> problem in my case was that one or more of the little unencapsulated mica
>> capacitors went west.
>>
>> There is no way to adjust the shunt capacitance of a Collins PTO. All you
>> can do is adjust the start point of the tuning slug and the little series
>> inductor that is used to trim the end point. This means that there is a
>> unique shunt capacitance which will make the tuning equation come out
> right
>> at both ends and the middle. If that shunt capacitance changes because of
>> aged components, no amount of fiddling the inductances will make the
> tuning
>> linear anywhere except at the end points.
>>
>> The procedure I followed was to adjust the start point and the end point
> as
>> per the manual. Then tune the PTO to the midpoint (500). If the 
>> oscillator
>> is high at the midpoint, add more shunt capacitance. If it is low, remove
>> shunt capacitance.
>>
>> Adjust both end points again per the manual and check the error at the
>> middle. If it is still off, repeat the process.
>>
>> We are only talking about 10 to 30 pf difference, but that small
> capacitance
>> difference can knock the daylights out of the linearity by putting an
> "error
>> bulge" in the middle.
>>
>> When you think you have the middle and both endpoints "spot on," check 
>> the
>> tuning error every 100 kHz. If there are two "error bumps" (at, say, 300
> and
>> 700) these can be washed out by simply over-compensating the middle so 
>> the
>> error runs in the other direction.
>>
>> By the way, I replaced the inner shield cover before each measurement.
>>
>> The main thing to bear in mind is that the corrector stack was set
> properly
>> at the factory. The main coil is heavily doped and is not likely to move
> or
>> change. What IS likely to happen is a change in the shunt capacitance.
>>
>> The shunt capacitors are intended to be temperature compensating, but 
>> this
>> never bothered me much. Ordinary NPO ceramics seem to work fine.
>>
>> Miles Anderson, K2CBY
>>
>
>
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