[R-390] Old Brown Beauty Statistics Lesson

Wed Jan 12 23:51:23 EST 2005

Ah hell lets just take all the resistors out and capacitors out and put them
in a big bag and shake them up and then throw them all on the work bench and
then put them  back in  the receiver.
Now that is a jig saw puzzle.
Matt
----- Original Message -----
From: "Bob Camp" <ham at cq.nu>
To: "Dallas Lankford" <dallas at bayou.com>; "R-390 HF Receiver List"
<r-390 at mailman.qth.net>
Sent: Wednesday, January 12, 2005 7:42 PM
Subject: Re: [R-390] Old Brown Beauty Statistics Lesson


> Hi
>
> A couple of thoughts about why the capacitors crack.
>
> There are a significant number of electronic parts out there that get a
> plastic coating over them when they are manufactured. Some of the
> coatings are relatively soft and will stretch a little. Others are
> fairly brittle and don't stretch much at all. Most plastics are made by
> cross linking long molecules together. The more linking you do the more
> brittle they get and in some cases the more they shrink.
>
> Other plastics are sensitive to humidity. A great example is Nylon. If
> you have ever seen nylon hardware that has been out in a hot dry
> location for a couple of years it's amazing how much the stuff shrinks.
>
> I have personally been involved in a couple of *very* painful recalls
> on epoxy coated parts over the last 30 years. In each case the epoxy
> slowly shrank over time. The net result in one case was thermistor that
> popped open like popping corn. The net result in the other cases where
> tuning capacitors that went out of alignment and shorted out. I suppose
> that if necessary I can dig up the GIDEP's on all of them. In all cases
> these were parts used in military equipment. Rockwell Collins, TI, and
> Magnavox (Ft. Wayne) were the end customers.
>
> Bottom line in each case:
>
> 1) The ratio of hardener and resin is pretty important in an epoxy. The
> normal methods mixing the stuff are surprisingly open to error. Is it
> equal parts by volume or by weight. Do you mix each can before
> measuring.
>
> 2) Regardless of the chemistry plastics continue to cross link after
> they are judged to be "fully cured". In some cases this is accelerated
> by humidity. In most cases it is accelerated by radiation. In all cases
> that I have been made aware of it is accelerated by heat.
>
> 3) Thermal cycling of the shrinking case wrapped around a hard inner
> core will also accelerate the cracking, crushing, or moving process.
>
> So much for the intro, but this is a thread for the long winded.
>
> Now for the variables:
>
> Radio A:
>
> Runs 24 hours a day in a tight rack for 20 years on board a ship. Nice
> salt content in the air even with the high temperature inside the rack.
>
> Radio B:
>
> Sits in a open air hut in far off land for 10 of those years and then
> is in storage in who knows where for 10 years. Used 6 hours a day 4
> days a week when in service. Radio is rack mounted, but the rack has
> wide open ventilation.
>
> Radio C:
>
> Run in the continental US, inside a climate controlled building. The
> guard guys run it two days a month for six hours. Radio is the only
> thing in the rack.
>
> All of these radios have a very plausible carrier in the military.
> Certainly we would all like to get radio C if we could. I would suggest
> that a lot more radios fall in the A and B categories. Somehow I doubt
> we are arguing about radio C so we'll simply drop the "baby doll"
> radios at this point.
>
> If you just look at time and temperature on radio A and radio B there
> is an enormous difference in what happened to each radio. This is not
> to say that is the only variable. It certainly isn't. Radio A saw 10X
> the time on power over the 20 years as compared to radio B. Common
> temperature measurements of the inside of the Navy racks put them up at
> about 60C or so. Best guess on the average hut would be 20C. If the
> acceleration factor is 2X per 10C rise (activation energy below 20C
> ...) then radio A gets 2^4 more stress. Net result is that radio A sees
> about 160 times more stress than radio B.
>
> I would humbly suggest that radio A is going to have a *different* set
> of problems than radio B. Most of us would be hard pressed to say that
> radio A has been more abused than radio B by visual inspection.
>
> Regardless of weather the caps crack or something else goes radio A is
> a lot more likely to have had problems of a certain type over it's life
> than radio B. So far I *hope* none of this is to controversial. It's
> pretty much straight out of MIL-HBK-217.
>
> Here's the part that makes for the problem.
>
> Depending on how good the maintenance on the radios was it's a total
> toss up as to weather radio A, B, or C is in better condition today.
>
> It is my contention (and I suspect that you *might* agree with this)
> that we routinely go well past the previous "standard" when it comes to
> working on these radios. I won't argue weather we are going above or
> below the standard, only that a lot of people these days do a lot more
> work on these radios than was done in the past.
>
> Is a leaky capacitor for instance a problem? That depends on who is
> doing the maintenance. In one case a radio that meets minimum
> specifications on the bands of interest never gets pulled or worked on.
> In another case the radio is worked on until it meets "bragging rights"
> specification levels. The first case probably has a very different
> opinion of what is a bad cap than the guy in the second case.
>
> Unless we agree on what is and isn't a bad capacitor I suspect we'll be
> at this for the next hundred years. Somehow I doubt we will come to
> anything other than an agreement to disagree ....
>
> Take Care!
>
> Bob Camp
> KB8TQ
>
>
>
>
>
>
> On Jan 12, 2005, at 9:20 AM, Dallas Lankford wrote:
>
> > Disclaimer:
> >
> > "If you all are talking about those rained-on, snowed-on, sun-baked,
> > frozen,
> > left in the parking lot for years, or otherwise abused R-390A's, then
> > my
> > remarks about capacitors do not apply to them.  I would not touch one
> > of
> > those with a 10 foot pole.  There are people who believe they can be
> > rebuilt.  I don't.  I wouldn't buy any R-390A nowadays that I couldn't
> > personally inspect before I bought it, or that didn't come with a
> > return
> > guarantee that it hasn't been abused."
> >
> >
> > The fact that your R-390A was an (old) Motorola [see below] does not
> > change
> > the fact that your claims violate the laws of statistics.
> >
> > Among the R-390A's that I have carefully inspected and rebuilt were
> > two 1956
> > Motorolas.  Let's see...  how many brown beauties were in those two.
> > I am
> > not sure.  There were about 12 each in the IF decks, and at least 1
> > each in
> > the RF deck.  That is a total of 26.  If on the average we would
> > expect  out
> > of 10 to be cracked (your claim), what is the probability that none
> > were
> > cracked (my observation)?  The answer is simple statistics.  Multiply
> > 3/10
> > by itself 26 times.  That is 2.5419 time 10 to the -14 power.  So the
> > odds
> > that I would observe none when you observed 70% cracked are 1 in
> > 254,190,000,000,000.  This violates the laws of statistics.
> >
> > The above does not include a 3rd Motorola IF deck that I still own, in
> > which
> > none of the brown beauties were cracked or bad.  If I included it, the
> > odds
> > would be even more outrageous, namely 1 in 1.3509 times 10 to the
> > minus 20.
> >
> > BTW, both of those Motorolas are alive and well (about 20 years after I
> > rebuilt them), with none of the brown beauties replaced (and none have
> > cracked or gone bad in the meantime).  The only problem which has
> > developed
> > in either is a switch which will not turn off in one of them.  The
> > owner,
> > who lives nerby, is too lazy to bring it by for me to fix.
> >
> > Dr. Dallas Lankford
> > retired Professor of Mathematics and Statistics
> > Louisiana Tech University
> >
> >
> >
> >
> > ----- Original Message -----
> > From: <N4BUQ at aol.com>
> > To: <r-390 at mailman.qth.net>
> > Sent: Wednesday, January 12, 2005 7:29 AM
> > Subject: Re: [R-390] Re: Caps and more caps
> >
> >
> > <cut>
> >> A large percentage of the "brown beauties" in my '56 Motorola had
> >> cracks
> > that could easily be seen.  I don't think mine was abused, but just
> > old.
> >>
> >> Barry(III) - N4BUQ
> >
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