[R-390] Old Brown Beauty Statistics Lesson

Thu Jan 13 18:15:46 EST 2005

Hi

The only parts being talked about here are leaking capacitors in high 
impedance circuits. The reason we know they leak is because we measure 
the leakage. We replace the capacitors that are broken.

Your position seems to be that a part that is broken (it is out of 
spec) AND is in a part of the circuit where that affects the 
performance of the radio should be left in place.

I simply do not understand this approach at all.

	Bob Camp
	KB8TQ

On Jan 12, 2005, at 11:51 PM, mparkinson1 wrote:

> 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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>>
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>
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