[R-390] Old Brown Beauty Statistics Lesson

[Dallas Lankford]

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