[R-390] Re-R390 Putting a Cap on Caps

[J.Byers]

This subject is getting out if hand and seems to have been been aided and abetted 
by a post making an overly simplistic statement on statistics.
(Accurate on  paper, perhaps, but not in practice)

With some trepidation I submit these observations.

1.    WE all know there were many manufacturers of the R390 series, apart from Collins

2.    Its HIGHLY unlikely the caps referred to were ALL sourced from the same 
        manufacturer so that its equally unlikely ALL R390's EVER made had caps fitted 
        which were came  from the same production line

3.    The quality of caps made by different factories WILL vary, even if the initial tests
        on these caps satisfied the militray procurement criteria.... which dictated a 
        'normal' service life of these components, and which ALL of them seemed to pass OK

4      These fine  receivers are all well past their (military) 'use by' date .

5..    SO ANY STATISTICS DONE MUST ALSO INCLUDE A 'Q'  FACTOR ALLOWING FOR 
        SUCH MANUFACTURING DIFFERENCES ON  THE EXTENDED LIFE OF THE 'C's' !
        Ie : Consistency of potting mix, property spread of dielectric material, sealing of wire ends
        to mould and probably many other things I don't know about to boot  !
 
6.    Dummer was  a respected UK scientist , specialising on component reliability, who published 
       a great deal of information on this subject  via UK HMSO.
       There were a few of his  articles in the old Wireless World on this and I remember reading 
       one of his text  books which discussed such moulded types as dicussed here..
       He mentions that a major source of failure was moisture creepage along the wire ends getting into
       the mould cavities (Ie cavities filled by wire and the 'C' guts).   
       The effect of corrosion caused by moisture is to pressurise the internals which can lead to cracking
       of the casing. This leads to more moisture ingress and is comulative.
        Improvements to wire sealing within the mould were one of his suggestions at the time.
        (Of course there are other failure modes apart from this)
 
      So any 'statistics' should  show runs of receivers with failures rates higher than others based on the
      MANUFACTURERS  FACTORY through which component sourcing may be traced.
      (I am merely repeating what an acknowledged expert on component reliability has already said)
      Indeed, it may be that a single R390 had MULTIPLE CAP SOURCES from different manufacturers
      in order to keep contract production rates going !  

SO:   WALTER  was quite correct in his statement.that such caps were consistently cracking.

And the others may well be correct  in thier observation that none had cracked !

Its likely that the spread band of good and bad 'C's  may well be due to effects as noted above
 (in addition to many other 'noise' factors as well, such as exposure and electrical overstressing, etc etc.)

NOW I am restoring my own R390.
  I WILL RETAIN AS MANY OF THESE COMPONENTS AS I CAN  simply because I like the look
 of them and they ARE part of the history of the receiver. !!!
YES,  ITS A NOSTALGIA THING !!

I can afford to do that as I am capable of and will be fixing it myself

(I will replace ALL critical 'C's' as gleaned from all you experenced fellows out there, which is what Mailmans
excellent service is all about)

BUT:  If I were given the job of restoring someone else R390,  I WOULD TEND TO REPLACE MOST, IF
NOT ALL these parts ,,,,,,,,,,,,  depending on what my clients wishes were.
After all, resistors and wire end capacitors are relatively inexpensive things to replace

And my reputation on doing a professional  job  of repair would depend on so doing

To check each Cap PROPERLY  you have to lift off at least one end, and to my mind once youve gone  that far 
your half way  to replacing  the thing with a modern, much more reliably made one
.
But.... and there's the rub.... its not 'original'  any more, is it?
That too may be important to both a repairer and a client.

Lets end this silly  bickering

BOTH sides  may just be  right enough in there own observations

John Byers