[Boatanchors] [BoatAnchors] "NOS" Caps: Don't Trust'em; Check'um!

[Richard Knoppow]

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: <boatanchors at mailman.qth.net>; 
<boatanchors at theporch.com>; <milsurplus at mailman.qth.net>
Sent: Friday, May 04, 2012 10:58 AM
Subject: Re: [Boatanchors] [BoatAnchors] "NOS" Caps: Don't 
Trust'em;Check'um!


> Arden said:
>
>> ..... A "NOS" paper cap is no better than one
>>wired into a rig at the factory...
> I actually agree.
> By "NOS," I didn't mean "old originals."
> These caps are "new production but unused and surplused"
> Polyester Film caps.
> Perhaps my definition of "NOS" needs correcting.
>
> As for really "old caps" like Micamolds and paper-
> I've thrown-away all the low-voltage ones and
> have, after long testing, successfully deployed a few
> hi-volters in low-voltage situations.  Some have been
> in service without trouble for years.  I remember one 
> failure,
> but at 45 volts B+ it did no harm.
> The RAAF AR8 receiver has all but one original WWII
> black-tar-and-paper cap still in it, running at 90V B+.
> I have left it on for 48 straight hours with no issues.
> It runs cool and stable.
> The AT5 transmitter had the same type caps and
> all but one of them was bad beyond hope.
> Go figure ;-) and "Each to his own."
>
> 73 DE Dave AB5S

     I've never found an authoritative answer to why old 
paper caps fail. A couple of things are mentioned: migration 
of moisture into the windings; impurities in the paper or 
impregnant when one is used.  The purpose of the wax or oil 
in some caps is partly to increase the dielectric constant 
but also to protect from moisture. I think caps in molded 
plastic cases were supposed to be more resistant to attack 
by moisture than the wax filled kind but they also seem to 
have a limited life.  The older books have derating charts 
for paper caps showing expected live versus voltage, the 
higher the voltage the shorter the life.  So, the limitation 
on life was known long ago.  I think this also explains why 
its common to find caps much over-rated for voltage, such as 
600V caps in applications where there is only perhaps 200V 
on the cap.
    New caps are just plain better. I have the means to 
measure value, dissipation factor, and dielectric resistance 
and new film caps are excellent.  Of course I don't know 
what the old paper caps were like when new but I suspect 
they were never that good.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com