[R-390] Info on Capacitors

[nryan at mchsi.com]

Hi, Don and group,

Great discussion going on here re capacitors.

The "self-healing" feature is small comfort for the R-390A's C553 capacitor.  That's the one which will take out your mechanical filters one by one if it shorts or leaks.  I use the Sprague/Vishay 715P (.01 uF @ 600VDC) and hope for the best.  Replacement (used) mechanical filters have virtually dried up on the market, so be forewarned herewith.

The BBODs routinely get replaced as almost of them all leak.  With the exception of C553, mentioned above, I retain the metal encased paper-in-oil capacitors as they usually test good.

C603 and C606 (plug-in electrolytics) we already know about -- monitor closely or restuff/replace them.  C609 (tantalum 8 uF @ 30VDC) frequently is found deteriorated.

Faulty silver micas are hard to pinpoint much less test in place.  Ceramics do well although I recently replaced a faulty C410 (.005uF @ 1KV).

Are there other capacitors to be especially vigilant about?  

73 de Norman, KG4SWM

Any activity becomes creative when the doer cares about doing it right, or doing it better. -- John Updike


----- Original Message -----
From: 2002tii <bmw2002tii at nerdshack.com>
To: R-390 at mailman.qth.net
Sent: Tue, 10 Jan 2012 20:17:30 -0600 (CST)
Subject: Re: [R-390] Info on Capacitors

Dave wrote:

>I read what I could find easily on self-healing, but the difficulty I'm
>having is this: These things are sold and used in commercial gear where
>failure to meet specs leads to liability lawsuits in a very large hurry.  If
>they aren't suitable at more than a fraction of the rated DC working
>voltage, how do the cap mfrs. stay in business?

Different applications can tolerate carrying the occasional (or not 
so occasional) clearing current more gracefully than others.  It's 
not a matter of not meeting spec -- manufacturers are very up-front 
about self-healing, and the applications in which careful designers 
use metallized film capacitors are ones that are able to withstand 
the abuse.  (Not so careful designers just use them because they are 
cheaper and smaller.  I've seen more than a few military and 
commercial designs that had serious reliability problems right from 
the start due to the unfortunate selection of metallized film capacitors.)

"Self healing" is a real case study in marketing -- turning a 
disadvantage (heck, call it what it is -- a recurring failure mode) 
into a "feature."  Hey, look -- our capacitors FIX 
THEMSELVES!!  Yeah, great.  Better to just buy capacitors that don't 
have a recurring failure mode in the first place.

Best regards,

Don

Copyright (c) 2012.  Not for redistribution