[R-390] Urban Legend Cap

[Wes Bolin]

I am a little confused after reading all these posts.  Can someone clarify for me:
1.  The self-healing film capacitors only heal with HV transits?
2.  How many HV transits do bypass capacitors see in a radio over time?
3.  Do self-healing film capacitors have a shorter MTBF than other tyoes?
4.  Is it wise to (or not) to use a High Voltage rated film capacitor in lower voltage circuits (like 1 KV rated in 250 VDC circuit)....basically
     will it have longer MTBF from less voltage?
I just re-capped my SP600 using ceramic discs in the RF deck, but metal film bypass capacitors in the rest of the radio.  Do I need to swap
out the metal film caps with ceramic discs (or metal foil, or???)?
Thanks for your comments.
Wes
 

________________________________
 From: Perry Sandeen <sandeenpa at yahoo.com>
To: r-390 at mailman.qth.net 
Sent: Sunday, April 28, 2013 7:53 PM
Subject: [R-390] Urban Legend Cap
  

Urban Legend Cap 

List,

Charles Steinmetz has commented extensively about problems with film foil caps.  In his usage situation these may pose a problem.

In group buys that I have made for the list over the last 9 or so years or so, I sold over 4,000 Vishay/Sprague 197LS.01 1KV 20% ceramic bypass caps for both R-390A and SP 600 radios.

On another buy I sold 1,000 .01/600 volt film bypass caps.

As the Vishay/Sprague price had risen dramatically, on the last buy I made for the group I bought 2,000 Murata DEB series .01 1KV -20 +80%.

Let me assume for a moment that only 50% have been installed so that makes 3,500 caps.

No one has ever told me that even one has failed.

With that type of track record I’m not going to stuff oversized and 8 time the price (in 1,000 pieces $1.22) caps in my equipment for bypass use. 

And to add insult to injury for those who are dead-set on using them, according to the current Mouser online info, they are being discontinued.

And AFAIK all the tubular bypass caps made up until the ‘60’s were all of foil construction. Correction welcomed.

Now for a bit of logic.  IF only foil construction capacitors met current HV needs, many manufactures would be making them.  Instead just the opposite is happening.  They are all making deposited metal coated plastic types.  So besides being cheaper and smaller they are accepted as reliable by the electronics industry for high voltages.

I don’t care if people choose to use OD’s for their radios.  It their preference. They can use Leyden jars if so inclined.

What I’m opposed too is this urban legend that will not die is that OD’s are necessary.  (Egor! Get the garlic, cross’s and wooden stakes!) We have new people coming to the list seeking information and are then mis-informed.   When one recaps a whole receiver, which is changing from desirable to mandatory as the sets get older, the economic difference between the two is probably around $100.  

I will wager USD $1,000.00 that no one on this list can prove that foil OD caps are measurably superior to the equivalent ceramic caps in bypass applications in BA receivers.  Any takers?

Regards,

Perrier



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