[Hallicrafters] BBODs

Sun Mar 8 16:00:20 EDT 2015

 I'll throw throw in one kick before the horse is buried..   In lieu of the recent BBOD discussion, here's something I wrote up on the Collins reflector a year or so ago.  Enjoy, and let the bullets fly!:
 To venture into dangerous territory, I will step out on a limb and counter your blanket statement- Paper caps were _not_ all junk when new. Paper dielectric capacitors are still available to this day, (at a justifiably high price!) in both metalized and non-metalized (foil instead of film) configurations for use not only in hi-fi equipment who want that paper/foil cap "sound", (Let's not get into that discussion, please), but for motor run and other various industrial and commercial settings. These are oil wetted/filled and capable of withstanding higher voltage and dissipating more heat without degradation, than a poly(x)ene film or foil caps. I'm sure Bill and others have Collins transmitters from the 30's with paper dielectric capacitors that still test just fine in the capacitance AND leakage department, even at full rated voltage. 

 The main problem with the molded paper Sprague "Black Beauty Telecaps" and so-called "Bumble Bee capacitors" (both referred to as BBOD, Black Beauties Of Death) is how well they are sealed- Contrary to popular belief, the genuine Sprague "Black Beauty Telecaps" were very high quality components in their time.  The  end seals on these caps are actuallyquite good- however the molded bakelite phenolic body by unintended design, is vulnerable to cracking which allows contaminating air and moisture (oober bad) inside, and allows the oil to vent out. Ever wonder why all 600+V (and some lower voltage) Black Beauty caps have what looks like a solder ball on one end? These caps were molded DRY. The end with the solder blob is actually a very small tube/eylet which allowed mineral oil insertion under high vacuum AFTER the capacitor was rolled and molded. A lead was then slid inside this tube, and soldered in place. By molding the capacitor dry and wetting the paper later, a much lower contamination level was able to be reached during the manufacturing process. 
 This is the same manor in which the much more expensive (and reliable) metal encased, glass "hermetically" sealed capacitors such as the Sprague "Vitamin Q" series were made. These caps are commonly found in the R-390A, and many other military radios from the mid 50's onward until plastic capacitors took over.  Of the ones I have removed, I have only tested ONE of these low value Vitamin Q caps to have under 500Mohm of leakage resistance, my "replacement threshold" being 200-500Mohm depending upon humidity, the phase of Mars, and just how lazy I'm feeling that day.  
 To the same point, I recently rebuilt a Detrola manufactured broadcast radio from about 1932 for a friend. The cabinet had been refinished so the goal was long term reliability, not originality- So all capacitors were replaced. For kicks, I decided to check out a few of the old "guaranteed to be junk" caps.  All but one of those paper decoupling/bypass capacitors were original to the set, and made by Solar. They were metal cans with rubber end seals, dipped in wax, and had a rolled paper covering. Guess what? EVERY SINGLE ONE tested within 10% of their stated value, and had leakage greater than 1000Mohm, most approaching infinity. This from "junk when new" paper capacitors that are (almost!) older than the eldest CCA member according to the recent informal survey. 

 I hope I work as well as those caps at 82....
 A counter to all said above, is a personal theory...  The paper used in these old capacitors may very well not be acid free- additionally it could vary in pH from batch to batch, manufacturer to manufacturer.  This could indeed present an age alone failure, as <7pH acidic paper breaks down (At what speed? Additionally at what speed when oil impregnated??) in addition to very possibly reacting with the high purity aluminum foil. If someone has scientific analysis equipment available to them, this would make for a VERY interesting study. 

 Moral of the story, not all paper caps are junk... The "shotgun replace 'em all" approach does regrettably have it's place- But should never be an automatic requirement for every piece of vintage gear- It should be done on an individual basis. Odds are if a cap passes the "life test" and still measures very good after the first 50 years, it may very well outlast you. 

 Regards,
 Chris kc9ieq
      From: Tom Dailey <radio at daileyservices.com>
 To: hallicrafters at mailman.qth.net 
 Sent: Saturday, March 7, 2015 9:14 AM
 Subject: [Hallicrafters] BBODs

BBODs - Black Beauties of Death -  have been discussed ad nauseum here, but 
point of fact is that they're BAD.  Period.  I've seen 'em split, 
DEAD-shorted, leaky, and every other thing.  You may not see immediate 
improvement, but not changing them will result in DEATH of many things; not 
the least being the B+ windings on your pwr xfmr.  Yes, there are many 
differences in caps, and the explanation by Jim, was spot on. 

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