[Hallicrafters] BBOD's and old caps in general

[Richard Knoppow]

----- Original Message ----- 
From: "TC Dailey" <daileyservices at qwest.net>
To: <hallicrafters at mailman.qth.net>
Sent: Saturday, January 05, 2013 9:16 AM
Subject: [Hallicrafters] BBOD's and old caps in general


>I frankly don't understand even BOTHERING with BBOD's 
>They're BAD, ipso facto.  A month ago, I began the repair 
>on an HRO-60, which had dull-red plates on the rectifier, 
>loud hum, and flashing at the function switch - somebody 
>had installed a BBOD from the audio-output screens point to 
>ground (C-124), so I checked IT first;.  The cap measured 8 
>ohms across, so basically a dead-short.  They're just bad.
>
> TC
    I completely agree. Evidently there was some 
manufacturing problem with BB caps, especially the ones that 
were coded with stripes. The ones I've dissected all had 
distorted windings even if the cases seemed to be intact, 
and none had any sign of oil left. There was a later version 
called a Telecap that was not oil filled. The oil filled 
ones can be told from others because they have one thick 
lead coming out of the case with the normal lead soldered 
into it. This lead is actually the filling tube. I have 
occasionally seen BBs with an oil slick on them.
    The problems evidently showed up pretty soon after 
manufacture because the modification work order for the 
SP-600 was issued sometime in the mid or late 1950s.  I 
remember an electronic engineer friend warning me about them 
nearly fifty years ago.
    Paper caps in general have a limited life in use. As 
long as they have voltage on them some degradation is taking 
place. The greater the ratio of rated voltage to service 
voltage the longer the life and the lower the operating 
temperature the longer the life.  The use of oil 
impregnation was supposed to mitigate this problem and good 
oil filled paper caps do indeed have a very long life. 
Several manufacturers began to make caps with paper coated 
or impregnated with plastic (not sure from memory which). 
This again was supposed to eliminate or at least reduce the 
effects of voltage on the paper plus increase the dielectric 
constant. The BB is of this type was sold as a high-quality 
cap.  Other caps using plastic and paper seem to have 
survived much better so its not the _type_ but something 
specific about the way the BBs were made.
     BTW, I suspect it is the leakage that the music people 
want since, when used as a coupling cap,  it upsets the bias 
on the stage following it. That results in distortion and I 
think they want the distortion.  It also kills tubes.


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