[Hammarlund] SP600 and BBODs

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: "Wes Bolin" <k5apl at yahoo.com>; "Ken Gordon" 
<kgordon2006 at frontier.com>; <hammarlund at mailman.qth.net>
Sent: Monday, March 04, 2013 8:08 PM
Subject: Re: [Hammarlund] SP600 and BBODs


Many of the 75A-2 and 75A-3 receivers have brown colored 
capacitors instead of black like the 75A-4. Of course, the 
75A-1 used bathtub capacitors. Although bathtub capacitors 
have lasted longer than other paper capacitors, they are 
going bad on a regular basis these days and I just replace 
all of them with modern capacitors.

Glen, K9STH


     All the paper caps have problems with long term 
exposure to voltage which tends to cause a chemical change 
in the paper.  The lower the voltage with reference to the 
rated voltage the longer the life.  I am not sure paper caps 
become degraded with just time.  Its usually worth replacing 
all of them.  I just went through recapping an RCA AR-88, 
this RX has nearly as many paper caps in it as the SP-600 
plus its older.  I tested all the caps I replaced for value, 
dissipation factor (AKA ESR) and parallel resistance 
(leakage).  The caps were of two types: oil filled bathtub 
paper caps, and Micamold paper caps.  The oil filled caps 
were leaking oil due to broken down seals. They were not as 
bad as one would think but were still electrically leaky. 
The Micamold caps varied from perhaps a 100 megohms to as 
little as 10 megohms parallel resistance, too much. 
Curiously, these are well made plastic impregnated paper. 
The dielectric looks similar to that in the Black Beauty but 
the construction of the element is quite different.  All the 
Black Beauty caps I've dissected (several) had distorted 
windings, those in the Micamold were folded and perfect. 
New plastic film caps do not have measurable dissipation 
factor on a General Radio bridge and do not have detectable 
leakage of parallel capacitance.  The RX worked before 
recapping which amazed me after testing what was in it. 
BTW, the Micamold caps appear to have been used in place of 
large value mica caps which were probably not available due 
to war-time shortages.  The older versions of the AR-88 have 
RCA- made mica caps which are lozenge shaped and of an odd 
pinkish purple color.
     Most makers of paper capacitors went to some form of 
molded case after about 1945.  The idea was that molded 
plastic, mostly Bakelite, would protect the winding from air 
and moisture. Some worked better than others. The flat 
Micamold caps, which look outwardly like mica caps, are 
pretty good. Something went wrong with the Black Beauty 
cases. Other makers have cases in white, brown, and some 
other colors.  While they might not have had the short term 
failures of the BB caps all should be replaced.


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