[Boatanchors] And Now for Something Competely Different: Zenith

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: "Glen Zook" <gzook at yahoo.com>; "Charlie , W5COV" 
<cvest at cox.net>; "David Stinson" <arc5 at ix.netcom.com>; 
<boatanchors at mailman.qth.net>
Sent: Saturday, March 30, 2013 11:02 AM
Subject: Re: [Boatanchors] And Now for Something Competely 
Different: Zenith


The quality and sound from these radios is pretty amazing. 
My theory is that after we restore them, they perform better 
than they have since they left the factory.

Here's why: back in the '30s, capacitors were a high-failure 
item even when new. A lot failed quickly and a lot more 
probably suffered leakage even then. A radio with a dozen or 
so caps that are leaky to one degree or another isn't going 
to sound as good as it should. It's sort of a cumulative 
degradation. If there was one particular cap that shorted or 
caused a noticeable problem, the repairman would replace it 
but I've never heard of any repairman back in the day 
replacing all the paper caps. It would probably have cost 
more than a new set. Instead, the set was written off as 
"old" and "tired" and relegated to a dusty attic.

Now, we come in and replace all the caps with high-quality 
modern caps. We replace the out-of-spec resistors and align 
the IFs with precision (thank you, frequency counter). The 
end result is that the radio performs the way it was 
designed to perform, probably for the first time since it 
left the factory.


Joe Connor


     It would be interesting to find any actual measurements 
on paper caps from that time. I suspect the dissipation 
factor or ESR would have been higher than on modern film 
caps where its so low as to be difficult to measure. Mica 
caps were used where low ESR was necessary.  The old 
handbooks say that paper caps degraded from applied voltage 
and for long life the rated voltage had to be very 
considerably higher than the applied voltage. I think that's 
why you often find 400V caps in applications where perhaps 
no more than 200 volts is across it.  The plastic 
impregnated caps which began to become available after WW-2 
such as the notorious Black Beauty caps were supposed to 
have much longer life than untreated paper. So were 
oil-filled caps.  The BB's should not be used to judge the 
average performance of caps of this type because I think 
they were defective in manufacture.  I found that the flat 
Micamold caps have a similar looking dielectric and were 
much longer lived.  Most of the old caps were impregnated 
with wax to seal out moisture. It must have been fairly 
effective since we are replacing caps that are often sixty 
or seventy years old.  I've measured many modern plastic 
caps using a General Radio bridge to measure dissipation 
factor and various methods to measure parallel resistance, 
AKA, leakage, both are so low that its hard to even detect 
them.
    I think around the 1960s that manufacturer's wanted 
something better than the usual paper caps so we began to 
see both plastic impregnated paper and ceramic caps being 
used.  Ceramic caps are very long lived but those with high 
dielectric constant material are not stable and can also 
degrade with time. Low K caps, like the zero temperature 
coefficient type, are extremely stable and can be used for 
virtually any application but tend to be expensive and are 
too large for modern surface mount use.


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