[Hallicrafters] Black Beauty capacitors Beware

[Greg Gore via Hallicrafters]

Thanks Jim,


Is this why you don't see much improvement replacing black beauties in the selectivity circuits of Hallicrafters 50KC systems? The capacitance of the molded capacitors has not changed much over the years while leakage at high voltages is almost always excessive and dangerous.


Regards, Greg



-----Original Message-----
From: Jim Liles <hallicrafterssr2000 at k9axn.com>
To: hallicrafters <hallicrafters at mailman.qth.net>
Sent: Fri, Mar 6, 2015 12:26 pm
Subject: [Hallicrafters] Black Beauty capacitors Beware


The ER article regarding Black Beauty capacitors has an assumption that needs
attention.

All of the 50 Kc I.F. systems in the Hallicrafters receivers
beginning from day 1 used Paper or Plastic film capacitors except one, the
flagship SX-115 which used Ceramic disc Z5U capacitors.  This was a disaster
that only becomes worse year after year.  

The selectivity circuits in the
SX-117 using film handily outperform the SX-115.  We talked about the reason for
this in an earlier post so won’t repeat all here.  

I have never seen a
schematic drawn by the Hallicrafters Co. that referred to a film or plastic
capacitor, just Paper, even though they switched to plastic film well before the
SX-100 was built.  The reference to paper is to assure that Ceramic is NOT used
in the noted circuit.  You will find paper and ceramic of the same value in
these radios so why distinguish?

There is a notion that the paper capacitors
had some peculiar property when used in the 50 Kc I.F. system or any other
circuit that would enhance the performance of the radio.

The purpose of the
capacitors in the 50 Kc selectivity circuits is but one.  Move the band pass
center away from the 50Kc carrier to a point where widening the band pass with
resistors would not require shifting the BFO.  Nothing more.

In the SX-115,
using Ceramic Z5U capacitors resulted in shifting the band pass centers to
anywhere but where they belong and it gets worse over time.

Why the problem
with the SX-115?  Find out for yourself.  Measure the capacitors.  The .01uf
caps will be approximately .0082uf.  That’s because Ceramic disc class 3
capacitors loose 5% of their capacity every decade.

Take it a step further. 
The Z5U has a +20 –56 temperature coefficient.  If you heat the caps to 20
degrees C over room temperature which is the expected temp deviation in the
radio, the capacitor will end up at about .0072uf to .0074uf.  The paper or film
will be .01uf.  

Result, the band passes are significantly misplaced. Alas,
lost in space looking for GPS coordinates!

In audio circuits the notion that
paper has some magic and mirrors in comparison to film is campfire talk.  In
comparison to ceramic class 2 and 3, it is no longer campfire talk but
irrefutable fact.

Kindest regards Jim
K9AXN
______________________________________________________________
Hallicrafters
mailing list
Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
Help:
http://mailman.qth.net/mmfaq.htm
Post:
mailto:Hallicrafters at mailman.qth.net

List Administrator: Duane Fischer,
W8DBF
** For Assistance: dfischer at usol.com **


This list hosted by:
http://www.qsl.net
Please help support this email list:
http://www.qsl.net/donate.html