[Hallicrafters] SX 42 capacitor question

[roy.morgan at nist.gov]

Quoting Steve & Sharon Nordentoft <sskn at msn.com>:
> 
> ----- Original Message -----
> From: chandlerh2 at aol.com 
> 
> i am recapping my sx 42 ... the caps that are in there look like mica caps.
> 
> they are about 3qtr by 3qtr square black  have the name TOBE on them are  
> 10000 MMF 

> Steve,
> 
> Those caps are mica. 

Steve and others,

I disagree, at least until further information is at hand:

Mica caps of that age are seldom found in the value of 0.01 uF. (10,000 mmf) 
These black caps may be of the same sort as the black, rectangular, beveled
cornered ones found in the BC-348 receivers, which caps are in fact paper
dielectric. not mica, and which caps are INVARIABLY leaky or shorted.

I have an SX-42 here and can have a look.  In addition, I can get to see two
other SX-42's, possibly today, to see what they have.  My memory of the other
two radios is that thy have cylindrical wax covered caps that are clearly paper
ones, not the black rectangular ones that are deceptive.

My suggestion is that you disconnect one or more of them and test for leakage. 
You can do this with the caps in place, power off or tube removed, with B+
applied from the set's supply or a separate supply, if you understand the
effect of a leaky cap in a screen bypass location, for instance.

My advice is that if you find even one of them to be leaky, do not stop till
they are all replaced.

A further note on the SX-42: 
Many or all of the IF cans contain small, red, rectangular caps that are easily
recognized as silver micas. These caps in at least one instance I know of were
shorted or open, and caused inability to tune the stage correctly during
alignment.  In some cases, the IF cans have wire leads extending from the
inside of the can to the rest of the circuitry. In the IF cans I have seen the
insides of, the cap is held in place by a stiff metal tab of the IF can
internal frame, and the (short) leads of the cap are used as the connection
point for both the litz wire of the coil and the wire lead that connects to the
rest of the radio.  All this means that replacing a cap is a delicate and
complicated operation, and is an opportunity to test your patience and
dexterity.

Opinions vary on whether to:
 1) Replace all the SM caps in the cans, a big job, or leave them all in unless
proven defective.  One veteran restorer of legendary equipment has decided to
"let 'em be", at least until further trouble appears.
 2) Replace any defective one with the can in the radio, or to remove the can
for surgery out on the bench.  One factor in this choice is whether the thing
has terminal points on the bottom or the integral wire leads.
 

Roy


Roy Morgan
7130 Panorama Dr.
Derwood MD 20855