[Hallicrafters] R45/ARR-7 Receiver by Hallicrafters

[Richard Knoppow]

     There is something called silver mica disease. Its caused by 
oxidation of the silver plating. That migrates across the layers 
causing shorts and if not shorts causes the capacitance to vary 
rapidly. In an oscillator it produces what is called 
scintillation. The silver can also form whiskers which cause the 
same problems. So, silver mica caps can cause trouble as you 
discovered. Non-silvered, or stacked mica caps are free of this 
problem but likely have others caused by moisture. Stacked mica 
caps are usually found as transmitting caps and an elaborate 
version was used by General Radio for secondary standards. They 
seem to be very stable.
    The problem with mica caps built into IF transformers seems 
to be fairly common especially with National and Hallicrafters 
stuff.
     The problem with paper caps seems to be the same kind of 
degradation that causes newsprint to turn brown. The 
manufacturers tried all sorts of things to lengthen the life but 
it seems to have been more than moisture. Since the expected life 
varied with the amount of DC applied it may have been partially 
an electrolytic problem. For a given applied voltage the life is 
longer where the rated voltage is higher. This perhaps explains 
why one finds 400V caps when there is no more than 100 V applied 
to them. Anyway, all of historical interest, probably all paper 
caps should be replaced and one should be suspicious of other 
kinds too.

On 3/5/2018 2:19 PM, w5jo at brightok.net wrote:
> I was given an SX 42 some time back that was cosmetically pretty 
> good and my resistance measurements looked good so I decided to 
> do the necessary work to get it going.
> 
> It played nicely for a while then suddenly went deaf.  It took 
> some time but I finally found one of those "postage stamp" micas 
> in an IF can that had developed a low resistance short to ground 
> which took the AGC line down.  so those mica caps can cause 
> trouble if the seal around where the wire enters the case becomes 
> compromised and allows moisture to enter.   This is not the first 
> receiver where that problem came up, I have seen it in IF cans in 
> several National NC-183D receivers among others.
> 
> So don't assume one way or the other but be aware the type of 
> material, paper or mica, may not matter and if I am working on an 
> area with those things, I go ahead and replace them  so I don't 
> have to get into tight spots a second time.
> 
> Jim
> W5JO


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL