[Hammarlund] Had more flakey micas!

[Barry L. Ornitz]

Phil, KO6BB, described the symptoms he found which he 
attributed to bad mica capacitors:

> This receiver appears to have sat unused for several years.  
> When I got it, with the exception of the cut wires to the 
> panel switches, it appeared to be nearly "virgin" from a 
> repair standpoint (other than tubes etc).  It doesn't
> appear that it has ever been soused with WD40 or other 
> solvent nor waterlogged.

The radio does not have to be waterlogged for the components 
to have absorbed moisture.

>  However, that doesn't preclude the fact that perhaps the 
> capacitors etc did absorb a lot of moisture that had to 
> "cook out".  The typical symptoms the set displayed were, 
> (except for band 6 which was terrible), when cold it'd
> be fine, but after getting heat soaked frequency would start 
> jumping up and down slightly.  Not BIG jumps, just enough to 
> be annoying, especially in sideband.  Replacing the 
> capacitors in the oscillator coil modules cured it.

This sounds exactly like moisture absorbed in the mica.  
Remember mica is a silicate material in the form of extremely 
thin stacked sheets.  Not much moisture (or anything else) can 
diffuse through the sheets themselves.  But moisture can 
readily travel between the sheets.  If that radio had been 
idle for many years, some of that moisture is inevitable.  It 
took quite a while for that moisture to diffuse in, and it 
does not bake out immediately.  I would be willing to bet that 
the minor jumps would have stopped after a few weeks of use.  
A hair dryer could probably speed things up a little.  Once 
dry, periodic use should keep it that way.  

By the way, mica that has absorbed considerable water when 
heated quickly will expand the mica producing vermiculite.  
You don't want this to happen in your radio!

In old radios, lots of components can absorb moisture besides 
the capacitors.  Probably the worst offender is phenolic 
impregnated paper coil forms.  The paper insulation in 
transformers is also a problem.  Most of the time, things 
return to normal after they have dried out properly.  But this 
is not the case with old composition resistors.  Here the 
ingress of moisture causes the carbon granules to not touch 
each other as well and the resistor value increases.  This is 
typically a permanent change.  If you find resistors that have 
increased in value, this is the normal cause and they should 
be replaced.  Resistors that have dropped in value have 
usually been overheated and also should be replaced.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net