[Hammarlund] Had more flakey micas!

[Barry L. Ornitz]

>From Barry Hauser's post:

> Phil wrote:
> <snip>
> > I'm beginning to think that a re-cap job should also 
> > include the mica caps, though I've had very little 
> > trouble with them in the past.
> 
> I don't know if it's a real trend or what, but I seem to 
> notice more reports of mica cap failures lately. Maybe 
> they're finally reaching their "half-life".  

I too have seen virtually no problems with mica capacitors 
WHEN THEY WERE USED PROPERLY.  And to this I should also add, 
IF THE RESTORATION IS DONE PROPERLY.

Silver mica capacitors do have a problem with electrolytic 
silver migration if a high DC voltage is applied and the 
silver is exposed to a moist environment.  Considering the age 
of most gear that used these capacitors, it is difficult to 
keep all of the moisture away from the capacitor internals.  
Keeping a good seal against moisture, especially around the 
leads, is difficult enough.  With advanced age, keeping this 
seal is almost impossible.

But the silver migration only occurs when a significant DC 
voltage is found across these capacitors.  When used in RF 
circuits, good design dictated that these capacitors rarely 
had much DC voltage applied to them.  Sometimes these 
capacitors had grid bias across them, but usually they were 
not called on to block much DC.  When used this way, these 
mica capacitors should be extremely reliable.  When used in a 
circuit with considerable DC across the capacitor, beware.

Another problem is that some manufacturers made paper 
capacitors that were cased in the same phenolic cases as 
"postage stamp" mica capacitors.  As with all paper 
capacitors, these fail at an alarming rate.  Going by the case 
and the markings, it is sometimes difficult to tell if these 
are really mica capacitors or not.  After you find a bad one, 
break it open and see if it is really mica inside.  One hint 
is the value.  True mica capacitors were uncommon (but not 
impossible) in values above 1000 pF (or uuF in those days).

Finally a new problem is surfacing.  This is the lazy jerk who 
feels that drenching entire radios with WD-40 and contact 
cleaners actually does the radios some good.  It does not!
And since most of the old phenolic capacitor cases have fine 
cracks or incomplete seals after all these years, the solvents 
and lubricating oils can get to the mica and foil inside and 
cause all sorts of problems.  At best, the dielectric constant 
of the assembly is changed causing a shift in capacitor value 
(which is not stable).  At worst, high leakage currents or 
shorting can result.

A little bit of moisture in a mica capacitor will lead to a 
shift in value, but as the radio is operated more and for 
longer periods of time, much of this moisture will leave by 
being baked out.  This can lead to the need for realignment 
after a period of time if the radio is quickly realigned after 
it is first restored.  Likewise if the radio sits for extended 
periods without being used, the moisture can return and shift 
the alignment a little.  Barring completely baking the 
radio for an extended time and then coating every component 
with a new moisture seal, little can be done here but to 
operate the old radios frequently!  Don't let those old 
wonders sit on a shelf after restoration - use them.

Carbon composition resistors are also prone to the same sort 
of problems.  Fortunately modern film resistors, especially 
the metal film resistors, do not have these problems.  Most 
modern capacitors, especially the film capacitors, are much 
better able to prevent moisture ingress too.

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