[Hammarlund] Had more flakey micas!

[Phil Atchley]

Good morning Barry (Ornitz);
Concerning mica capacitor reliability, I'd have to say that I've always
considered them to be very reliable and rarely replaced them.  But now =
I'm
starting to get a different opinion of them.

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.

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.

BUT, even if it was due to moisture that was absorbed over the years, I
STILL say that is not acceptable behavior and that those capacitors are
unreliable, for whatever reason!   =20

73 de Phil, KO6BB
DX begins at the noise floor!
k06bb@elite.net
Merced, California
37.18N  120.29W  CM97sh

>From Barry Hauser's post:

> Phil wrote:
> <snip>
> > I'm beginning to think that a re-cap job should also=20
> > include the mica caps, though I've had very little=20
> > trouble with them in the past.
>=20
> I don't know if it's a real trend or what, but I seem to=20
> notice more reports of mica cap failures lately. Maybe=20
> they're finally reaching their "half-life". =20

I too have seen virtually no problems with mica capacitors=20
WHEN THEY WERE USED PROPERLY.  And to this I should also add,=20
IF THE RESTORATION IS DONE PROPERLY.

Silver mica capacitors do have a problem with electrolytic=20
silver migration if a high DC voltage is applied and the=20
silver is exposed to a moist environment.  Considering the age=20
of most gear that used these capacitors, it is difficult to=20
keep all of the moisture away from the capacitor internals. =20
Keeping a good seal against moisture, especially around the=20
leads, is difficult enough.  With advanced age, keeping this=20
seal is almost impossible.

But the silver migration only occurs when a significant DC=20
voltage is found across these capacitors.  When used in RF=20
circuits, good design dictated that these capacitors rarely=20
had much DC voltage applied to them.  Sometimes these=20
capacitors had grid bias across them, but usually they were=20
not called on to block much DC.  When used this way, these=20
mica capacitors should be extremely reliable.  When used in a=20
circuit with considerable DC across the capacitor, beware.

Another problem is that some manufacturers made paper=20
capacitors that were cased in the same phenolic cases as=20
"postage stamp" mica capacitors.  As with all paper=20
capacitors, these fail at an alarming rate.  Going by the case=20
and the markings, it is sometimes difficult to tell if these=20
are really mica capacitors or not.  After you find a bad one,=20
break it open and see if it is really mica inside.  One hint=20
is the value.  True mica capacitors were uncommon (but not=20
impossible) in values above 1000 pF (or uuF in those days).

Finally a new problem is surfacing.  This is the lazy jerk who=20
feels that drenching entire radios with WD-40 and contact=20
cleaners actually does the radios some good.  It does not!
And since most of the old phenolic capacitor cases have fine=20
cracks or incomplete seals after all these years, the solvents=20
and lubricating oils can get to the mica and foil inside and=20
cause all sorts of problems.  At best, the dielectric constant=20
of the assembly is changed causing a shift in capacitor value=20
(which is not stable).  At worst, high leakage currents or=20
shorting can result.

A little bit of moisture in a mica capacitor will lead to a=20
shift in value, but as the radio is operated more and for=20
longer periods of time, much of this moisture will leave by=20
being baked out.  This can lead to the need for realignment=20
after a period of time if the radio is quickly realigned after=20
it is first restored.  Likewise if the radio sits for extended=20
periods without being used, the moisture can return and shift=20
the alignment a little.  Barring completely baking the=20
radio for an extended time and then coating every component=20
with a new moisture seal, little can be done here but to=20
operate the old radios frequently!  Don't let those old=20
wonders sit on a shelf after restoration - use them.

Carbon composition resistors are also prone to the same sort=20
of problems.  Fortunately modern film resistors, especially=20
the metal film resistors, do not have these problems.  Most=20
modern capacitors, especially the film capacitors, are much=20
better able to prevent moisture ingress too.

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




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