FW: [Hallicrafters] sx-28 Silver Mica Re-cap kit

[Rodger Singley]

Hi Rob and list,

Generally silver mica caps are very reliable and the only ones I have had
fail are ones that have B+ across them (such as on the input side of an IF
transformer).  Before replacing all of them (and there is always some risk
of damaging the IF transformers during the replacement process) you might
check a few things:

First, is the xtal filter not working as in very low sensitivity or just a
lack of selectivity?

Can you find any peak for the crystal?  Often it is up to 1.5 khz either
side of the nominal 455 khz IF frequency and you will have to realign the IF
to the actual crystal peak.

I have had to clean the crystal element in my SX-16, SX-42 and one of my
SX-62 units to get them to work.  If your xtal seems not to be working, you
can CAREFULLY disassemble and clean the actual xtal element with isopropyl
alcohol.  Be sure and make note of the parts of the holder as you take it
apart.  Dry the element before reassembly.  Some crystal units seem to be
very critical as to the tension placed on the element by the case screws; I
had to adjust the screw tension on my SX-16 unit while watching the scope
display for best "activity".

An easy way to test the actual crystal is to connect your signal generator
"hot" lead to one side of the crystal and a scope probe to the other crystal
pin.  Connnect the ground lead from the signal generator to the ground lead
of the scope probe.  Slowly sweep the signal generator near the marked
frequency and you should see a very sharp peak for a good crystal.  You can
do before/after testing if you decide to clean the crystal element.

If the phasing cap is dirty or shorting, it will have a major impact on the
function of the filter so make sure that this capacitor is okay.

I have had to replace a few mica caps; there is one in the discriminator
transformer in the SX-42/62 series that always seems to cause trouble.
Also, the unsealed mica caps built as part of the IF transformers in early
Hammarlund HQ-170/180 receivers are very failure prone due to dust which
leads to carbon tracking.  The major failure mode for mica caps seems to be
a sort of silver migration across the mica layer which usually leads to very
high leakage or intermittent shorting; this is commonly seen in the input
caps in the phasing networks of CE phasing rigs. However, the migration
doesn't occur unless there is a large voltage potential from one side of the
cap to the other; older Tektronix scopes suffer a similar issue with silver
migration and carbon tracking along their ceramic tie points if they are
exposed to a dusty and humid environment.  Mica, unlike paper and similar
materials, doesn't suffer from leakage due to minor moisture infiltration
over the years.

Good luck with your SX-28.  I went through mine several years ago and it is
one of my favorite receivers-it is currently paired with a Johnson Viking 1.

73, Rodger WQ9E  

> -----Original Message-----
> From: hallicrafters-bounces at mailman.qth.net [mailto:hallicrafters-
> bounces at mailman.qth.net] On Behalf Of Rob Copeland
> Sent: Friday, November 03, 2006 5:04 PM
> To: Hallicrafters at mailman.qth.net
> Subject: [Hallicrafters] sx-28 Silver Mica Re-cap kit
> 
> Hi Everyone,
> 
> I have rebuilt my sx-28 to sx-28a specs everything minus the silver mica
> caps that are in the IF cans and the rest of the circuit minus those that
> are in the RF section those were done when the RF section was rebuilt. The
> radio works great except for The Crystal Filter. I am suspecting that the
> silver micas are to blame seeing how that is the only thing I have not
> replaced in the radio. All caps and resistors plus the silver micas in the
> rf section have been replaced.
> 
> So I am curious if anyone has a re-cap kit for the silver micas minus the
> RF
> section?
> 
> Thanks,
> 
> Rob Copeland
> 
> 
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