[Hallicrafters] SX-28 sensitivity problems, another thought

[Mike Everette]

Something that should be considered while puzzling
over "deafness" in the SX-28, is the overall condition
of the RF box.  And, the switches; read on.

Look for dirty tube sockets.  It may indeed be that
"stuff" has collected between the pins that forms a
leakage path.  Check top and bottom.

Look at the bandswitch wafers as well.  What's between
the stator contacts?

Be really careful when cleaning.  Go easy and use
cotton swabs.  Don't "hose" a phenolic switch wafer
with contact cleaner.  It will absorb the stuff and
you might have a destructive flare-up that will
destroy the wafer.

Most contact cleaners are highly flammable.

Even the wiring itself might be suspect if it is
dirty.  A little equipment-degreaser could be the
cure.  BUT:

Let the whole thing dry very thoroughly before you
fire it up.  Cloth covered wiring also absorbs these
liquids.

My SX-28 had suffered from being stored in a sooty
environment -- coal or oil, I can't tell.  The stuff
that collected on the chassis -- and to a degree,
under it -- kept it from rusting, but it got into a
lot of nooks and crannies.  I found the bandswitch and
the IF switch to be rather nasty.  This cleaning task
has been an adventure.  I hope it is worth the effort
when I am finished with the radio.

One lesson I learned, and a trick for cleaning rotary
switches, from avionics work -- some of the older
nav-com radios (Narco comes to mind) used a rotary
switch rotor contact as a step-inductor.  The cure for
sensitivity problems, as well as microphonics, believe
it or not was to use a typewriter eraser -- the
pencil-type that must be sharpened -- to thoroughly
clean the rotor contact until it was shiny bright. 
Contact cleaner alone would not begin to get the job
done.  You could do all the tweaking and tube-subbing
in the world, to no avail.  The "step inductor" switch
was almost always the problem and until it was clean,
you were grounded.  Only then did the contact cleaner
come into play, to get rid of the eraser residue.

Now, I know some of you are going to take me to task
for this next point; but:

The "really adventurous" SX-28 people might want to do
some further work while inside the RF box and replace
the plastic/bakelite sockets with ceramic or steatite
types -- especially the first RF stage, if you can
find them.  If you have the coils and baffles out, it
would not be that much harder, and putting in a new
"clean" socket is often easier than trying to
unsolder/resolder an old one.  I myself have not done
this yet, but will consider it if I come across any
such sockets that are an exact match in the way they
mount to the chassis.

Look at it like this -- it is what Hallicrafters
should have done in the first place, but the bean
counters said no.  That doesn't stop us.  And if the
tubes are in place it is not very noticeable.  Think
of the SX-28 like a Volkswagen. "We don't change it to
make it look different.  Only to make it work better."

73

Mike
WA4DLF

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