[Hallicrafters] SX-28 sensitivity and other issues

[Mike Everette]

Admittedly, I haven't done this -- yet -- but probably
will:

It seems to me that replacing the mica capacitors in
the SX-28 front end would be a good idea, and not
"that" much more work once you are into the RF box. 
After all, you'll already be replacing all the wax
caps; and more than likely, most if not all the
resistors.  Mica caps are not immune to leakage and
value change (especially in silver mica types).

Some time back the organization I work for had a large
fleet (over 1000) of Aerotron mobile radios.  The
receiver front end boards and the transmit multiplier
boards were full of brown molded silver mica caps made
by Cornell-Dubilier.  Unfortunately these caps had an
extreme tendency to shift value whenever the
temperature and humidity changed.  The symptoms
included poor receive sensitivity, reduced local
oscillator injection, and/or severe or total loss of
transmit power output.  Realigning the affected boards
would solve the problem temporarily, at least until
the little beasties changed value again.  

I recall changing numerous caps, but this didn't do
any good until we started receiving disc ceramic caps
as replacement parts from the manufacturer.  

Aerotron never, ever (to my knowledge -- I never saw
one) issued any service bulletins regarding this; they
just quietly began shipping the new type caps.  It
turned out to be the cure-all; but replacing all those
caps on hundreds of PC boards was a daunting,
time-consuming task and it never did get done in every
unit.

Why no service bulletins?  Hmm... seems Aerotron had
committed a colossal FUBAR by buying "inexpensive"
silver micas in an attempt to cut costs. 
Cornell-Dubilier had manufactured this line of
cheapie-cheapies with recycled photographic silver.

Not to suggest that the foregoing is the source of the
problem you may find in the SX-28 front end -- but,
after all, Halli and most other manufacturers were
always looking for ways to meet a selling price for
their products, and maintain a profit margin.

I have seen references on at least 2 web sites to
people having encountered problems traced to off-value
mica caps in the IF cans of the SX-28.  

I also know from experience that a lot of attention
paid to cleaning dirt, scum and grime from the surface
of components -- and their leads -- in the front end
of the SX-28, or any old receiver, will pay dividends
by eliminating leakage paths; but it is no substitute
for replacing bad components.

Get out the aerosol degreaser and Q-tips and have at
it.  Clean the bandswitch wafers too, but be careful
not to soak them with cleaner.  The bakelite or
phenolic abosrbs the stuff.  They might arc over and
burn up.  Then you REALLY have a mess.

73

Mike
WA4DLF



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