[Hallicrafters] SX-28 sensitivity and other issues

[George KB2Z]

Mike,
Good food for thought! I've always been in the silver mica caps rarely 
go bad bad camp.  I believed they were all high quality, silver mica.
Thanks, G

Mike Everette wrote:
> 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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