[Hallicrafters] SX-42 Hum(bug)

[Mark Shaum]

Mike,

A couple suggestions on the '42 - I've spent dozens of hours on mine
chasing down various bugs, and am currently doing bandswitch surgery to
cure a reoccuring high impedance leakage problem in the first RF stage.

First, get all of the various schematics you can locate, either through
manual services or on the web.  I have three distinctly different
schematics for my 42, none of which exactly match my receiver.  Any one
of them is useful enough for general troubleshooting, however.  Having
the right one to match your set definitely reduces the frustration
levels (... what's THAT resistor doing there? ...)

Check the web for sites with troubleshooting tips on both the SX42 and
SX62 series.  The two are very similar in circuitry.

Beg, borrow or steal a mutual conductance tube tester, not one of the
go/no-go shorts and emission types I often see at hamfests.  A low-gain
tube will generally not silence a receiver, so it need not be
immediately replaced.

You will need a 'scope for any in depth troubleshooting to determine
what signals are where at roughly what frequency.  Start at the speaker
end of the audio stages, and work back towards the antenna one stage at
a time.  Verify that tube voltages are at least somewhat near the
voltages called for in the service manual.  If you find a stage where
voltages are way off, dig into it further, checking capacitors for
leakage and off-tolerance resistors.  Sooner or later you will find a
stage in the receiver that has an input signal (either from the receiver
or a signal you are injecting) but no or reduced output.

Quick fix department - some of the front panel toggle switches on my 42
and pair of 62's have been intermittant.  A bad receive/standby switch
will definitely cause no output except the base hum level.  I've also
had pots go bad, which is unusual.   One of my 62's had an RF gain pot
that measured 500 ohms (instead of 10k) but ONLY after the set had been
running for several hours.  Leave it sit for a day and it would be fine
again, until powered up for a few hours.

Except for shorted and/or leaky capacitors or way-off resistors causing
abnormal voltages at a given tube pin, I would NOT suggest starting with
any mass component replacement.  As long as nothing is overheating, try
to get the receiver in some state of basic operation before doing any
massive resistor and capacitor swaps.  Doing all of the capacitors is
probably a good idea if you plan to keep and use the set for years, but
is not easy.  In the 42, several capacitors are barely visible under the
bandswitch and other places, and are a pain to replace.

The above does not apply to the power supply electrolytics in the B+
lines.  These should be either replaced, or removed and reformed prior
to use.  Having one short and take your power transformer with it makes
for a very bad day.  You can remove wiring from existing filter
electrolytics and "tack in" new replacements for the duration of
troubleshooting while you locate new can type caps or reform the old
ones.

Hope all of the above makes some sense and is at least a bit helpful.
Pardons if I'm repeating a bunch of stuff you already know, you didn't
really indicate how much experience you have had with tube gear
troubleshooting when you posted to the Halli list.

73! - Mark
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Mark Shaum K9TR
email: k9tr at dtnspeed.net
http://www.qsl.net/k9tr
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