[Hallicrafters] Restoration hints for the SX-24 and SX-25

[jeremy-ca]

Very good info Mike.

I recently had to walk a relative novice thru a SX-25 rebuild and it got a 
bit interesting at times when he got frustrated.

When I was a tech in the National Radio Service Dept we used to make many of 
our own tools to get into tight places. For the SX-25 above I heated and 
bent long nose pliers (just like the old days) and mailed to him. Wire picks 
bent at various angles can be made from steel rod; coat hangers can be used 
in a pinch but they are brittle and its easy to break the point. 
Screwdrivers can be compound bent to reach impossible places to break a 
screw loose.

Hallicrafters transformers are always at the edge of failure it seems and it 
is due to their design. If you run one on a Variac you'll see that the 
output voltage becomes non linear right around 123-125V and the core is into 
full saturation at 130V. Even partial saturation results in excessive 
temperature.

Ive not seen the need to send good transformers out, however all Hallis and 
some other brands are run at 110V with either a Variac controlled bench 
strip or a 12.6V bucking transformer.

National claimed in the 30's that the HRO could be run at 135-185 VDC with 
full performance if speaker audio wasnt needed. Recent S+N/N testing has 
confirmed that dropping the AC down to 110V results in a reduction in 
backround hiss with a resultant better S+N/N. These tests were on a SX-28, 
SX-9, HQ-129X, and NC-240D. I havent had time to see if it is the same with 
miniature tubes.

Carl
KM1H
National Radio 1963-69


----- Original Message ----- 
From: "Mike Everette" <radiocompass at yahoo.com>
To: <hallicrafters at mailman.qth.net>
Sent: Monday, January 07, 2008 11:17 PM
Subject: [Hallicrafters] Restoration hints for the SX-24 and SX-25


> While working on my SX-25, I learned a few things that
> may help some other people who are into these and the
> SX-24's.
>
> Recapping the 25's front end (and I suspect the 24 as
> well) will be much easier if you temporarily remove
> the U-shaped metal piece that runs from the front to
> the rear of the chassis and is anchored to the metal
> baffles which hold the goils and band switch sections.
> With this out of the way, it is a lot easier to get
> to many of the caps that are connected to the coils.
> BE VERY CAREFUL of stressing the lugs during soldering
> or old-part removal.  They aren't hard to break, or
> pull away from the coil forms.
>
> Don't try to remove the baffles!  They are anchored to
> the chassis with sheet-metal screws, and these screws
> are often brittle from age.  Using any real torque on
> them, even with a hand held nut driver, may snap the
> heads off (been there/done that etc).  They are close
> to a #6 size with a 1/4 inch hex head, but are
> actually a rather odd thread by modern standards, and
> I haven't found any source of new ones.  BUT, if you
> have access to any junked Motorola control heads from
> tube type radios or even stuff up to and including the
> MICOR series, these same type screws -- or something
> much closer to the Halli screws than anything else
> I've seen -- were used to hold them together.  In any
> given Motorola head, you'll probably find 6 to 10 such
> screws.
>
> If you have the misfortune to break a screw head off,
> use a very long and slim pair of heavy-duty needle
> nose pliers to remove the screw by grabbing it on the
> top side of the chassis and slowly turning it so it
> will come on through to the top side.  (BE CAREFUL of
> the tuning condenser!) Then replace the screw from the
> bottom side, just like the original.
>
> The best thing to do, though, is LEAVE THEM ALONE in
> the first place!
>
> The hardest caps to replace in the 24/25, in my
> opinion, are the cathode and screen bypasses on the
> 6K8 oscillator-mixer tube socket.  If you use
> conventional tools, they are probably impossible
> without damaging something else, like a coil in the
> neighborhood.  BUT,  if you obtain 2 special tools --
> a pair of bent-nose longnose pliers, preferably 30
> degree bent; and a pair of angle-tip cutting pliers --
> it CAN be done.  Note, I didn't say it was easy, but
> it's a lot less difficult!
>
> The reason some 24/25 receivers may be a bit deaf on
> the high band MIGHT be that the cathode resistor on
> the osc-mixer 6K8 has gone high in value.  Early runs
> of the 25 used a 400 ohm resistor; later runs used 250
> ohms.  The one in my 25 was marked 400, but measured
> 750 ohms.  My 24 hasn't yet been restored but a
> resistance check showed its 6K8 cathode resistor to be
> high as well.  When replacing the cathode resistor in
> the SX-25, I used a 270 ohm 1 watt type which is more
> in line with the later production run.  Nothing else
> in the 6K8 circuit appears different between runs.
>
> I also found a Velleman 60 watt solder station useful
> on this job.  It has a thin pencil iron, great for
> tight spots.  Also, once the tips get hot, they can
> CAREFULLY be bent to about a 30 degree angle which
> proves quite handy for working in the front end area.
>
> If you replace the caps around this socket, get both
> the cathode and screen bypasses AND the cathode
> resistor at the same time.  My 25 used some long leads
> on one of the caps to tie some pins together and it
> was not that hard once I figured out what to unsolder
> and what to leave alone.  Some of that "tie wire" can
> be left in place.  But above all, BE CAREFUL in this
> area.
>
> At least 80% of the resistors in my SX-25 were found
> to be significantly beyond a 20% tolerance limit.
> Most were much higher than marked.  This included one
> of the two large 15K carbon voltage divider resistors,
> which measured almost 25K.  I replaced both of these
> with 15K 5 watt wire wound types.  The other resistors
> were replaced with 5% components.
>
> I highly recommend having the power transformer,
> filter choke and audio output transformers in these
> receivers megger-tested and vacuum-tanked by Gary
> Brown, WZ1M, to restore the insulation and help
> prevent a catastrophic problem.  The line voltage in
> my area rose to 135 volts one afternoon while I was
> testing some work on my SX-25, and the power
> transformer got really hot and started smelling like
> it was about to fry.  I pulled the iron and sent it to
> Gary; no more problems after it was reinstalled, and I
> for sure run a Variac and voltmeter on the bench at
> all times now.
>
> My SX-25 had a severe stability issue until I
> discovered another of those infamous sheet metal
> screws to be broken.  This was one of the two screws
> at the front of the chassis that hold the band switch
> detent in place.  If you just looked hard at the band
> switch knob, the radio would jump frequency like a
> nervous flea.  Replacing BOTH screws with new ones
> cured 97% of the problem;  the rest of it is probably
> the designed-in "Halli bug" we've all encountered.
>
> The more I get into them, I'm convinced that these
> radios were never intended to be worked on beyond a
> certain point.  Surely the repairmen of that era
> didn't have the sort of tools we enjoy today and which
> are so necessary to work on these sets.  Hallicrafters
> (and others) probably hoped the owner would just buy a
> replacement radio.  Hmm, not unlike today's junk....
>
> 73
>
> Mike
> WA4DLF
>
>
> 
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