Hallicrafters Model SX-42 Communications Receiver (1947)

>He hot-wired the set so that I could play it on BC, at least.
>This was done by wiring a jumper wire between two little
>terminal blocks mounted near the back, one underneath
>each 6AG5 RF tube. I haven't traced it exactly, but it
>looks like the jumper connects to about pin 6 of each
>6AG5 through a resistor.
Phil's Old Radios: Home: Gallery: Communications
Hallicrafters Model SX-42 Communications Receiver (1947)
Owner's Manual
The Big Kahuna! Widowmaker! If you have ever tried to lift a radio this size, you know
what I'm talking about. This superlative communications receiver represents a peak of
post-World War II radio technology. And it weighs about as much as a 1947 Buick.
Description
Model SX-42 was the top-of-the-line Hallicrafters radio of its time, and the company
promoted it accordingly. The March, 1947 front cover of Radio News magazine showed
an SX-42 as it would have been used in the Gatti Expedition to South Africa. The SX-42
was also featured in full-page ads in the April, 1947 and December, 1946 issues of Radio
News. (Click the thumbnails below to view the full-size magazine pages; you can view
many more magazine images in our Literature section.)
I think the magazine cover is a real hoot. Intended to look authentic, it seems awfully
stagy to me, with a fez-hatted native peering curiously through the window at an
earnest young radio operator. And the magazine's scene dresser couldn't resist a little bit
of self-promotion. If you look closely at the magazine cover, you'll see the December,
1946 issue of Radio News on the desk under the young ham's left arm.
As the Hallicrafters ad indicates, the SX-42 sold for $275, a princely sum in those days.
Its features also appealed to the most demanding radio listeners. Using 15 tubes, this
radio covered the AM frequencies in six bands from 540 Khz, the bottom of the standard
AM broadcast band, to 110 Mhz. It also offered FM reception in the two highest bands:
27-55 Mhz and 55-110 Mhz.
Hallicrafters touted the SX-42's wide coverage, stating it had the "greatest continuous
frequency coverage of any communications receiver." That was doubly true of its FM
coverage. Prior to World War II, FM radio broadcasts were found in the 42-50 Mhz range.
After the war, FM broadcasts were shifted upward, to today's 88-108 Mhz range. By
designing the SX-42 to receive FM in the radio's two highest bands (27-110 Mhz),
Hallicrafters covered both of those standard ranges, and then some! You can read more
about the change in FM frequencies in my page on the Philco 42-350.
The next three photos show the SX-42 along with an interesting companion set, the
model T-54 television. The first photo is from Hallicrafters literature. The second shows
my SX-42 and the third shows my unrestored T-54. The design for the SX-42 and T-54 is
credited to the famous Machine Age designer, Raymond Loewy. You can read more about
early Hallicrafters televisions in my articles about the T-54 and model 505.
Hallicrafters offered a matching speaker for the SX-42, the model R-42 "reproducer," as
well as an optional tilt-swivel base. The swivel base is extremely rare. The photo above
shows the radio with my R-42 on top. Although it looks nice for a photo, I don't
recommend keeping the speaker on top of the cabinet permanently. It's heavy and it
might interfere with ventilation.
In the Radio News magazine cover shown above, you can see an earlier-model speaker
(PM-23) to the right of the young ham's SX-42. The PM-23 speaker is electronically
compatible with the SX-42 but it was designed to match earlier receivers such as the SX-
28.
This complex receiver has a lot of controls. Rather than paraphrase the original source, I
have provided a separate page with the complete Detailed and Technical Operating
Instructions from the SX-42 Owner's Manual.
Here is the lineup of the receiver's fifteen tubes:
Tube Type Function
V1 6AG5 1st RF amplifier
V2 6AG5 2nd RF amplifier
V3 7F8 Converter
V4 6SK7 1st IF amplifier
V5 6SG7 2nd IF amplifier
V6 6H6 AM detector/Noise limiter
V7 7H7 1st limiter
V8 7H7 2nd limiter
V9 6H6 Discriminator
V10 7A4 BFO/S meter amp
V11 6SL7GT Audio inverter
V12 6V6GT Power output
V13 6V6GT Power output
V14 5U4G Rectifier
V15
0D3/VR150 Voltage regulator
The rear view shown below gives a peek into the SX-42's interior. This shot is taken from
the rear. At lower left is the large power transformer, a black rectangular object with the
Hallicrafters "h" logo molded in the top of the case.
To the right are the huge tuning capacitors. The main tuner lies to the right in this view,
and the bandspread tuner to the left. This section is normally covered from above by a
large rectangular metal shielding plate. I removed the plate for this photo, but you can
see three of its mounting posts in this view. If you shop for an SX-42, pay attention to
whether this plate is present. It's an easy thing to lose, and the designers put it there
for a reason.
The SX-42 is especially attractive to me because I like to listen to FM and shortwave late
at night, and I appreciate good audio quality. It's hard to find any vintage tube radio—
much less a boatanchor—that offers all those features in one package. (One exception is
the luxurious Scott 800B6.)
If you go shopping for an SX-42, don't confuse it with a model SX-43. The two radios
date from around the same time, and look very similar, but the SX-43 is a cheaper 11-
tube model, selling for $105 less than the SX-42. The simplest way to tell them apart is
to look at the main tuning dial on the left front. The SX-43's dial is plain on the bottom
and has the Hallicrafters "h" logo on its left side. The SX-42's main dial, as you can see
in the front view, has the Hallicrafters logo on its right side, and the distinctive
bandspread dial is "notched" into the main dial's lower right edge.
Like its bigger brother, the SX-43 covers FM bands, both the prewar 42-50 Mhz band and
the modern 88-108 Mhz band. I don't mean to imply that the SX-43 is not a good radio.
Just make sure you understand which radio you're looking at. All things being equal, an
SX-42 would be worth more than an SX-43.
A few other Hallicrafters sets combine FM with shortwave. Model SX-62, made from
1949-1953, is a somewhat more consumer-oriented version of the SX-42, lacking a few
"boatanchor" features such as bandspread.
Model S-47, introduced in 1947, is a 15-tube AM/FM/SW chassis designed for consumer
use, offered in various console cabinets or sold as a bare chassis for installation in a
custom cabinet. These seem to be quite rare.
Much later, in 1966-1967, Hallicrafters sold a few cheaper tabletop radios—some solidstate—that
covered the standard FM broadcast band as well as some shortwave bands.
The SX-42 stands alone, however, in offering continuous coverage over a very wide
range, as well as high-end audio.
First Look
As found, this radio worked—sort of. It sounded fine on the standard broadcast band,
but had a serious shorting problem when you moved the bandswitch to FM. (The seller
had warned me of this condition, so it didn't come as a surprise.)
Cosmetically, the set was not perfect. The front panel was starting to develop some light
surface rust, but it looked quite presentable after cleaning, and the lettering was in good
shape for a 52-year old radio.
The hinged top cover dips slightly in the middle, causing its front edges to stick out very
slightly to the side when closed. That's a common problem in sets with hinged covers,
probably caused when someone parked another heavy piece of equipment on the cover.
I suspect that I can straighten the cover by carefully clamping it sideways in a long
woodworking clamp.
The first thing I did after receiving the radio was to remove it from the chassis, brush off
the surface dirt, and clean all the controls with DeOxit. I also examined all the interior
wiring for obvious loose connections, wiring breaks, or solder bridges.
Then, with the bandswitch set to the BC band, I hooked it up to my variac and slowly
powered it up under increasing voltage. That procedure lets you watch for problems
during a gradual "smoke test."
On BC, the radio performed pretty well, as the seller had claimed. The shorting problem
on FM was still there, however. Powering down, I decided it was time for closer
examination.
There are two controls that might be at fault here: the complex bandswitch, which
chooses a particular frequency range, and the much simpler Reception control, which
chooses different reception modes: CW (code), AM, FM, or Phono. Using a ohmmeter
and following the schematic, I quickly determined that there was nothing wrong with the
Reception control.
That left the bandswitch, which is not like any switch I had seen before. This is not
something that you can remove in one piece and rebuild on the bench. It's an elevengang
switch assembly running the entire length of the radio, around which many other
components are wired, in a sort of dense electronic forest. Simply finding the problem
would be quite a challenge. Fixing it might be even harder.
Time to call on a higher power! Reloading my checkbook, I hauled the boatanchor to a
local repair shop, the same one that had fixed up my Hallicrafters SX-122. When I
brought the set in, the proprietor grumbled, but said he would look into the problem,
giving no guarantees. "I might have to charge you more than the radio is worth," he
warned.
When I phoned for an update one week later, the news was not good. "Come to pick it
up and we'll talk then," he said. When I arrived at his shop, the radio was playing
wonderfully on AM. Then he delivered the bad news. "There's a part of the switch that's
completely burned away. You don't want me to tear apart that whole end of the radio,"
he said. "I haven't got time for that anyway. I bridged the burned section of the
bandswitch so that it can play on AM. I won't charge you for all the time I spent—just
pay me $25 and we'll call it square. It's a nice radio, but if I were you, I'd try to swap for
one that works."
A Tale of Two Switches
Handing over the $25, I sadly hauled my SX-42 home, vowing that this would not be the
end of the story. Against hope, I posted a message in the rec.antiques.radio+phono
newsgroup, asking for advice, or at least condolences. Then I packed the radio onto a
shelf in the garage.
A couple of weeks later, I got an email message from a guy in California who had a
mostly-stripped SX-42 chassis with an intact bandswitch. If I paid the shipping cost, he
said, he'd be willing to box it up and send it to me gratis.
Yes, I replied! One week later, for a cost of about $12, I had a second SX-42 chassis with
a good switch. By that time, my family was in the process of moving to a new house—
not a good time to have a boatanchor in pieces on the workbench—so I shelved both
units for the time being.
Months later, we were finally settled in our new house, and I found some time to work
on this project.
The first step was to replace the old paper capacitors. This set has a lot of capacitors. I
stopped counting after a while, but I believe I replaced about three dozen of them.
I call the next photo "Beauty and the Beast." It shows the newly recapped SX-42 on the
left, and the junker chassis on the right.
In this view, the bandswitch is at the top of both chassis. The long rod running the
length of the chassis is one of the side supporting shafts for this very large switch.
The repair guy told me that one wafer of the switch was burned beyond repair, so I
assumed I would have to replace that wafer. To find out how hard this would be, I
started to disassemble the switch on the junker chassis. The next photo shows one of
two wafers which I removed from the junker chassis.
Although the wafer is electronically identical in both chassis, the newer wafer differs in
some construction details. Notably, where the wafers in my older SX-42 use round rivets
to secure central rotors through to the other side of the wafer, this newer design uses
split crimps.
The switch is built around a center shaft and two side shafts. The center shaft turns the
center rotors, and the side shafts secure the stationary, outer portion of each wafer. The
side shafts are simply long brass screws running through all eleven switch wafers. To
replace any of the wafers, you need to pull out all three shafts. The following photo
shows the shafts from the junker's bandswitch.
Also visible in the photo are a rectangular side plate, which is removable for aligning the
radio, and a dish containing all of the nuts and little pieces of metal collar which fall
loose when you withdraw the side shafts.
On newer SX-42 models, such as my junker, the center shaft is made of two pieces,
which you can disassemble by loosening two screws near the detent gizmo. This allows
you to pull the center shaft out through an access hole in the rear of the chassis. The
side shafts are also held with nuts on both ends, and can be drawn out through access
holes in the front of the chassis.
On older SX-42s, such as my "good" one, the side shafts and center shaft are riveted on
to the detent gizmo at the knob end. To remove any wafer, I would have to drill out the
rivets before removing the shafts. When I reassembled the switch, I would substitute
the side shafts and two-piece center shaft from the newer radio, as well as the newer
detent gizmo.
You can understand why Hallicrafters made this design improvement. The older switch
design makes it very hard to replace a wafer!
To gain better access to the switch, I removed the entire side panel from the radio. I
also bought an extension light with a large magnifying lens, to get a better view into this
dizzying switch. The following photo shows the radio with side plate removed and the
magnifier giving a closeup view.
Removing the Front Panel
Assuming that I would eventually have to remove the shafts, I removed the radio's front
panel to access to their front ends. The next two photos illustrate this process. First, you
need to remove all of the knobs. The smaller knobs are secured with small Phillips-head
screws, two setscrews per knob.
The bandswitch knob and tuner knob are secured with 5/64-inch hex setscrews. Located
at the end of deep, narrow holes, these setscrews are hard to see. I used a flashlight
and a bright extension light to spot them.
The tuner knob has three parts: the main tuner, the bandspread tuner, and the small
winged lock in the center recess of the bandspread knob. (The lock lets you clamp the
main tuner while using the bandspread, and vice versa.)
You can remove the locking piece by simply unscrewing it counterclockwise. This
exposes a tiny locking collar at the shaft of the bandspread knob; the collar can be
removed by carefully prying it loose with a small screwdriver or needle-nose pliers. This
collar is very small and it has a tendency to spring free when released. Don't lose it!
To remove the bandspread knob, you need to rotate the main tuner so that an access
hole in the tuner knob lines up with each of the two setscrews in the shaft of the
bandspread knob. This is where a flashlight helps a lot! To aid in refastening this one, I
scribed two tiny lines in the center part, pointing to where the setscrews should go.
The previous photo shows the SX-42 with all knobs removed except the main tuner. This
knob is also secured with two hex screws on its shaft, but since part of the knob lies
behind the dial cover, it can't be pulled completely free until you loosen the front panel.
Once the knobs are all free, you can remove the four large screws from the sides of the
panel and the securing nut for the headphone jack. The last step is to remove the radio's
S-meter (at upper right in the previous photo).
The S-meter is secured with a thin locking collar behind the front panel. After removing
the collar, you also need to remove three small screws around the edge of the meter
case. This lets you slip the case out through the front of the panel while you slide the
meter itself back from the rear.
Now you can take off the front panel, gaining access to a number of otherwise
unreachable components. Before going any further, I reassembled the S-meter to
prevent damage. The meter is completely unsupported when the panel is off, so I used a
plastic tie to temporarily hold it to the chassis side panel. The next photo shows the
radio with its front panel removed.
Although the radio loooks quite naked in this state, it can still be operated as usual. To
allow testing during repair, I reattached all the knobs and set the radio on its side. I also
hooked up an antenna and speaker.
Happy Birthday, SX-42!
On the back of the front panel, I found a date stamp that looks like May 6, 1947. The
next photo shows a closeup.
Strictly speaking, I suppose this date tells you when the front panel was made, not
necessarily when this particular radio left the assembly line. But in the absence of any
other evidence, I'm using it as this radio's birthday.
I've been told there are at least two variations of the SX-42, with differences in the front
end and AVC circuits. Mine is clearly an earlier model, with serial number HA-77712. The
junker is a later model, with serial number HA-93256. In addition to the bandswitch
changes, several other small differences are visible in what's left of my junker.
Help Through the Ether
At this stage, I had invested considerable effort without getting any closer to my goal,
although I had learned plenty about how to bare an SX-42 chassis for repairs. While I
had the front panel off, I thought I might look into repainting it. I posted a question to
the rec.radio.amateur.boatanchors newsgroup, asking if anyone knew a way to repaint
the front panel including the lettering. Fellow SX-42 owner A.B Bonds sent this reply:
-------------------------------------------
From: A. B. Bonds
To: Phil Nelson
Date: Tuesday, October 06, 1998 7:15 AM
Subject: Re: repainting boatanchor front panel WITH lettering?
There is no practical way to repaint the lettering on the SX-42 panel.
It's multicolor, silkscreened. To reproduce it would cost several hundred
bucks. If you can get paint that is a close match, you can feather the new
paint around the existing lettering. One of my SX-42's has had that done
(dunno who did it) and it looks mighty fine. I did this with an S-76. It
involved masking out the old lettering (to make sure), cleaning the old
surface with a mild solvent, then applying the paint with an airbrush,
staying reasonably far away from the masked areas so as not to make a line.
After letting it dry for several days, I rubbed the new paint out to
de-emphasize the boundary with the old paint, then polished with a swirl
remover. Car polishes are ideal for this.
Couldn't help notice your reference to "the cussed bandswitch". I posted
about four notices on my experiences with this critter on the boatanchors
mailing list. The third wafer from the rear carries both B+ for the first
RF tube and grid signal. When bypass caps fail, the B+ current goes up,
heating the switch contacts. On mine this had carbonized the wafer. Even
the slightest amount of conductivity biases the grid positive. I ended up
fixing it in place, but it was a trial. Basically scraped out all the
black with a dental tool, then refilled with epoxy. Took about three
evenings. A truly cussed design. Did you actually remove your bandswitch?
Sheesh.
73 A. B. Bonds
-------------------------------------------
Finding anyone who had wrestled with an SX-42 bandswitch seemed like a godsend.
Over the next few days, I got even more good advice from Mr. Bonds. The email
exchange speaks for itself.
-------------------------------------------
From: "Phil Nelson"
To: "A. B. Bonds"
Date: Tue, 6 Oct 1998 09:23:16 -0700
Hi, thanks for the useful information. You're the first person
giving advice who has actually worked on an SX-42 !
The panel on mine isn't in horrible shape, but it's not
great, either. If I had not already put so much time into the
set, I'd probably polish it up and leave it alone. Your
technique would work, I guess. Can I ask what sort of
paint you used, and how you went about matching the
color?
Regarding the switch, this has been a long, long trial
for me. The fellow who sold me the set said it had some
sparking problem in the bandswitch, but I had no idea
at the time just how beastly complicated and crowded
that switch is. I certainly wouldn't have bought it, had I
known.
When I bought the set, it played OK on BC, but shorted
when moved to other bands. After getting a look at the
switch, I hauled it to a local repair guy who had done some
work on an SX-122 for me. He spent some time on it and
decided not to tackle that switch. According to him, there
is at least one contact burnt completely away, so you
would have to replace the whole wafer. He said it would
cost me more than the radio is worth, just to get the
switch working. Plus, he really didn't want to do the job. So
I brought it back home again.
He hot-wired the set so that I could play it on BC, at least.
This was done by wiring a jumper wire between two little
terminal blocks mounted near the back, one underneath
each 6AG5 RF tube. I haven't traced it exactly, but it
looks like the jumper connects to about pin 6 of each
6AG5 through a resistor.
Meanwhile, I have completely recapped the set. I think
there were almost 35 paper or plastic capacitors! It now
sounds very, very good on BC. No trace of hum, so I haven't
messed with the filter caps.
Last year, I got a junker SX-42 chassis that has a good
bandswitch. Yesterday I partially disassembled that switch to see
just how awful the job would be. The answer seems to be,
PRETTY AWFUL. I'd be worried about wrecking the coils
crammed in there, for one thing. And I'm not sure how
I'd get a snipper back in there to disconnect some components,
short of starting at one end and tearing out every wafer until
I got to the bad one.
In the course of recapping, I also noticed that someone (maybe
my repair guy) had snipped a couple of connections at
the tone control switch. After I restored them, the radio
plays fine with the tone set to Hi-Fi or Bass. It shorts out
when switched to Low or Med, which seems very weird to me.
Needless to say, I would appreciate any advice you could
offer. The set is sitting on my workbench right now, with
front panel and side panel removed, ready for me to tear
into that switch. If you can remember exactly which part(s)
of which wafer was bad on your set, maybe that would help
me know where to start. I know its original shorting problem
was in one of the rear wafers, but now I'm having a hard time
seeing anything down in there. And the original shorting
problem seems to have gone away! Maybe because I
replaced the bad cap(s) that caused it in the first place.
Cheers.
Phil Nelson
-------------------------------------------
From: "A. B. Bonds"
At 09:23 AM 10/6/98 -0700, you wrote:
>Hi, thanks for the useful information. You're the first person
>giving advice who has actually worked on an SX-42 !
Been there, done that. I am greatly sympathetic. First, don't give up
hope. Once fixed, it's a very hot (sensitive) radio that sounds _great_
what with the push-pull output stage.
FM performance is quite nice, and it deserves a good speaker. I have mine
playing through a Rat Shack small Linaeum speaker (which is currently on
clearance sale) and it sounds mighty fine.
>Can I ask what sort of paint you used, and how you went about
>matching the color?
I didn't do my SX-42, I did an S-76 that uses a satin black, used Rustoleum
satin black. There is a guy (I think the outfit is RR designs) who sells
custom-blended paints for boatanchor rx's. I'll try to find his net
address. It's a good bet to try an auto paint supplier, they have machines
that look at the color and give a recipe for blending the color. Be aware
that the finish is not a gloss, it's kind of a flat grey.
>I certainly wouldn't have bought it, had I known.
Sounds familiar. But now you are owner of the problem, and it's important
to try to do your best to fix it.
>When I bought the set, it played OK on BC, but shorted
>when moved to other bands.
That's because the first two bands (BC and the next one) do not use the
first RF stage, it only kicks in on band 3 and above.
>After getting a look at the
>switch, I hauled it to a local repair guy who had done some
>work on an SX-122 for me. He spent some time on it and
>decided not to tackle that switch. According to him, there
>is at least one contact burnt completely away, so you
>would have to replace the whole wafer.
Probably an accurate diagnosis. Note however that if a contact is burnt
away, it is possible to replace that contact without necessarily removing
the bandswitch.
The wafer that usually is problematic is (I believe) third from the rear
end. To access it, you need to remove the wide shield nearest the rear
(there is a smaller one behind it, but this does not need to be moved).
This involves desoldering the big copper strap connecting the shield to the
rear of the chassis and desoldering the components on the circuit strip
attached to the shield, as well as one capacito ground near the copper
strap. In addition, to get at one of the screws holding the shield down
(nearest the edge) you have to remove the screen circuit components from
the single terminal phenolic strip. This is accessible through the
(removed) side cover. You also need to remove the entire side plate, since
the shield is attached to it with a braid. This will give you pretty good
access.
I used a magnifying loupe to examine the switch closely. The damage on my
switch was limited to burnt phenolic, both on the outer rim of the switch
as well as the rotating part. I used a dental excavator (little tool with
a very tiny sharp scoop on the end) to scrape out the carbonized area in
the outer wafer and the rotor. You have to be very careful here. Even
when your meter reads 20 meg + resistance, when you slap 250 v on the
wafer, microamps can leak over and improperly bias your first RF tube. One
way of telling if your repair is satisfactory is to look at the screen
voltage (with screen circuit clipped together). For the first RF tube, it
should be about 150 v with the sensitivity turned up and no signal (only
measurable on bands 3 and above). If this voltage is low, you have grid
leakage. If you can sneak a probe into the grid pin, it should read very
close to 0 v. Even + 1 v on that RF tube will cause problems. Once the
offending carbon has been removed, I locally strengthened the switch by
standing the radio on end (nose down) and spreading on some partially set 5
min epoxy (so it won't run). After about ten minutes for setting work the
switch to make sure it is loose and to permit removal of excess.
If you really have a burnt (outer) contact (the little springy thing), you
can use a Dremel tool to grind off the attaching rivet, remove the
individual contact and mount a replacement off your junker switch with a
short 2-56 screw and nut to hold it in place. If your rotor contact is
burned as well, I am at a loss.....