Cambridge Communications and Technical Services Inc.
Hallicrafters SX42 Restoration

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The Hallicrafters SX42 was one of the first post WW2 general coverage receivers to incorporate not only FM reception
but FM reception in the then new 88 to 108 MHZ FM broadcast band.  

Few people remember that the original FM broadcast band was assigned to 42 to 50 mHz known as the Apex FM band.

The SX42 was one of the first to be able to decode not only all the AM and short-wave bands but also the then new 88 to 108 mHz
FM band.  It is one of the most complex radios of its time.

 

My understanding is that it was manufactured from mid to late 1946 through 1947.

 


  

Many of the first photos are simply a compilation of the radio as I found it or more accurately how it found me.    This is a photo of the radio
on its way to Vermont.  Original and only owner was Edwin Thurston Green, owner of Green Instruments in Cambridge Mass.

 
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This is the underside of the radio.

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 There  are certainly capacitors in trouble here..


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Power Transformer area.
 
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The following few photos are of the band switch section.
 
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 This is the main tuning capacitor and the band spread tuning capacitor. These next two photos makes it look worse than it actually is.....

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The Rear of the chassis....
 
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Left side of the front panel, the main tuning indicator.
 
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 Looks like a hand written note by Edwin on the top inside cover. The camera caught it better than the eye.
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Original  inspection tag from the power cord.  May 24, 1947.
 
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 Various views of the top cover after bead blasting......

 

 

 

 

Power Transformer cover bead blasted.

 

 

Just a few replacement capacitors.

 

Case front view, lots of rust a pitting on the right side.

 

Case, right view.  Lots of rust and pitting of the metal.  I suspect the radio was stored for a long period on this side on concrete.

 

Case, left side.  The paint is starting to blister.

 

Case bottom view.

 

Case top view.

 

Top cover after priming.

 

Transformer cover after priming.

 

 

SX42 on the shop bench. In the foreground, Weller 3000 series solder / de-solder station, Heath TT1 tube tester and then the SX42.

Various other treats in the background.

 

Testing and power the power transformer.  High Voltage on the transformer, no load. The rectifier is out of circuit.

 

 

Primary current, this is with all filaments running and any functioning dial lights.

   

Photo of the test set up.  Notice the variable transformer to slowly bring the voltage up. We don't want any shower o sparks.....

Power Transformer Test: Primary .047 amps, Total HV 568 V No Load 5U4 Tube Pulled 

Panel lit up on its own power. Looks like the lower light to illuminate the tuning knob is out.  I'm checking the warranty on this.....

 

 

  Power Transformer Test Set Up

Photos of the case being primed.....

 

 

 

 

This is the top cover after being painted with my Volvo "Moondust" paint left over from the 1995 850. 
An extremely close match.  I could not find anyone that could do an accurate paint match on perforated 
material, so I used what I had....

But the truth is I was driving down the road after looking at this (it was in the trunk) and I'm looking at my 
car over the hood and it struck me. I have that paint!  (Actually my mechanic had some left over and gave it to me.)

 

 

Hallicrafters SX 42 and Bondo?   ..........   Yup..........  Filled up those pitted parts.

 

Added a little Bondo to the front also

 

OK Sand it and prime it again..........

 

 

Again this was the original............

 

This was the worst side of the cabinet... as you can see in the previous photo.  I didn't try to take out any dents or the bow in this side
other than filling in the rust pits.  You can see that in a previous photo, and you can end up with a worse looking job trying to 
modify and make everything perfect. The paint is of a slight hammer tone but the color was matched very closely using the color match
spectrophotometer at one of the paint stores.

 

 

Top after painting. Can still see a little pitting in the top, but the flash in the camera brings that out also.  Didn't want to try the
Bondo trick on the perf top.  Figured the Volvo paint I used would be  enough... and when the R42  speaker is on it......  you won't
see it in the living room, where if Eva does not kill me... it will be

 

 

Front looking down after painting.

 

Right Side Photo

 

Back.  Didn't totally Bondo the lower corner.  The goal was to stop the rust and preserve, not make it is 
something it is not.  It is after all a 1947.  It deserves some respect.


 

This was taken with a flash. Makes the color look lighter than it is


Band Switch with most capacitors replaced.  The band switch had 5 rotary positions.  
It has 10 sections. Some Sections have a front and back set of switches.

 

Buried hard to get to capacitor was removed and replacement moved.

 

In this photo, if you look closely you will see 3 resistors connected in parallel.  Look for the brown tubular 
with brown, green and an orange band. Actually not a bad job for someone trying to get the proper resistance
when they don't have the actual part they need.  And the resistance value was
not far off.  Someone knew ohms law.

 

 

This photo is after most of the capacitors have been replaced. I need to replace about 4 more but ran 
out of that value component.

This is a pile of the capacitors removed...........

 

I made the unfortunate decision, according to some purists, to replace the power cord with a grounded type 
with a three prong plug AND drill a hole and add a fuse holder.  I think anything at "our" age ought to be fused..............

But then I don't like things electrical that smoke... (don't ever let smoke out of anything electrical...it is hard to 
get the smoke back in). So about the only modification other than replacing capacitors and resistors that have 
gone bad is the addition of a grounded cord and the addition of a fuse holder. The original radio was not fused.

 

While replacing bulbs, it was discovered that the bulb for the "S" meter was stuck in its rubber grommet.  
The meter was removed, cleaned with NOVUS #1 polish.  Note the grime on the rag.  
Always use a soft cotton cloth as paper towels are abrasive and will scratch plastic.

The meter cleaned up to almost new condition. The rubber grommet and the bulb were replaced.

 

 

Radio Chassis with S meter removed.

 

The bench is getting a little cluttered.  This view is with all tubes and S meter removed.  
Note the cover on the transformer has been painted.

This is the R42 Speaker that originally was an option for the SX 42.  Sounds real nice. Photo does not do it justice.

Thursday December 13, 2012.  Here is the SX 42 actually powered up and operating, S Meter re-installed. Some of the bands 
don't seem to operate, however I do not have a proper antenna.  

This will require more work and a full alignment, but the good news is that it did power up just fine, voltages are normal. 
Listened to Toronto, Cleveland, Washington DC and Germany.   Washington DC was the least interesting.

 

The following night I made a couple small adjustments and now Band 1, which is basically the AM broadcast band
plus....   This band other than a minor dial frequency calibration seems to work fine.  One band out of 5 down.

The audio output stage of this receiver is exceptional for its time.  Typically a radio of this era would be a single tube 
that would deliver a very few watts. This receiver uses two 6V6 tubes operating in what is known as
push - pull operation.  The set up can probably deliver up to 20 watts of audio power to the speaker, although this radio
is not rated for quite that much.

Shown below is the pair of new matched 6V6 audio output tubes by Groove Tubes.  This particular
audio output structure is used in a number of classic Fender guitar amplifiers.

OK, not the greatest photo, but you can see the tubes and lamps glowing. AM band  working great now.  But then...  
I flipped the band switch and back and then nothing......  Thinking OH Crap after all this the band switch has somehow failed somewhere
and this is no easy thing to troubleshoot.....  Well I decided I got a couple hours of listening and this old friend so far has been good to me
that I just went to bed.  But it did bug me.

So at 3 AM (when we are all up), I went to the shop to try to figure it out.. I first looked at the band switch and 
visually could not find anything wrong. But then thought maybe something else, and started wiggling tubes and bang!  The tube on the
lower left had some dirty contacts and is directly connected to the band switch. So some DeOxit cleaner on that socket and we seem OK.

Now to start on the other  things that need repair, but I think I'll do all the tube sockets with DeOxit cleaner... DUH!?

 

Printed out a schematic I can read.  This  is an  ASNI D or a 2 foot by 3 foot  print of the electrical schematic.
The nice thing about this one is that it is all in one piece rather than in segments. 

 

Band switch knob before restoration

 

Band switch knob after restoration

 

Radio with front panel removed

 

Front panel removed and ready for new paint and silk screening.





Front Panel assembled after new paint and silk screen. And in this photo the radio is 95% aligned and all bands are functioning properly.

I still need to do a frequency sweep of the 10.7 mhz intermediate frequency (IF) amplifiers and double check the crystal phasing control.

This is one nice sounding radio

Now I need a decent antenna for short wave listening.

When all alignment is done, it will be installed in the refinished case and moved from the shop to the office.




Finally got the sweep set up to properly align the IF (intermediate frequency) stages. The two upper bands were "peaked"
according to the three somewhat contradictory sets of instructions, but were "peaked" somewhat off center frequency. The band pass was not flat at all.

I don't have the exact marker generator I need for this, but using another channel of the scope and a diode detector,
and the Marconi signal generator, was able to generate pretty accurate markers and set the scope for 100 khz per horizontal division.

This is the set up. Generator coupled through a .1 micro farad capacitor to the grid of the mixer tube.
Terminating the sweep generator or not made no difference in the sweep pattern. The generator output is sufficiently
low impedance and the grid is such a high impedance,there is no loading effect. The reason the generator is terminated is so
that it is accurate on ouput level.



The radio is getting sensitive enough so that a clip lead across the antenna terminals acts as an antenna on the upper bands.
A direct short and tuning to no known radio channel is required.






The HP 8601A sweep generator. Built like a rock.





The Marconi used as a marker generator......



This is the 10.7 mHz band pass after alignment. Each horizontal division is 100 kHz. The lower trace zero beat is exactly 10.7 mHz.
I think the injection level to the mixer was -66dBm..... thus some noise in the display. It was dead full quiet with that injection level.
Not bad for a 1947 tube radio. I think it started to get noisy at around -75 dBm......




Another photo..... Actually the 7L13 was not being used in this set up. I had used it
to verify the sweep generator output previously.

Channel 1 of the scope is fed by a combined AUX RF out of the sweep generator and the RF out of the Marconi 2018A
through a diode detector to generate a marker. The sweep pattern is taken through a X10 probe
off the AM detector of the radio, X1 or X10 probe loading made no difference in the sweep pattern.





It makes a real difference in audio quality especially on weaker signals if the band pass is symmetrical around the IF frequency.


4-7-2015 ... Finally did another alingment check of the 455 kc IF. Put the radio in its case and moved it out of the shop into the office.
Dilbert is happy. Now I need to invent something for an antenna.




 

 


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