[Hallicrafters] Response to the SX-88 Value Controversy

[WA1KBQ at aol.com]

The following discourse is based upon my personal  observations and practical 
experience from owning seven SX-88s during  the past ten years. In that time 
I have extensively restored four, sold  three and still have four.
 
The question of which equipment manufacturer actually made the best overall  
receiver is a very difficult, subjective and extremely controversial subject.  
Raymond S. Moore pursued this question with a lot of effort several years  
ago which resulted in extensive research and testing and the culmination of a  
lot of data on even the most obscure brands eventually being summed up in four  
editions he authored on the subject entitled "Communications Receivers of the 
 Vacuum Tube Era."
 
Raymond S Moore's answer: (drum roll please...) "it all depends!"
 
I have managed to obtain over the course of the past 14 years one or more  of 
nearly all the vintage receivers Moore mentions and after working on and with 
 many of them my hands down favorite is the Hallicrafters SX-88. This is  
after a lot of careful work (including removing all RF modular sections and the  
second converter deck for access) to replace original capacitors and drifted  
resistors which then allows a precise alignment. To me there is nothing that  
matches the audio quality of a wide open (10KC bandwidth) SX-88 but when 
nearby  QRM makes copy difficult just start clipping the sidebands all the way down 
to  250 cycles if need be which is accomplished without the associated 
"ringing" of  a crystal filter. The SX-88 utilizes special litz wound 50KC IF 
transformers  with ferrite sleeves and threaded ferrite cores which results in an 
impressive  "Q" of over 180. While certain other Hallicrafters models also have 
a 50KC IF  scheme none has IF transformers quite like the SX-88. S-76, SX-100, 
SX-101,  SX-122, SX-117, and even the SX-115 all use single strand wound 
transformers  with threaded brass rod adjusters which achieve a "Q" of no more 
than 140.
 
In addition the SX-88 was the first commercially produced receiver to have  
an "SSB" function marked on its front panel. Admittedly this was very early SSB 
 technology and the later developed Product Detectors were better than the  
SX-88's amplified BFO.
 
Still, all things considered a carefully refurbished and precisely aligned  
SX-88 is usually the receiver of choice here in spite of having many others 
also  set up and ready to go. Maybe it's because after a very short 15 minute 
warm-up  period there is zero drift which is very much appreciated when so many 
others  require a constant and annoying retouching of the tuning setting every 
couple of  minutes even after an hour!
 
I say: "Long live the King!!"
 
"It doesn't live up to all the hype" some of you say for whatever  reason...
 
Or, "works pretty good but not like some others I have here" you might say.  
But does it work correctly as designed and specified by the original 
engineering  team? How would your receiver score in a performance test compared to it's 
 original design specifications? Do you have one that was restored  
electrically and correctly adjusted for everyone to judge fairly? My  experience with 
the SX-88 is quite different. 
 
As is the case with most vintage receivers usually offered for sale few  will 
be found properly restored and adjusted to represent anything near what it  
once was. To attempt a critical "performance evaluation" report using an old  
receiver found for sale at the local swapmeet is often no more representative 
of  its true potential than taking a worn out fifty year old car out for a 
drive  test and reporting on its faults and shortcomings. I will agree with you 
that a  typical SX-88 found in the usual as-is, as-found condition will 
disappoint. But  then so will all the others. 
 
An R-390A by today's standards is an anachronism of mechanical complexity  
and will not be satisfactory either mechanically or otherwise until it is  
completely disassembled, cleaned, lubricated and adjusted.
 
Hallicrafters SX-88 Design and Engineering Features
 
1) The Hallicrafters SX-88 is actually an extensively revised and  completely 
updated SX-28!
 
2) First commercially produced communications receiver to have an SSB  
function marked on its front panel
 
3) Full frequency coverage of .535 - 33.0 MC in six bands
 
4) 20 tubes, two RF amplifiers and four IF amplifiers
 
5) Dual conversion, 2075 for excellent image rejection on high bands and  
50KC 2nd IF with special 180 "Q" litz wound transformers capable of superior  
selectivity. 
 
6) Precision flywheel weighted inertia type reduction gear tuning with all  
gears running on ball bearings and with cam actuated positive end stops and  
friction dial locks on both tuning shafts.
 
7) Tuning dial indicators resettable from front panel.
 
 Six position bandwidth control selectable from 10KC down to 250  cycles 
without a crystal filter!
 
9) 5 position audio filter.
 
10) Current regulated filament supply and voltage regulated plate supply  for 
local oscillator stability.
 
11) Crystal controlled 2nd Conversion oscillators 
 
12) Amplified and delayed AVC.
 
13) 10 watts of push-pull 6V6 audio with inverse feedback.
 
14) On-board 100KC crystal calibrator.
 
15) Auxiliary sensitivity control to monitor transmission in the stand-by  
position.
 
16) 50KC cathode follower with output jack for oscilloscope  connection.
 
17) Robust construction for mechanical stability. Chassis triangulated and  
braced for rigidity.
 
The I.F. Amplifier (from 1954 QST review)
As an "old skirt-selectivity  man," the writer was particularly interested in 
the 50-kc. i.f. amplifier, and  we'll take a little time here to tell you how 
the SX-88 gets its wide range of  bandwidths. The heart of it is, of course, 
the tuned circuits that are used.  These are special coils tuned by a ferrite 
slug and surrounded by a ferrite  sleeve. The special design gives a coil with 
a Q of 175 to 185 at 50 kc., as  compared with the Q of 100 of the coils used 
in the S-76. An interesting  sidelight is that it was found impossible to 
obtain a Q of higher than 130 until  a metal screw was removed from the ferrite 
core and a means was found for  threading the glass-hard and glass-brittle 
ferrite.
 
Any skirt-selectivity man can build a sharp i.f. amplifier if someone hands  
him a bunch of Q = 180 coils, but the SX-88 i.f. has the various bandwidths  
mentioned earlier. This poses quite a problem, because the frequency must not 
be  changed radically by the bandwidth-variation method, and the gain must be 
held  substantially constant. This was accomplished by the Hallicrafters 
engineer in  the general way shown in figure 2A below. 
 
_http://www.antiqueradio.org/art/qstfig2.gif_ 
(http://www.antiqueradio.org/art/qstfig2.gif) 

Figure 2. (A) Basic circuit of the variable-bandwidth i.f. used in the  
SX-88. The coupling is increased as C is made smaller, and the Q of L2C2 is  
reduced as R is increased. The stage gain is held constant with changes in  
bandwidth by tapping the grid and plate up or down on the coils. (B) The effect  of 
varying C and R in (A) is that the passband "grows" out to a higher  frequency, 
as illustrated here. 
 
This simplified diagram shows a variable condenser ganged with a variable  
resistor—in the actual receiver these are step-switched, of course. It can be  
seen that the smaller the capacity of C, the tighter will be the coupling  
between the two tuned circuits, L1C1, and L2C2. Furthermore, the larger the  value 
of R is made, the lower becomes the Q of the grid tuned circuit, L2C2. By  
proper proportioning of the various values of C and R (at different switch  
positions), the wide range, in bandwidth variation is obtained. One of the three  
50-kc. i.f. stages has taps on the coils, as represented in Fig. 2A by the 
leads  to S1, and this enables the gain of the i.f. amplifier to be held 
relatively  constant over the entire range.
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