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The SX-28 Introduction

Hallicrafters announced the SX-28 "Super Skyrider" in July of 1940. The receiver's ultimate design was the result of the analysis of more than 600 requested reports, including input from government engineers. Twelve Hallicrafters' engineers were assigned the project of creating a receiver that not only satisfied government and commercial users but also gave the hams a receiver that performed better than any previous Hallicrafters. Additionally, the SX-28's modern, 1940 styling was to compliment the receiver's great performance. The circuit utilized 15 tubes in a double preselection front-end on the top four bands and single preselection on the lower two bands. The frequency coverage was .55 to 43MC in six bands. Amplified AVC, Lamb Noise Silencer, Calibrated bandspread, Push-Pull Audio were some of the features incorporated into the design. The SX-28 would become an all-time ham favorite, famous for incredible audio coupled with amazing sensitivity, stability and selectivity - all at a reasonable selling price.

I was a 15 year old aspiring ham when I first encountered an SX-28A. My ham "Elmer" told me of a friend of his that was going to sell an SX-28A. The price was $100 (this was in 1965) and was way beyond my capabilities to pay, so I settled for a 1934 Patterson PR-10 (it was only $15.) By the time I was in my twenties and having been a ham for several years, I happened onto another SX-28A. By this time the prices had dropped significantly to where this decent example was only $25. I did a few repairs, a cosmetic restoration and used it for a few years but eventually traded it for an early 1920s radio. Yet another SX-28A turned up in the 1980s ($40, this time) and this one was given the full rebuild treatment and performance was first-rate. I used it on several occasions for AM nets and general listening around. Over the next several years I managed to "pick-up" six more SX-28s (all "non-A" versions.) Noteworthy were, receiver H-151197, which was purchased in superb original condition from its original owner. Also, receiver H119051, which was a very early version, also in great shape. Receiver H-130170 came complete with its original R-12 Jensen-Hallicrafters bass reflex speaker. Finally, in 2007, another SX-28A was acquired - though this one is actually a military version, the AN/GRR-2, SN HA-2703. 

Dating the SX-28 by Serial Number

Why is it so difficult to find production information on the Hallicrafters' receivers? After all, they were one of the "big three" (Hallicrafters, National and Hammarlund) from the late thirties up into the 1960s. Unlike National, a company that just recently went out of business (1991) or Collins, a company still in business (though owned by Rockwell,) Hallicrafters was unfortunate enough to have been purchased by Wilcox Instruments, a division of Northrop, in the early 1970s. Wilcox was also known as Wilcox Electric and was located in Kansas City. One of the first orders of business at Wilcox was to DESTROY ALL of the Hallicrafters records and archives. I wrote a letter to Hallicrafters in 1975 that was forwarded to Wilcox Instruments. They did reply to my letter but made it very clear that no records or archives existed anymore. I have also had visitors in our museum that worked at Wilcox Instruments in the 1970s and were present when the orders were issued to destroy the Hallicrafters' archives. There were protests from the employees ordered to carry out the destruction but it was to no avail. Some records were "smuggled" out of the plant but it was only a fraction of what was destroyed. Most of these surviving records are probably in private collections. Since there are no company records available for reference to production dates or quantities, we have to use other methods to reconstruct what most likely took place during the SX-28 production period.

HALLICRAFTERS' SERIAL NUMBERING METHODS UP TO WWII: Hallicrafters' serial numbers on early equipment consists of a number sequence in addition to a manufacturer code number. This was because prior to late 1936, Hallicrafters' receivers were built by contractors, such as Howard Radio Company or other contractor companies. When Hallicrafters was able to build their own equipment (late 1936) they began assigning a serial number consisting of an "H" prefix followed by five numbers. The serial numbers were assigned sequentially to all products as they left production and not exclusively to any model line. There are exceptions to the "H" followed by a five digit number format. Accessories and smaller items that were serialized sometimes have an "H" followed by a four digit number.

 Estimating a dynamic like production is difficult and we are going by assigned serial numbers compared to inspection tag dates and date codes on components. Inspection tags tie a serial number directly to a known manufacturing date. Date codes can be used as a double check to assure a production estimate "makes sense." Additionally, many radio manufacturing companies reduced or stopped their production during the early summer months, at least prior to WWII, so we are not counting May or June as "full" production months until 1942. Most companies used this time for design and tooling changes along with new product introduction.

By mid-1938, Hallicrafters' serial numbers were up to H-80,000. A dated inspection tag shows SN H-83879 was assigned on November 11, 1938. Another dated inspection tag shows that serial number H-85531 was assigned on December 19, 1938. These two tags are separated by approximately one month and show that 1652 serial numbers were assigned during that time period. By late-1939, the numbers were in the H-100,000 range. By estimating that Hallicrafters assigned between 1600 and 2000 serial numbers per month at this time, the SX-28 production should start (approximately) with serial number H-115,000. VE3CSQ owns SX-28 H-116368 which happens to still have its original inspection tag that is dated September 27, 1940. This is the earliest SX-28 inspection tag that has turned up (so far.) SX-28 SN H-119051 was assigned on October 29, 1940, about one month later and shows that 2683 serial numbers were assigned (mostly in the month of October.) This shows that Hallicrafters was increasing their output, perhaps preparing for the 1940 Christmas sales season and also probably due to increased demand as the company grew. It appears that from August 1940 up to about May 1941, production was approximately 2000 to 2500 average assigned serial numbers per month. Variations in output occurred since there was always a rush in the late fall towards Christmas and then a subsequent slowdown in the winter to spring season. During the summer of 1941, Hallicrafters must have again increased their manufacturing capacity because, starting in September 1941, production seems to be at a rate of about 2,500 to 3,000 assigned serial numbers per month. By February 1942, the serial numbers were up to H-150,000. SX-28 SN H-151197 was assigned on February 21, 1942. Remember, these numbers are for all Hallicrafters' products and while they can determine a probable date of manufacture for a particular receiver, it requires other data to determine what percentage of production the SX-28, or any other product, represented.

WWII SX-28 Production Serial Numbers and the Introduction of the SX-28A - 1942 to 1944

During WWII, Hallicrafters' output again increased significantly, however the nature of the production changed from amateur receivers and transmitters to equipment required by the military. After about April 1942, it was impossible for civilian amateurs to buy receivers like the SX-28, as everything produced at Hallicrafters was destined for military or government use. Consequently, SX-28 and other "amateur" receivers accounted for less and less of the total production output. Towards the end of 1943, the serial numbers up to H-180,000. Hallicrafters decided to end the "H" prefix and start a new "HA" prefix with numbers beginning at 1000. It appears that the change over to the "HA" prefix happened around December 1943 or possibly January 1944. As a result, SX-28 serial numbers beginning with the "H" prefix end around H-183,000 but some very late production SX-28s will have serial numbers with the "HA" prefix. In fact, three SX-28s have turned up that have the "HA" prefix serial number (HA-2126 is one of them.) HA-2126 does have the old style SX-28 coils installed in the old style RF box, indicating that it is indeed an SX-28 and not an SX-28A (the other two receivers did not show the underside of the chassis and the RF coils so they could not be positively identified as SX-28s though all of the other SX-28 indicators were present.) This receiver shows that SX-28 production continued into early 1944 with the introduction of the SX-28A probably occurring around February or March 1944. Certainly, Hallicrafters was in the process of designing the "SX-28A" upgrades at the time of the change over to "HA" serial numbers but the indications are that the "HA" prefix was not specifically for the SX-28A model. The AN/GRR-2 (Signal Corps SX-28A) receiver with SN HA-2703 (with the Hi-Q Micro Set coils installed) is MFP dated March 22, 1944, indicating that the SX-28A production was in place by that time. It is possible that Hallicrafters intended for the SX-28A production to coincide with the beginning of the "HA" serial numbering sequence but the new coils and harnesses required new assembly procedures/models and new test/alignment methods which may have delayed the SX-28A introduction for a short time.  

SX-28A Post-War Production and Serial Number Dates for SX-28A on Front Panel

When WWII ended in August, 1945, Hallicrafters almost immediately started civilian production of several pieces of equipment that had formerly been for the military. The civilian SX-28A production started in September 1945 and most dealers at that time were offering to take "advance orders" for when they actually got the SX-28As in stock. Though the use of serial numbers for the SX-28A had been rather conservative during the war, the production use of the HA serial numbers for civilian amateur equipment market from September 1945-on was fairly rapid. It is likely that all SX-28As with serial numbers higher than about HA 20000 are probably post-war production.

The appearance of  "SX-28A" on the front panel seems occur near the end of 1945. KF4TP owns two SX-28A receivers, one is HA-27742 without the "A" on the front panel. His second receiver is HA-27748 which does have the "A" on the front panel. With a separation of only six serial numbers, this almost certainly dates the "SX-28A" front panel identification to December, 1945.  It is likely that post-war civilian customer complaints about the receiver's old ID "SX-28" on a brand-new SX-28A prompted Hallicrafters to finally have the front panel die changed. Approximately 3800 SX-28A receivers were produced with the "A" designation on the front panel. This estimate is based on earliest serial number encountered with "SX-28A" on panel (HA-27748) to latest SX-28A serial number encountered (HA-53404) resulting in 25656 serial numbers issued with 15% assigned to SX-28A receivers equals 3840 receivers with "SX-28A" on front panel. Since this is an approximation, the number is rounded off to about 3800 units. Also, since this estimate is just for the post-WWII production, 15% is used for SX-28A percentage of assigned serial numbers since there were several other models also using the "HA" serial numbers.

By July 1946, the new 1947 models were introduced with the SX-42 taking the "flagship" position that the SX-28 had occupied for the past six years. The SX-28  had provided pre-war hams with a great receiver at a bargain price, while during WWII the SX-28 filled many different occupations from intercept to entertainment. The SX-28A eased front end manufacture and alignment while conserving necessary materials and, after the war, hams still had an opportunity to purchase a brand new version of a receiver they may have become familiar with while serving in the military. Revered, admired and respected,...the SX-28 was and still is one of the great receiver creations.

1945 SX-28A, SN HA-25583 An early example of the civilian SX-28A (probable build date October 1945) with the "SX-28" designation on the front panel. Civilian production started in September 1945 and continued until around July 1946 when the new 1947 models were introduced. The front panel identification was changed to "SX-28A" in December 1945.  (I  purchased this receiver at a CHRS swap meet in the 1980s. I restored the receiver and repainted the cabinet with Illinois Bronze "French Gray" Wrinkle Finish paint - too bad it's no longer available. I used this SX-28A  for several years but reluctantly sold it in 2004)

  Identifying SX-28As that have "SX-28" on the Front Panel

From its introduction in early 1944 until the front panel designation change in December 1945, all SX-28A receivers are identified as "SX-28" - at least according to the front panel. As far as Hallicrafters was concerned, the designation "SX-28A" applied only to receivers that had the Hi-Q, Micro Set coils in the front-end. Nowadays, all Hallicrafters enthusiasts agree with this definition. However, there are several characteristics that can be used to identify an SX-28A without removing the bottom panel to look at the coils. These are known as the SX-28A "indicator parts." They are not entirely reliable for ID but, generally, it's a quick way to tell from a distance what the receiver is. All SX-28A receivers will have webbed tuning and bandspread knobs, a heavily textured front panel, a clip-on louvered condenser box cover and a serial number prefix "HA." While all SX-28As have these parts, unfortunately we can't say that none of the SX-28s would have had these parts installed. It is known that some of the very late SX-28s were fitted with webbed knobs. Certainly, some of the late SX-28s have a louvered condenser box cover though it was mounted with screws (not clip-on.) It is known that the heavy-textured front panel also made its appearance prior to the introduction of the SX-28A. The changeover to these later parts seems to be on SX-28 serial numbers higher than H-180,000, or in late 1943. The change to "HA" prefixes began around January 1944 (possibly as early as December 1943) and seems to only predate the introduction of the SX-28A by a couple of months. This series of events would account for some late SN SX-28s that have webbed knobs (H-181958, HA-2126), louvered condenser cover (H-173611, H-174842, HA-2126) and heavy textured front panel (H-178848, H-181715, HA-2126.) Since these parts were incorporated into late SX-28 production, we can't be 100% sure just by looking at these parts if a receiver is an "A" or non "A."  Even the appearance of the "HA" prefix in the serial number is not a sure indicator. Three SX-28s have turned up with the "HA" prefix serial numbers (HA-2126 is one, the other two have not been confirmed.) These receivers have all of the SX-28A indicator parts (louvered condenser cover is screw mounted though) but the coils are the old SX-28 style and the RF box is the old SX-28 style. Though it is likely to change as more SX-28s are photographed, it does appear for now that the SX-28A exclusively uses the clip-on condenser box cover and does not have the auxiliary relay socket in addition to the Hi-Q Micro Set coils. Only the Hi-Q Micro Set coils are the 100% sure indicator since they were the reason for the designation change. Fortunately, if it's really important to know in advance of purchase whether the receiver in question is an "A" or non-"A",  you can turn the receiver on its side and, if you've seen what the Hi-Q Micro Set coils look like, they are visible thru the openings of the bottom cover.

Component Date Codes for Build Date Confirmation

There are three date-coded parts used in the pre-war SX-28 that provide specific information as to when the part was made. From that, one can certainly assume that the receiver was built later than that date. The audio output transformer is usually stamped (on top) with a numeral month and year, e.g., "2 Stc 41", or February, 1941 (the "STC" separating the month and year is the logo for Stancor.) The filter choke is also sometimes stamped in the same format, however use of a date code on this part seems to have been sporadic. These parts are only a source for early SX-28 models as, by early 1942, Stancor had eliminated the date code information. The bass choke is another part stamped with a date code. SX-28 SN H-119051, built 10-29-40, has a date-coded bass choke "10 Stc 40" showing that it is possible for newly arrived parts to be immediately used by production. The date coded bass choke disappeared about the same time as the dated coded filter choke.

Here is a photo of the top of the bass choke showing the date code information. The logo is for Standard Transformer Corporation (Stancor.) 55-010 is the part number. 10 is the month and 40 is the year of manufacture. This bass choke is from SX-28 SN H-119051 (10-29-40.)

On some SX-28A and GRR-2 models a date was ink-stamped on the back of the condenser box when (and if) the receiver was fungicide treated. A stenciled form was applied to the condenser box with all of the treatment information including the year. The actual date was rubber stamped in a space provided. Usually, the format is a small "month-day" - MAR 7, for example that was rubber stamped and then a larger, two numeral year, e.g. "45" for 1945, which was part of the stencil.

Here is a photo of the fungicide treatment stencil which is usually located on the back of the condenser box. The date of "45" is part of the stencil. The smeared rubber stamped date is "MAR 7." This stencil is on Military Rack Mount SX-28A SN HA-11774.

Assigned Serial Numbers - August 1940 to June 1946

The following chart is based on observed serial numbers compared to dated inspection cards and date coded parts or date coded stamps on SX-28, SX-28A and GRR-2 receivers. It is an approximation that takes into account the slow increase in production in last part of 1940, the dramatic increase in production through 1941 and into the first third of 1942, the drop in production during WWII with the subsequent increase of civilian SX-28A production starting in September 1945 and running through the first part of 1946. Dated inspection card confirms H-116368 was assigned September 27, 1940 and another dated inspection card confirms HA-53513 (on S-40 receiver) was assigned on June 18, 1946.

Earliest SX-28 serial number encountered:  H-115254 (seen on eBay)

Latest SX-28 "H" prefix serial number encountered: H-181958 (seen on eBay)

Latest SX-28 serial number encountered: HA-2126 (seen on eBay)

Aug 1940  to  Dec 1940 . . . . . . . . . . H-115,000    to    H-124,000

Jan 1941  to  Dec 1941  . . . . . . . . . .   H-124,000   to   H-145,500

Jan 1942  to  Dec 1942 . . . . . . . . . . . H-145,500    to   H-166,500

                Jan 1943  to  Dec 1943 . . . . . . . . . . . H-166,500    to   H-183,000/HA-1000

Jan 1944  to  Dec 1944 . . . . . . . . . . . . HA-1000    to    HA-8,500

Jan 1945  to  Dec 1945 . . . . . . . . . . .  HA-8,500   to   HA 29,500

Jan 1946  to  June 1946 . . . . . . . . . . . HA-29,500   to   HA-55,000

Earliest SX-28A serial number encountered: HA-2703 (owned by W7HTR)

Earliest SX-28A serial number with "SX-28A" on front panel: HA-27748 (owned by KF4TP)

Latest SX-28A serial number encountered: HA-53404 (owned by VE6XI)

Estimated Production Figures for the SX-28 & SX-28A

PRODUCTION BY SERIAL NUMBERS: By using the total quantity of serial numbers issued between August 1940 and March 1942, one has about 40,000 numbers. Assuming that SX-28s accounted for about 15% of the numbers assigned, one arrives at a quantity of 6,000 receivers. One has to consider that the SX-25, SX-24, S-20R, Sky Buddies and later the S-27, SX-32, etc., all went into the production serial number use. Small tube-count, inexpensive receivers were much better "sellers" than expensive sets like the SX-28 and accounted for a much larger percentage of production and serial number assignments. An interesting advertisement in August 1941 QST seems to confirm that the SX-28 did not account for a large percentage of orders. The advertisement is for "Bob Henry W9ARA" in Butler, MO, a major ham equipment dealer at that time, (later became Henry Radio.) It shows a telegram from Bill Halligan congratulating Bob Henry on the placing a very large order for Hallicrafters equipment on June 26, 1941. Of the 140 receivers ordered by "Bob Henry W9ARA" only 20 are SX-28s, or about 14%!

From April 1942 through January 1944, manufacture of SX-28 receivers was at a much lower rate than in the pre-war days. However, since many of the less expensive, pre-war Hallicrafters' receivers were now not part of production, the percentage of serial numbers issued to SX-28 receivers increased. It is probable that the ratio of assignment of serial numbers to SX-28s compared to other equipment increased. It is possible that between 30% and 40% of the assigned serial numbers went to SX-28s during this period. Serial numbers assigned during this period should run from H-155000 up to H-185000 and from HA-1000 up to HA-2000. If we assume that 35% of the serial numbers went for SX-28s with about 31,000 serial numbers issued during this period, this would account for about 10,850 SX-28s built between April 1942 and January 1944. This would bring the total SX-28 production to about 16,850 receivers, (6,000 pre-war SX-28s plus 10,850 built during the war for a total of 16,850 receivers.)

The use of HA serial numbers started slowly in 1944 and progressed through August 1945 at a steady pace. With the end of WWII in August 1945 and the return to amateur radio production in September, Hallicrafters assignment of "HA" serial numbers increased rapidly. Hallicrafters enthusiasts agree that SX-28A production was significantly less than that of the SX-28 production. The SX-28A production started around February 1944 with serial numbers around HA-2500. Highest SX-28A serial numbers encountered are about HA-55000. If the SX-28A accounted for an average of 20% of the assigned numbers that results in about 10,500 SX-28A receivers - a reasonable figure. Again, other Hallicrafters receivers were accounting for the remaining 80% of the serial number assignments, e.g., S-40, SX-25, S-38, S-36A, etc. Total SX-28 and SX-28A production should be around 27,350 receivers, (16,850 SX-28s plus 10,500 SX-28As.) However, in a post-war advertisement, Hallicrafters stated that over 50,000 SX-28 and SX-28A receivers had been produced. To achieve this quantity of receiver, nearly 40% of all serial numbers assigned between 1940 and 1946 would have been assigned to SX-28 and SX-28A receivers - not likely. A more reasonable production estimate, that seems to reflect how many SX-28s and SX-28As are encountered today, is around 16,850 SX-28s and 10,500 SX-28As for a total of 27,350 receivers produced.

1945 Military-Commercial SX-28A Rack Mount Receiver, SN HA-11774 The military and some government users purchased the SX-28A in the rack mount configuration, with characteristic rack mount top dust cover and (though it can't be seen) bottom cover. Some are fungicide treated which sometimes leaves a red colored coating on solder joints and around tube sockets. The circuit is identical to the standard cabinet SX-28A though some of the rack mounted SX-28As had plexiglass windows in the dial covers. These receivers are different than the US Army Signal Corps AN/GRR-2 receiver in that they will have the standard audio output transformer and choke along with the Hallicrafters' power transformer. Additionally, these receivers have the dial string drive on the bandspread dial. Also, long handle toggle switches are used along with the metal cased S-meter.   This receiver is owned by: K0DWC, Charles Cusick, Va. City, Nevada

SX-28 and SX-28A Engineering and Production Changes - 1940 to 1946

1940 Production Construction - August 1940 to Dec. 1940

Though the SX-28 was announced in July 1940's QST, manufacturing didn't start until August, 1940. Most of the major dealers began advertising the SX-28 was "in stock" in September, 1940's QST.

These are some of the differences in very early production as compared to 1941 and later production.

1st RF amp tube is 6SK7 - SN H-116268 uses 6SK7 - changed to a 6AB7 by SN H-119051, 10-29-40

1st RF coils have trimmers (Bands 3-6) - This was probably a schematic error although trimmers are shown on 1940 and 1941 versions. Even the earliest manual's photo of the chassis underside show that no trimmers were used on these four coils.

No Cathode Return bypass condenser C87 (.25mfd) - this may be another schematic error as C87 is found in all early examples of the SX-28 but C87 was not added to the schematic until after 1942

2RF and Mixer coils use air trimmers

Hex head screws used to mount front-end coil chassis

Front panel painted blue-gray - latest SN seen with gray panel, H-124230 - paint color changed to medium black before SN H-124417 (12/40)

No panel to cabinet screws used at the top-center of panel, flanking main dial bezel - these mounting screws were being used by SN H-124230 (12/40)

RF Gain potentiometer mounted to chassis - mounted to panel by SN H-124230

T3 tuned with air trimmers (last IF transformer)

C41 listed as 10mfd (1AF Cathode By-pass) - this may be an error on the parts list although it remained listed as 10mfd until the 1943 parts list. Even the earliest SX-28s seem to have a 40mfd capacitor installed for C41

AVC Amp input source from 1st IF Grid

ANL input source from Amp AVC

6SK7 Noise Amp tube - changed to 6AB7 before SN H-124230

T5 design uses variable inductance with only a single adjustment on top of can

CH3 used - choke in ANL, mounted rear chassis next to S-meter pot

Antenna Trim not used on Band 1 and 2

Gear Driven Bandspread Tuning

Selling price was $159.50

1941 Engineering & Production Changes

(The following changes are not necessarily shown on the schematic or the parts list in the 1941 Manual)

Front panel paint changed to medium black - end of 1940, latest SN seen with gray panel, H-124230 and earliest SN seen with black panel, H-124417 - Only 187 serial numbers separate this particular change, dating it to Dec. 1940 (though intermixed stock may account for some variability.)

Upper Panel to Cabinet mounting screws (flanking main tuning dial) are being used by SN H-124230 (12/40) - Interestingly, even the earliest cabinets always did have the two tabs to accept these screws however the front panel was not drilled for these two screws until around 12/40.

Cathode Return Bypass (C87) installed on inside front panel by RF Gain control which was moved from chassis mounting to panel mounting - before SN H-124230

AVC Amp input source moved to Mixer Plate

ANL Noise Amp changed to 6AB7, design of T5 changed to variable capacitance with two trimmers on top of can, CH3 replaced with wave trap (CH4 & C55) and ANL input moved to Mixer Plate, C38 deleted (shown on parts list but not on schematic and not installed in circuit,) C37 changed from 100pf  to 50pf, R24 changed from 50K to 100K, SW5 changed from SPST to DPST (ANL switch,) shielded cables to SW5 - all part of the redesigned Lamb Noise Silencer circuit (12-40)

Hex screws used to mount front-end coil chassis changed to Phillips head screws

Red Indicator Set Points on Main Tuning Dial for correct Band Spread Dial calibration - Early dials may have had defective ink used for the red markers. Many early SX-28s will appear without red BS indicators, however close examination of the main tuning dial will usually show some traces of the original red ink. By 1941, the red ink formula was probably changed as the problem of faded red BS indicators is normally not encountered after ~ SN H-128000.

C31, C30 and C29 (Xtal Filter compensation) changed from compression trimmers to ceramic trimmers

Front-end trimmers in RF2 and Mixer stages changed from air trimmers to ceramic trimmers

ANL wave-trap (CH4 & C55) construction changed (early style has a vertical mounted coil, later is horiz. coil)

T3 trimmers changed from air trimmers to ceramic trimmers (last IF transformer)

New manual - still dated 1941 but has recent changes added to circuit description, schematic and alignment procedure (8-41)

Selling price increased to $179.50 (QST 9-41)

Gear driven Bandspread changed to dial cord - mid to late 1941 change, it is still gear driven at SN H-130170 but is cord driven by H-151197 (2-21-42) this was certainly a cost reduction change as it eliminates all of the bandspread gearbox and replaces it with a pulley and dial cord. NOTE: The WWII military AN/GRR-2 (SX-28A) models used the gearbox driven bandspread dial (1944-45.)

1942 Engineering and Production Changes

(These changes are not necessarily shown in the schematic or parts list of the manuals included with 1942 models)

Fuse and fuse holder added - initially mounted atop chassis adjacent to 5Z3, later installed on back chassis apron

Location and orientation of Auxilary Stand-by Switch outlet (looks like AC receptacle) changed when fuse holder is mounted to rear chassis - initially, located above the Phono Input jack with horizontal orientation - later moved next to fuse holder with vertical orientation.

Connector insulators on rear chassis changed from brown R-39 type material to black bakelite (by SN H-158997)

Bracket added to rear of Selectivity switch and screwed to chassis to strengthen mounting

Fiber shaft used for Antenna Trim, eliminates the copper grounding springs used on the earlier metal shafts

R25 and R26 (250K each) replaced with a single resistor, R25 (500K)

Shielded cables no longer have the shields soldered to the RF box corner - SN H-158997

Sleeved shielded cables (to SW-5) that were part of the redesigned ANL and were routed above the wiring harness are now installed next to the chassis and with the wiring harness. There are other minor changes in the wiring harness. A sleeved braided cable connects the 1st RF Amp section shield to the rear of the main chassis (mounted with screws and nuts on each end.) Seen on SN H-158997

Antenna Trimmer circuit modified to have C6 (Ant Trim) functional on Bands 1 & 2 (required a new Band Switch wafer to be added at the rear of the assembly) - This change appears by SN H-158997 (ca. 5-42) but is not on SN H-151197 (2-21-42)

Amplified AVC modified to allow a lower AVC voltage to be used on the RF amplifiers on all bands except Band 1, which still used the higher AVC voltage (required a new Band Switch wafer to be added) - ca. mid-1942, not on SN H-158997

Manual updated with new schematic and under chassis photos to reflect recent updates from 1942 - Part values are shown for capacitors inside IF transformers in new schematic - This manual probably dates from August 1942 (the previous updated manual was dated Aug 1941 and the initial manual was dated Aug 1940)

There is a possibility that the front panel paint was changed toward the end of 1942 - SN H-161034 has a bluish-gray-black (SX-28A color) front panel with light texturing

1943 Engineering and Production Changes

Condenser box cover modified by enlarging the tube cover panel for better access to tubes (seen on SX-28s SN H-167827, H-167872)

A louvered condenser box cover, similar to the SX-28A cover, started to be used by mid-1943 (seen on SX-28s SN H-173611, H-174842, HA-2126) - this cover is mounted to the condenser box with screws

Front panel texture changed to deeper convolutions, painted "bluish-gray-black" (seen on SX-28s SN H-178848, H-181715, HA-2126)

Knobs for MAIN TUNING and BANDSPREAD changed to "webbed" style (SN H-181958, HA-2126)

Serial number "HA" series starts (~12-43 to 1-44)

1944 Engineering and Production Changes

SX-28 production stopped (~1-44 to 2-44) to introduce SX-28A into production

SX-28A production starts (~2-44) - earliest encountered SX-28A (AN/GRR-2), SN HA-2703, fungicide treated date: Mar 22, 1944

SX-28A Manual dated April 10, 1944

Most parts changed to JAN type values

Tube IDs changed from engraved on sockets to painted ID on chassis

Louvered lid for condenser box changed to clip-on mounting

Auxiliary Stand-by Switch outlet on rear chassis eliminated

Front-end coils changed to smaller "Hi-Q, Micro Set" types, coil chassis sheet metal changed for new coils along with wire routing changes, eliminates old RF box construction

Amplified AVC transformer has added 25pf capacitor in parallel with secondary to prevent oscillation

The above changes were in place when the SX-28A production began and are on all examples found

1945 Engineering and Production Changes

Glass in dial windows changed to Plexiglas on some models (seen in rack mounted models)

Shafts for ANT TRIM and XTAL FILTER PHASING changed to plastic on some models

Designation on front panel changed to "SX-28A" (earliest SN seen with "SX-28A" on panel - HA-27748 ~12/45)

1946 Engineering and Production Changes

Selling price listed as $223.00

2nd RF Amplifier tube changed to 6AB7 (seen on HA-53212)

SX-28 and SX-28A Variations Seen

SX-28 Navy Version made in late 1943 does not use the standard front panel but has a black wrinkle finish panel with white nomenclature. No "Super Skyrider" or "hallicrafters" on front panel - a military ID plate is mounted on the panel instead. Heavy duty construction in some areas under chassis. Cabinet is similar to AN/GRR-2. These receivers are issued standard Hallicrafters' serial numbers carried on the tag mounted to the rear chassis of the receiver. Whether the introduction of the AN/GRR-2 in February 1944 eliminated the Navy version SX-28 is unknown. The "AN" designates "Army-Navy" so perhaps the SX-28A Navy versions are the AN/GRR-2 receivers.

Canadian Versions - distributors like Rogers-Majestic sold SX-28s in Canada. These receivers will have the original Hallicrafters' serial number tag with the standard Hallicrafters' serial number assigned and an additional serial number tag with a distributor-assigned serial number installed by the distributor in Canada. Also, the receivers usually have a tag or decal from CSA, which is similar to USA's UL.

The FCC Variation - had a metal tag mounted on the front panel or on the back of the receiver for FCC identification. These receivers also carry the standard Hallicrafters' serial number tag with standard Hallicrafters' serial number assigned. These SX-28s were built for FCC monitoring posts where several receiving stations would be in operation. Each operator would be using earphones to prevent listening interference to an adjacent monitoring station. Since the audio requirements were minimal, the Push-Pull audio output 6V6s were replaced with a single-ended 6V6 audio output. The unused 6V6 tube socket mounting hole has a push-in metal plug installed. (Thanks to WB4TGG)

SX-28U and SX-28AU were special order receivers with a dual primary power transformer for operation on either 120vac or 240vac. The transformer has a slide switch on top of the transformer cover for selecting the operating voltage

Send-Receive and Bass In/Out Switches - short handle, ball-end toggle switches used on SN H-130170 - long handles were normally used, although the military AN/GRR-2 receiver is usually encountered with ball-end toggle switches.

T3 inductively tuned - seen on SX-28A Rack Mount SN HA-11774

White front panel, supposedly used aboard a hospital ship. Silver front panel - unknown reason. Many of these custom panels seem to have some story of their "official" origin but "from the factory" is doubtful. Official front panel colors were blue-gray in 1940, black in 1941 through 1943 and blue-gray-black in 1943 through 1946.

SX-32 & S-35, though they aren't really variations, the SX-32 is very similar to the SX-28 but doesn't have the Lamb Noise Silencer or Bass Boost and uses 13 tubes. It was priced at $149.50. The SX-32 was introduced in July 1941 and, by September, the SX-28's price was increased from $159.50 to $179.50. The S-35 was advertised as a "panoramic receiver" and was an SX-28 or SX-28A with a panadapter with 5" CRT mounted in a single, tall table cabinet. The 1946 price of $375 probably kept most S-35s out of the amateur's shack.

Prototype SX-28 Cabinet - This cabinet is obviously for the SX-28 chassis however there are several differences between it and the standard production cabinet. Apparent are the lack of vent holes in the top and the lack of a removable louvered panel on the underside. Only three screw holes per side are available for securing the front panel to the cabinet. On the underside, only two, enlongated holes are provided to secure the receiver chassis. The top edge on each side is a single piece of metal with a smooth radius on the edge - the standard cabinet is a two piece construction with spot welds. Overall though, the cabinet is undoubtedly for the SX-28 and is more than likely one of the concept or prototype cabinets built for finalizing the actual production design. At present, this cabinet has a thick coat of gray paint over the original wrinkle finish.

Restoration Methods & Suggestions

MANUALS: If you don't have a manual, get one. Remember, there are several variations in the manuals with different schematics and different under chassis layouts (for alignment.) The earliest manual was printed on "slick" paper in August of 1940 and is numbered "07292840." This manual has the earliest schematic with the two 6SK7 RF amplifiers. The next version is from August of 1941 and is printed on standard matte paper. It is numbered "01092841" and has most of the 1941 changes shown on the schematic and in the circuit descriptions. The schematic and alignment information in Rider's Perpetual Troubleshooting Manual VOL. XII is the same as "01092841" Hallicrafters' manual. Most reproduction manuals that are available are broken down into "standard" early version (01092841), mid-production version (no number-1942) and SX-28A (April 10, 1944.) Most production SX-28s won't agree entirely with any manual as the engineering changes were incorporated immediately while documentation usually took some time. Sometimes addemdums turn-up, so Hallicrafters was attempting to keep their documentation current. The mid-production manuals are sometimes dated "1943" but many manuals at this time weren't dated at all. Many of the 1943 manuals have thick covers rather than standard paper (more variations!) If the schematic shows the AVC and ANT TRIM changes, then it is a mid-production manual from August, 1942 (or later.) The SX-28A manuals are so-marked and Rider's VOL. XVI contains a schematic and alignment information on the SX-28A. There is also an AN/GRR-2 (military SX-28A) manual that has much more detail than its civilian counterparts. The AN/GRR-2 manual is TM-11-874. There is an error in the GRR-2 schematic in TM-11-874...R17 (1K) and its connection to pins 4 and 5 of the 6L7 first IF amplifier are missing. This part of the circuit is shown correctly in the standard SX-28A manual's schematic.

WORKMANSHIP: The SX-28 or the SX-28A are not easy receivers to restore. There are around forty paper-wax capacitors to replace along with five electrolytics. Many of the capacitors are difficult to access and several parts need to be dismounted or disassembled to remove old capacitors and install the new capacitors. Additionally, many of the resistors can't be measured accurately while in the circuit. This requires that one end be unsoldered for accurate measurement if the resistor is suspect. How well the receiver is going to work after its rebuild depends on the level of workmanship (or skill) of the rebuilder. Checking yourself and rechecking what you are doing is second nature to experienced technicians. Quality soldering technique again is something acquired with experience. Rushing through the job is not recommended as this leads to mistakes and poor workmanship. All of these are important skills that are necessary when reworking (or considering reworking) any receiver as challenging as the SX-28 or SX-28A.

ELECTRONICS - CAPACITORS: For best reliability, all paper-wax capacitors and all electrolytic capacitors should be replaced. From the manual, go through the parts list and order the capacitors in advance. Having them all on-hand makes the job much easier. Whether you use "orange-drops" or "yellow jackets" is up to you. Both types are excellent quality caps that are much better than the originals were when they were new. I find it easier to work a section at a time starting at the power supply then moving to the IF/ANL side and then to the front part with AVC/DET and BFO circuits. Also, I install the electrolytics inside the original cans. This is just for aesthetic reasons and results in a nicer looking job. When that is finished, then it is time for the receiver front-end.

RF BOX CAPACITOR REPLACEMENT - SX-28: There are usually eleven capacitors that should be changed in the RF box. Though some people can work around with special tools to replace the caps without disassembly, I can't imagine how they accomplish it without breaking something. Disassembly of the RF box is necessary, along with removal of both RF coil chassis. This will require desoldering several connections to the tuning and bandspread condensers, desoldering wires from the coil chassis to connections in the receiver chassis and a few wires that interconnect between the coil chassis. Usually the Mixer coil chassis can be worked on while still in place, however a long tip soldering iron will be required to reach the chassis connection on one of the capacitors on earlier versions. Read the procedure on restoring an SX-28, in particular the section on rebuilding the RF box, at Phil's Old Radios (link provided in references, bottom of page.) This is a step-by-step guide that can be followed as you rework the RF box. Remember, not all SX-28s are like the one described in that procedure. It is a guide and you will find some variations in any SX-28 encountered. Watch the height of the caps when mounting the replacements as clearance is tight. I use a Makita driver to disassemble the RF box - it's a lot faster and the bits seem to fit the Phillips head screws better. The early SX-28s used hex head screws to mount the coil chassis. These make clearance between the coils and dividers very tight so care has to be taken to avoid damaging the coils. A straight sided, .25" hex, driver bit extension can be used as a nut driver for extra clearance.

The underside of SX-28 SN H-119051 showing the construction of the RF box and the layout of the various coils and trimmers. There are several noticeable differences in this early model, e.g., the chassis mounted RF Gain control, the absence of the shielded cables for the ANL switch, air trimmers used in the 2nd RF and Mixer coil assemblies. Also, noticeable are the SBE Orange Drop replacement capacitors.

SX-28A FRONT END: The SX-28A is similar to rework, that is, requiring desoldering of the condenser connections, removal of three to four wires per coil chassis terminal board and a few wires that interconnect each section to allow removal of each assembly for capacitor replacement. The coils are mounted to fiber boards that are screwed to brackets that are part of the coil chassis of each section. The Second RF and Mixer coil chassis have two fiber boards with three coils mounted on each board. The First RF chassis has the four coils mounted to brackets that are part of the coil chassis. Unlike the SX-28, the tube sockets are mounted to the coil chassis and this construction has some of the capacitors not visible (since they are under the fiber boards) until the coil chassis is removed. While it might be possible to leave the coil chassis mounted and try to remove each fiber board to access the capacitors, it is easier in the long run to just remove both RF chassis, along with the Mixer chassis, for capacitor replacement. Even then, the 2nd RF coil chassis requires the removal of both fiber boards to access two of the paper-wax capacitors. The SX-28A has no RF box per se, as the coil chassis make up the entire front-end shield assembly when mounted to the receiver main chassis. Also, the band switch shaft has a coupler between the oscillator section and the mixer section so only the rear band switch shaft needs to be removed. So, even though the SX-28A is considered an easier candidate for front-end rework, don't be surprised if it takes just as long and is just as difficult as the SX-28 front-end.

The underside of SX-28A SN HA-11774 showing the construction of the new Hi-Q Micro-set coils and the coil chassis that comprise the shielding when the front end is completely assembled. Note the replacement capacitors. Also, note that the original power transformer was replaced sometime in this receiver's past. The coax from the mixer output to the accessory plug for operation of a panadaptor. This SX-28A is owned by: K0DWC, Charles Cusick of Va. City, Nevada

A close-up of the 2nd RF coil assembly from an SX-28 removed for rebuilding. Shown are the large coils that are characteristic for that model. Since this is an early version, note the air capacitor trimmers. The old capacitors have been replaced with "orange drops" and this assembly is ready to re-install. The masking tape on the condenser leads are aids for proper connections.

This is a close-up of the 2nd RF coil assembly from an SX-28A removed for rebuilding. Note the difference in the size and shape of the Hi-Q Micro-set coils compared to the old style coils of the SX-28. Shown are the fiber boards that the coils mount to and the tube socket that is part of the coil assembly. Though they can't be seen in the photo, the condenser leads are underneath the assembly. The capacitors have been replaced on this coil chassis.

ALIGNMENT: Though re-capping is certainly an important facet of the electronic rebuild, a thorough alignment is what really allows the rebuilt SX-28 to perform to its full capabilities. The manual's alignment instructions are tedious and never changed (after 1941) throughout the SX-28 and SX-28A production. Even TM-11-874 for the AN/GRR-2 uses the same procedure. Essentially, the procedure tells you to "rough" align the IF at 455KC and then determine the exact crystal frequency. Realign the IF at this frequency and balance the crystal filter response for its three positions using the trimmers provided. If you have aligned communications receivers before, the procedure will be a familiar process. A resistive load is required for alignment of the 1941 Lamb Noise Silencer (the earlier version does not require a load for alignment.) The remainder of the alignment is standard procedure. The better your alignment skills are and the better your equipment is, the better your SX-28 will perform after alignment.

Sometimes the slugs on the Hi-Q Micro-set coils used on the SX-28A are waxed in place making adjustment difficult. Normally, the slugs will show excessive wear in the slot for the adjustment tool. You have to proceed carefully if adjustment is necessary as the slugs break easily. Sometimes it will be impossible to move a stuck slug without breaking it or breaking the coil mounting, which is not an option. Try a small amount of localized heat (small soldering iron tip on slug) to loosen a stuck slug. You should see the wax begin to bubble near the edge of the slug. Alternately, keep trying to move the slug and then apply the heat. Eventually, the slug should loosen. If the slug is hopelessly stuck, try to equalize the error across the band using the trimmer adjustment.

S-METER POT:  A common problem with SX-28s is the S-meter potentiometer. Quite a few will show a short to chassis. In most cases these are easy to repair. Remove the pot from the circuit and the chassis. Remove the rear cover from the pot. This is a tight press-fit but if you support the pot body in a small vise, the rear cover can be removed by using a small flat metal drift and a small hammer. Don't be too aggressive, the cover will come off with gentle taps. Once the rear cover is off, you will probably find that the rear cover's inside metal plating has peeled off and several strands of metal plating are all around the windings of the pot. These can be blown out with air or brushed out with a small paint brush. Once the strands are removed the pot will no longer show a short to case (or chassis.) Reinstall the rear cover (after wire brushing the remaining plating off of the inside of the cover) and reinstall the pot into the circuit and chassis.

AUDIO OUTPUT TRANSFORMER: The audio output transformer used in the SX-28 and SX-28A was unique to Hallicrafters' designs. It is 9K Z CT on the plate side and 100, 500 and 5000 ohms Z on the output side. Nowadays, quite a few SX-28s are missing their original audio transformer and a universal replacement has been installed in its place. The replacement will not have the correct output Z and will compromise the correct operation of original Hallicrafters' speakers that have the 5000 to 8 ohm Z matching transformer built-in. Fortunately, the Peter W. Dahl Co. does make a correct replacement audio transformer for the SX-28 (Peter Dahl website listed below in references.) It is $125.00 and its installation will provide the correct matching Z for the P-P 6V6 amplifiers and allow the SX-28 to be used with an original Hallicrafters' speaker, such as the PM-23 or R-12. The physical size and construction is identical to the original, however the appearance is somewhat different in that Dahl's transformer is painted black. In operation, it provides excellent audio that is identical to the original transformer.

 NOTE: There has been a lot of e-mail, postings and other sources of information on the web that suggest that the Peter Dahl Company is going out of business as of  January, 2008. As of 2/20/2008, the Peter Dahl website is still up and appears the same as usual (although the on-line catalog doesn't work.) This is such a wide spread rumor that it is probably true and, if so, another great source of components to keep vintage equipment operational will have been lost.       

COSMETICS - CABINET: Many SX-28s are in fairly rough condition after years of storage in less than ideal environments. Before stripping paint, take the receiver out of the cabinet and remove the front panel. This will allow a constant flow of water and cleaning liquid to flush away the years of grime and dirt that are ingrained into the wrinkle finish of the cabinet and panel. I use a large paint brush to work the cleaner into the convolutions of the wrinkle finish and then flush with water. It will probably take two or three sessions to get the cabinet and panel thoroughly clean. When everything is clean and dry it is a lot easier to assess what needs to be done. Many times, after a good cleaning, it is apparent that just a touch-up will fix what are now just minor paint problems. I use artist's acrylic paint to mix an exact match for touch-ups. Remember that you can't get the correct color "off of the shelf" because every SX-28 has aged somewhat differently and the color will vary from cabinet to cabinet. If you have the color matched at a paint shop, it won't be wrinkle finish. It will be fine for touch-ups but not for a complete repaint. There are automotive shops that specialize in various types of powder coating that give good results and can look pretty close to wrinkle finish if you have cabinet paint that is beyond saving. When reinstalling the receiver chassis into the cabinet, be sure to install all of the screws that secure the chassis to the bottom of the cabinet. If the screws are left out (which is how many unrestored SX-28s are found) overall stability of the receiver suffers. This is a mechanical issue and without the screws any movement of the cabinet results in flexing of the chassis. When all of the screws are installed and tight, the SX-28 chassis is rigidly mounted and the cabinet can be "jarred" with very little (if any) frequency change resulting.

FRONT PANEL: The front panels are "medium" blue-gray on very early SX-28s. The front panel color was changed to a "charcoal" black in December 1940 and remained that color until late 1943 when the new, heavily textured front panel was introduced on some of the last SX-28s produced. There is some evidence that the front panel paint color was changed sometime before the heavy textured panel introduction as at least one SX-28 has turned-up with the bluish-gray-black paint on a front panel with the light texture (SN H-161034 - late 1942.) This same color was also found on a 1944 AN/GRR-2 receiver panel. In comparisons with other, later SX-28A receiver panels, it does appear to be a slightly different color, however more examples of later, post-war SX-28As have to be examined to determine if this color was used after the end of WWII. Usually, the late-1945 to 1946 SX-28A front panels are sort of a gunmetal-type blue-black. The color difference between the WWII panels and post-WWII panels is subtle and only noticeable when compared directly with other panels.

When a poor condition panel is encountered and it is necessary to repaint, a good condition original panel is always helpful for comparison to available colors for matching. In most cases the back of the panel will provide the best condition source for the correct color match. Note that the nomenclature on the panel is NOT white but is correctly SILVER. Careful examination of a good original panel will show the fill color to be a matte finish silver paint. Over the years, the silver fill paint has discolored so that it looks like "dirty-smoky-white" but it is actually silver. Also, the red fill is not a bright red but should be a very dark shade of red.

A close-up showing the matte silver paint fill used for the nomenclature of an excellent condition original SX-28. A white card was placed in the photo as a reference. Note the red is a very dark shade.

DIALS: Never let excessive moisture get on the front of either of the dials or the meter scale. The ink used is somewhat water soluble and if aggressive wet-cleaning is attempted it will be the end of the dial nomenclature. Never use "Windex" or similar ammonia-based glass cleaners on the nomenclature side of the dial. These types of cleaners dissolve the ink almost immediately. If the dials are in good condition, use only a dry camel hair brush to dust off the surface - lightly. The back of the dials and the meter scale can be wet-cleaned and it will help the overall brightness when illuminated. If the dials are severely discolored due to smoke and dirt, their appearance may be improved using a "foaming" plastic cleaner. These plastic cleaners contain very little water and no ammonia. I have had some success using this type of cleaner applied with a cotton cloth (never use paper towels on plastic.) The cloth should be sprayed, not the dial. With the cloth slightly damp, apply the cleaner gently. This has to be performed with a light touch while watching the dial nomenclature for any thinning. Avoid the red BS indicators. You will only be able to remove some of the oxidation - don't be too aggressive or you will begin to remove the dial nomenclature. Careful cleaning will help "even-out" the discoloration due to dirt and smoke. This method will also help somewhat with the discoloration due to exposure to light. The photosensitive color change is actually into the plastic and it is impossible to return the dial to its original color. Most of the dials have darkened considerably over the years and aggressive cleaning will surely damage the dial nomenclature. It is unfortunate but that was the type of plastics that were around in 1940. The correct dial color is light yellow-amber but most are now a dark orange-amber. Since the dials were photosensitive, you probably will find that the part of the dial that was exposed to light through the dial glass will be much darker than the unexposed sections. Also, the main dial has red markers for setting the main dial for bandspread use. The early SX-28s had a problem with the red ink used that results in the indicators fading to the point of invisibility. Even the later formula red ink is super-sensitive to light, wet, rubbing - just about anything. Leave the red markers alone.

Two dials showing the typical darkening that occurs with exposure to light. On the left is an excellent original dial with virtually no darkening. On the right is an original with the typical darkening that takes place when exposed to moderate light levels over the years.

Modifications or Upgrades

The SX-28 is a great receiver - but it was designed in 1940 and, as such, its performance is dated when compared to a modern receiver's performance. In its day the SX-28 was the "top of the line" from Hallicrafters and, as a new SX-28 owner, you should want to experience what the original owners enjoyed about the receiver's performance - something accomplished with a good rebuild and alignment - not modifications. Granted that modern signals, like SSB, weren't part of that original owner's experience but the SX-28 is perfectly capable of receiving SSB signals without modifications. Of course the SX-28 drifts,...and you have to "ride" the RF Gain because you can't have the AVC on with the BFO,...and the dial accuracy is vague - all pre-WWII receivers have the same performance characteristics - it's part of the nostalgia that vintage equipment owners want to experience. Modifying a vintage classic receiver in an effort to "modernize" its design seems to go against the very idea of preserving and operating vintage equipment in the first place. Here is a list of some of the common mods you might encounter - and have to remove - in the SX-28 or SX-28A receivers.

Hallicrafters joined in on the "modification mania" of the fifties and issued an upgrade to the Lamb Noise Silencer in August 1951. Titled as "Service Bulletin from Hallicrafters - Bulletin 1951-30," it consists of four pages printed on two sheets of yellow paper. The first mod removed the 6H6 rectifier from the Lamb Noise Silencer and replaced it with a 6AL5 dual diode (but only one section was used.) The 6AL5 is supposed to be mounted on a bracket under the chassis close to the detector tube. The rectifier is reconfigured to operate in series rather than the original shunt arrangement. The bulletin continues that since you have removed the 6H6 ANL Rectifier tube, now you can use that tube socket to add a 6SL7 cathode follower for a panoramic adaptor or oscilloscope - however, the connections are to the last IF amplifier stage where the IF selectivity will severely limit the bandwidth of a panoramic adaptor display. Additionally, the mod has you use the other section of the 6SL7 as a 100KC crystal oscillator for calibration. The bulletin concludes with the statement that if the rework is beyond your capabilities, a Hallicrafters Service Center will perform the job and also perform a thorough "overhaul" of  your receiver - that must have really been something to witness. I have only seen one SX-28 that had this ANL mod installed and it is doubtful that the "modifier" was happy with the results. The Lamb Noise Silencer was a great circuit and it was the only pre-war tuned noise blanker used in any receiver. It was the only ANL circuit of the time that worked in CW and AM. As for the Panadaptor and Crystal Oscillator mods - they are of dubious value.

Over the past few years, several questions about the detector load and audio distortion while using AVC have been poised and this has resulted in a couple of mods being published to reduce "perceived" audio distortion. These are usually minor modifications involving resistor changes and improvements are certainly subjective. The SX-28's 1940 design selected the diode load resistor value as a balance between best sensitivity with the least audio distortion. As is the case with most amateur modifications, enhancing one aspect of performance (audio) will subtract from another (sensitivity.) Remember that when the Hallicrafters' engineers designed the SX-28 it was as a communications receiver first and to have great audio fidelity second. Be sure that the receiver you are judging has been completely rebuilt and aligned before making a hasty decision regarding its audio quality.

An ubiquitous mod is the 6AC7 sub for the 6AB7 in the 1st RF stage. This is a "plug-in" modification with no alignment issues and at least it does no harm to the receiver. The theory is that the 6AC7 has a greater transconductance figure than the 6AB7 and should therefore result in more sensitivity. Then why didn't Hallicrafters install the 6AC7 in the first place? It was possible that a 6AC7 RF stage could oscillate under certain conditions which weren't entirely predictable by Hallicrafters. Also, the receiver could be more susceptible to cross-modulation with the 6AC7 as the first RF amp. Also, one of the last engineering upgrades to the SX-28A was to change the 2nd RF amplifier tube to a 6AB7! Obviously, Hallicrafters' engineers believed the 6AB7 was giving the SX-28A maximum usable sensitivity. I have operated SX-28s with the 6AC7 1st RF amp and with the original 6AB7 for long periods of time and noticed very little difference. My H-119051 receiver is running the 6AB7 and I notice very little difference in sensitivity when compared to H-130170 which has the 6AC7 installed. My AN/GRR-2 receiver HA-2703 is also running the 6AB7 as the 1st RF amplifier with excellent sensitivity. All three receivers have been completely rebuilt and aligned. Many SX-28s and SX-28As encountered will already have the 6AC7 installed as it was a very popular mod that was probably installed after the receiver had aged several years and really would have benefited from a new set of tubes and a full alignment instead. On the subject of tubes, be sure that all of the tubes used in your SX-28 test "as new," especially the 6SA7, 6B8 and 6SK7 tubes. Overall performance suffers noticeably with marginal tubes.

SX-28 Performance Expectations

Using the SX-28 as a Vintage Communications Receiver:   Some hams are reluctant to use a pre-war receiver in actual "on-the-air" operations for fear that adjacent frequency QRM will limit their ability to successfully copy stations and that they will be unable to complete QSOs or Vintage Net operations. The SX-28 might have problems today coping with adjacent frequency interference from powerful SSB signals when the user is trying to copy an AM signal (and this will be the case for almost all vintage communications receivers.) Even narrowing the Selectivity or using the crystal filter doesn't seem to help much. However, most of the problem is caused by the user operating the receiver as a "broadcast radio" rather than as a "communications receiver." Most manuals will direct users to operate the receiver in the "standard AM mode," that is with AVC on, the RF Gain at maximum and the Audio Gain rather low. This allows the receiver's front end gain to be controlled by the AVC in response to whatever the strongest "signal" happens to be. This works fine when listening to strong broadcast signals. But many times the desired signals are weak and the stronger energy will be atmospheric RF noise. Often times it is adjacent frequency interference. The best solution is to switch off the AVC, increase the Audio Gain to near maximum and operate the receiver's front end gain manually with the RF Gain reduced to the minimum necessary to copy the desired AM signal. You will find that now the adjacent frequency interference is greatly reduced since its strength compared to the tuned signal is reduced. Also, the AVC-controlled gain is no longer responding to variations in what it "sees" as a signal - the "pulsating" and "popping" of adjacent frequency signals within the passband. Additional selectivity using either the Selectivity switch control and/or the crystal filter will also help to reduce the ratio of adjacent frequency interference to tuned signal. You will have to tune the desired signal "on the nose" for good copy. A couple of losses in this method of operation will be the S-meter function but it is really not very useful and only provides a relative measurement anyway. Also, broad audio fidelity cannot be enjoyed in this mode. But, the goal is successful copy and a completed AM QSO, in other words - communications. What about in actual use? I use only vintage receivers for operations on the ham bands and many times I'll use the AN/GRR-2 (SX-28A) as the station receiver. Using the "communications receiver" method of operation has allowed me successful copy of all stations checking into the vintage AM nets regardless of the adjacent frequency QRM. Weak signals can be a challenge but switching in the crystal filter usually provides the extra edge necessary for successful copy. Of course, it isn't enjoyable, "arm-chair" copy but it is a successful completion of the QSO or of the net operations, which is the goal. Unfortunately, deliberate on-frequency interference is another matter and no receiver, no matter how sophisticated the design, can cope with that type of intentional QRM.    

SX-28 vs SX-28A: Most Hallicrafters enthusiasts have various opinions about the merits of either receiver. A commonly heard statement is that the "...pre-war SX-28 is better than the SX-28A." In some regards this is true. Throughout production, Hallicrafters was cutting costs (normal business procedure) by using less expensive parts and construction. For example, the gear-driven bandspread on the early SX-28 (and AN/GRR-2) has a better feel and a better ratio than the dial-cord drive used on the late SX-28 and SX-28As. Other SX-28 cost cutters were the elimination of air trimmers in the front end and last IF transformer. On the down-side, the very early SX-28s have a slightly less effective version of the Lamb Noise Silencer and an Antenna Trim that only functions on the top four bands. Certainly the SX-28s seem to align better than SX-28As. The Hi-Q Micro-Set coils used in the SX-28A were another cost cutting and conservation measure requiring no brass parts in construction and a time savings on the alignment, maintenance and rework. Nowadays they are always a source of problems because of slugs that are broken, missing, stuck or so loose they won't hold alignment. Military SX-28s, which are most of the receivers made between 1942 and 1945, sometimes have significant wear in the tuning gears. Additionally, the military examples are usually fungicide treated which makes any rework (soldering) difficult. When it comes to performance though, the differences between a rebuilt SX-28 and a rebuilt SX-28A are subtle. You will probably find that the SX-28 will have a more accurate dial readout but both receivers have strong, bassy audio with good sensitivity and stability.

A fully rebuilt and aligned SX-28 or SX-28A receiver is a pleasure to operate. The audio is incredible on AM-BC stations and on strong AM-SW stations. No modifications to the original design are necessary for great performance on AM, CW or even SSB signals. Sensitivity, selectivity and stability are quite good considering that the design is over 60 years old. CW/SSB signals are no problem to copy but give the receiver time to "warm-up" if you plan on monitoring a SSB net. The receiver will drift until it has thermally stabilized which takes about 30 to 60 minutes. For informal listening and tuning around, a few minutes "warm-up" are all that is necessary. Since the SX-28 is using a standard envelope detector with BFO injection accomplished by a "gimmick", one has to "ride" the RF GAIN since the AVC cannot be used. This was standard for all communications receivers up to the time when product detectors came along in the mid-fifties. Typically, the RF GAIN will be set around "5" or less and the AF GAIN set to "5" or more. If an SSB signal sounds distorted, the RF GAIN is too high. Keeping the RF Gain low will provide the detector with the proper ratio of signal to BFO injection for good SSB demodulation. The CRYSTAL FILTER can be used for increased selectivity, for heterodyne rejection or for "peaking" a particular audio frequency when copying CW. The Lamb Noise Silencer is a "tuned noise blanker" that must be aligned to the 455kc IF to operate correctly. It is an excellent noise limiter when functioning and is the only noise limiter circuit from pre-WWII that actually works for CW. It is normal for the ANL to be in the 6 to 8 range before noise reduction is apparent. The top band covers 20-43MC and this is where most of the problems in tracking and lack of sensitivity or stability are encountered. The SX-28 performs no worse than any other 1940 receiver at those frequencies.

One should remember that the design of the SX-28 is over 65 years old but in its day, its performance was equal to or (in some cases) better than the competition's receivers. It can't compare to the performance of a modern ICOM, Yaesu or Kenwood. Sixty years of receiver design and electronic evolution have taken place since the SX-28 was new. It is fair, though, to compare the SX-28 to its 1940 contemporaries.

Competition Comparisons

The National HRO Senior was the top receiver from National from early 1935 up to the last tube-type variant of the mid-1950s, the HRO-60. The HRO Sr. is an excellent CW receiver with great sensitivity, low-front-end noise and incredible bandspread. The HRO Sr. also provides double pre-selection on all coil assemblies, so two RF amplifiers are used at all times. A separate power supply is required and the HRO lacks a powerful audio section utilizing only a single-ended 42 (or 2A5 on early models.) It has great a crystal filter for selectivity, which works wonders on either AM or CW, however few amateurs ever used it enough to appreciate it. No Noise Limiter was used in the HRO until the post-war HRO-5TA1 was introduced. No Antenna Trim is provided on the HRO so the 1st RF Amp stage on each coil has to be aligned to the station antenna used on that band. The Sperry Gyroscope based Micrometer dial is fabulous but provides no direct calibration leaving the operator to check the dial readout versus a graph. Resetability of the dial to a known logged frequency is precise, however the dial is not illuminated. The Plug-in coil assemblies were part of what gave the HRO its low-noise figure but they are a hassle to store when not in use and changing bands is a pain. Also, when changing coil assemblies from general coverage to bandspread, four small screws have to be removed and screwed in different holes. No doubt, National figured that the hams would leave the coils set for bandspread. The HRO was always promoted in QST, with every issue featuring photos of ham stations showing the HRO center-stage, perhaps giving a somewhat biased perspective of just how popular the HRO was. After all, it was expensive, required several accessories (like Power Supply, Coils, Speaker) and lacked easy versatility. On the positive side, the ham bandspread is the best that can be found on a vintage receiver and the sensitivity unbeatable. If you are interested in working CW-DX, the HRO Sr. is a fabulous choice.

The National HRO Senior from about 1940. There are several physical variations from early to later models and collector interest is mainly in the very early examples. With tremendous bandspread and low noise with high sensitivity, the HRO Sr. is a great CW receiver. For AM, it lacks a powerhouse audio output stage but, to borrow the CCA argument,..."it is a communications receiver."

The Hammarlund Super-Pro was an expensive receiver designed for professional use and by the time the company introduced the 200 Series, in October 1939, the price had been reduced and several features enhanced or added. If a ham could afford a National HRO with all the accessories, he could probably afford a Super-Pro. Like the HRO, the Super-Pro uses a separate power supply and speaker. A special design cam-operated band switch, special tuning and bandspread condensers and twenty lab-tuned coils on Isolantite bases make up the front-end of the receiver. Double preselection is used on all bands regardless of model or variation. The front-end of a Super-Pro is a magnificent construction effort and it is testament to its quality that 65 years later very few Super-Pros ever have problems with their front-ends. This superior quality effort resulted in dial readout accuracy of 0.5% and even today an aligned Super-Pro will still meet or exceed that spec. Another special design was the variable-coupled IF section that gave continuously adjustable bandwidth from 16kc down to 3kc. This is great for AM stations whether BC or ham. Additionally, a crystal filter gave heterodyne relief and increased selectivity. Finally, the audio section is powerful with a 6F6 driving a push-pull pair of 6F6s giving about 14 watts of high fidelity audio. Since there is no tone control on the 200 Series Super-Pro (other than cutting the highs by reducing bandwidth,) the better the speaker quality is, the better the audio will sound. So, with all of these great features, why is the Super-Pro seldom encountered on the AM ham bands today? First, the frequency coverage is somewhat limited. Only the SP-200SX covers all of the HF ham bands (1.25-40MC) but it is the rarest of the variants. The next best is the standard SP-200X, covering .54-20MC. One has to remember, in 1940, most hams were on 160M to 20M, so the SP-200X satisfied the majority of the ham's frequency requirements. The BC-779 (SP-200LX) has two LW bands, 100-200KC and 200-400KC and covers 2.5 -20MC on the top three bands. It is a more specialized communications receiver in that it doesn't tune the AMBC band or 160M. Next is the bandspread which is only calibrated in a 0-100 scale and only operates on the top three bands. The SX variation does provide bandspread on all five bands however due to the way this is accomplished some of the ham bands are covered with only about 50% of the bandspread range. The SP-200X version had the bandspread condenser designed so that all ham bands were covered with about 90% of the bandspread range. Finally, the Super-Pro can have a tendency to be noisy in the front-end when used with certain kinds of antennas. It does take a very long time to stabilize but, as a commercial/military receiver, it was designed to be left on continuously therefore eliminating the drifting issue. The Hammarlund Super-Pro was never very popular in ham shacks due to its high selling price but today its great audio and continuously variable IF bandwidth make it ideal for vintage AM operation.

The Hammarlund SP-200SX. This is an early version with an aluminum front panel and a toggle switch selecting speaker or phones. The SP-200SX is the only Super-Pro that covers 160M through 10M and has bandspread on all five bands. Frequency coverage is 1.25MC to 40 MC. With 18 tubes, continuously variable IF bandwidth, high fidelity audio, the SP-200SX is ideal for vintage AM.

The Hallicrafters SX-28 has a lot going for it when compared to these two "flagship" competitors. Bandswitching frequency coverage is greater than any of the Super-Pro variations and would require optional coils to equal with the HRO. The SX-28 has a built-in power supply - both the HRO and the Super-Pro use separate a power supply. Direct readout calibration on the main tuning dial and the bandspread dial is used on the SX-28 - the Super-Pro's bandspread dial is 0-100 and the HRO requires micrometer vs graph to determine frequency. Switch selectable IF bandwidth with Crystal Filter - although the Super-Pro's continuously variable IF bandwidth is better, the HRO has just the Crystal Filter. High Fidelity audio - so does the Super-Pro along with more audio power (14 watts vs the SX-28's 8 watts.) Tone control - although one can reduce high frequency audio by limiting the bandwidth on the Super-Pro, the HRO has no tone control. Antenna Trim control - neither the HRO or the Super-Pro provide antenna compensation requiring the 1st RF Amp on each coil assembly or band to be aligned to the station antenna for best performance. The SX-28 has a tuned ANL designed by James Lamb - the Super-Pro has a clipper type ANL but the HRO has no noise limiter until after WWII. Amplified AVC - so does the Super-Pro, the HRO has standard AVC. Now what about the SX-28 disadvantages. The SX-28 only uses double preselection on the top four bands, this leaves the AMBC and 1.6-3.0MC range only using one RF amp. Hallicrafters only used two RF amplifiers for the increased image rejection necessary on higher frequencies. Double preselection is used on all bands on both the Super-Pro and the HRO, perhaps unnecessary but AM BC performance on the Super Pro is phenomenal. SX-28's dial accuracy is pretty good if aligned correctly - but it doesn't compare to the Super-Pro's 0.5% dial accuracy, however there's no contest when compared to the HRO which requires graphs and micrometer readouts. Hallicrafters' speakers require 5000 ohm Z audio, which the SX-28 provides, but if you want to use some other speaker you would probably opt for the 500 ohm Z output with a matching transformer. The Super-Pro is usually 600 ohm Z and the HRO is 7000 ohm Z requiring an audio output transformer (usually mounted on the speaker frame in National speakers) so there's no real advantage to any of these receivers when trying to match a standard 8 ohm Z speaker.

Then there were the original selling prices. SX-28s sold for $159.50 without the speaker in 1940 (PM-23 was $12.) In September 1941, the price increased to $179.50 with the PM-23 speaker then selling for $15.00. Though the list prices were much higher, a Hammarlund Super-Pro 200 could be purchased from most dealers for $275.00 with power supply and speaker included. The basic HRO sold for around $195.00 which included four coils sets but did not include the power supply or speaker. These necessary accessories would push the total HRO package price up to about $225.00. These are generally pre-war prices and, after WWII, everyone raised their prices. The SX-28 was a bargain and provided the most features with the least amount of accessories and added expense. Without doubt, selling price was the primary factor in most ham's decision to purchase an SX-28 for their station receiver. Performance was certainly competitive with the HRO or the Super-Pro but it was clearly not superior. A low selling price for the high level of performance was the SX-28's major advantage.

Prices nowadays are skewed due to the popularity of all Hallicrafters and, in particular, the SX-28 and SX-28A. A good condition, complete, unrestored SX-28 will sell in the $250+ range. If it has been completely rebuilt, the price will be astounding, with excellent examples having sold for $1000+ in this condition. There are exceptions for exceptional receivers and some SX-28s have sky-rocketed over $2500 - but these are exceptions. The demand for the Super-Pro is difficult to predict as there are many variations that are more desirable than others. The early Super-Pros, like the SP-10 and SP-100s are very rare and consequently, very expensive. The SP-200 Series was produced in large quantities although many of the surviving examples are in poor condition. A great condition, complete SP-200SX, with 160M through 10M coverage, may sell for well over $500 (when's the last time you've seen the SP-200SX version for sale?) The SP-200X will depend on condition, whether it is civilian or military and the paint type used on the front panel. The SP-200X usually will not sell for much more than $250. Since the BC-779 (SP-200LX) is a specialized communications receiver with LW coverage it sometimes sells for less than the other model Super-Pros. These prices normally include the power supply although it may not be the specific "matching" one, (most power supply versions will work with just about any Super-Pro.) The National HRO is also dependent on the particular version with early examples (pre-1936) selling for very high prices due to collector interest. The typical 1939-1941 HRO Sr., in good condition with power supply and four coils will usually sell in the $400 range. There does seem to be a trend in all pre-WWII receivers selling for well above $250 and up to $450+ depending on condition and rarity. These prices assume that the buyer will be shopping on eBay and can't go to ham swap meets where the prices are about half of "eBay."

Continuing Research

With virtually nothing available as far as official company records are concerned, Hallicrafters enthusiasts have to depend on each other for observations, recollections and experience to rediscover the information that has been lost. Serial numbers are easy to find, easy to share and they do offer meaningful information when combined with detailed observations of the receiver that the serial number belongs to. Another very important document is the final inspection tag that came with each Hallicrafters receiver. These cards have sometimes survived and they have the serial number and the EXACT date that the receiver left the Hallicrafters' plant. This not only applies to SX-28s but to any of the other Hallicrafters' products. Since all products were serialized sequentially, a dated inspection card carrying a numerically close serial number can be compared to another product that doesn't have this information and a probable build date established for the latter. EBay is an excellent source for serial numbers and sometimes even inspection cards. Many sellers do take the time to carefully macro-photograph the tags or ID plate and this is invaluable information that is easy to retrieve. Fortunately, hams are always good at communications and by sharing this information we can preserve much of what has been lost to business expedience.

We are always updating this webpage for greatest accuracy. We depend on the information supplied to us by interested hams and Hallicrafters enthusiasts to form conclusions as to Hallicrafters' manufacturing process during the SX-28/SX-28A period. We are particularly looking for any information about the following:

1. Any SX-28 with a serial number higher than H-180,000 or any SX-28 with "HA" prefix serial number - we are interested in the type and color of front panel used, the type of condenser box cover installed, the type of tuning and bandspread knobs used and any other late changes noted.

2. Any SX-28A with a four digit serial number earlier than HA-2700 -  Indicates when SX-28A production started.

3. Any SX-28A with serial number later than HA-54,000 - verification as to how late the SX-28A was produced.

4. Any SX-28 with serial number earlier than H-116,000 - verification as to when production began.

5. Any SX-28 or SX-28A that is original and DOES NOT have the standard serial number plate installed on the rear of the chassis - verification that ALL SX-28s and SX-28As were serialized at Hallicrafters.

6. Any other interesting variations seen on original SX-28 or SX-28A receivers - hopefully will include serial number.

7. Any Hallicrafters inspection tag information - these are paper-wire tags that are attached by twisting the wire wrapper onto the power cord. These tags are dated and carry the assigned serial number. Additionally, there will be several inspector stamps or initials. Since all serial numbers were assigned sequentially, it doesn't have to be an SX-28 - ANY model Hallicrafters' tag provides important build-date information.

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REFERENCES

1. SX-28 and SX-28A Manuals, also VOLs. XII and XVI of Riders Perpetual Troubleshooting Manual have information and schematics on the SX-28 (1941 version) and the SX-28A

2. AN/GRR-2  Manual, Army # TM-11-874, military SX-28A, this manual has much more detail than the civilian counterparts including a resistance chart for troubleshooting and detailed circuit descriptions not in other manuals

3. QST - July,1940 to mid-1946, ARRL Handbook 1946 - QSTs have ads that date certain changes and verify introduction date while the 1946 ARRL Handbook has detailed info on the SX-28A

4. Electric Radio - Article by Bill Kleronomos, KD0HG, June 1990 - general information and some incredible measurements of the SX-28 performance specifications. Article by N6PY, Bill Feldmann, Part 1, Feb 2005 and Part 2, Mar 2005 - part 1 details many early changes in SX-28 construction and design with circuit analysis while part 2 details modifications to modernize the SX-28 performance (this part should be for reference only - modifications to the SX-28 are unnecessary.)

5. Phil's Old Radios website has a detailed procedure (with photos) on restoring an SX-28 and specifically on rebuilding the SX-28 RF box. It is an invaluable reference on proper disassembly, capacitor installation and reassembly of this difficult phase of restoration. Click here to go to Phil's Old Radios-SX-28 Restoration

6. Peter W. Dahl Co. will build a new audio output transformer for the SX-28 with the correct impedances and proper physical size. Click here to go to Peter W. Dahl Co.    NOTE: There has been a lot of e-mail, postings and other sources of information on the web that suggest that the Peter Dahl Company is going out of business as of  January, 2008. As of 2/20/2008, the Peter Dahl website is still up and appears the same as usual (although the on-line catalog doesn't work.) This is such a wide spread rumor that it is probably true and, if so, another great source of components to keep vintage equipment operational will have been lost.

 Copyright © Henry Rogers Feb.2005

New info added:  June 2005, July 2005, Feb 2006, Nov 2006, Mar 2007, Feb 2008 

Western Historic Radio Museum

Vintage Radio Equipment and Memorabilia

1910 through the 1950s

P.O. Box 73, Virginia City, Nevada 89440

owners/operators: Henry (W7HTR) and Sharon (KK7EI) Rogers

copyright: March 2005/2008