Radio Boulevard
Western Historic Radio Museum


the
hallicrafters inc.

 
SX-28

"a pre-war masterpiece"

 

(includes SX-28, SX-28A, AN/GRR-2 & R-45/ARR-7)

PART 2 - Engineering & Production Changes, Restoration Hints & Suggestions,
Performance Expectations, Competition Comparisons

by: Henry Rogers - WA7YBS/WHRM

 

PART 2

 

 

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.

 

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-125569 - earliest SN with medium black panel H-124417 (12/40) - With these kinds of changes, expect some overlapping in later or earlier serial numbers with gray or black panels due to intermixed stock. 1152 serial numbers separate latest gray panel with earliest black panel. Since SNs were not exclusive it's hard to say just how many SX-28 receivers were built until all of the gray panels were used up. So, expect either color panel to have been used 12/40 thru 1/41.

Front panel texture from 1940 up to late-1943 will have shallow wrinkles or convolutions. These wrinkles were stamped into the metal panel during manufacture.

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-120692 (11/40)

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

1st IF Amplifier Cathode Resistor R-16 is 300 ohms - single resistor with no switching.

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-124389 and earliest SN seen with black panel, H-124417 - Only 26 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

1st IF Amplifier Cathode Resistance - Early in production, a switch wafer is added to allow a parallel combination to be switched into the cathode of the 1st IF Amplifier. 600 ohms is parallel with 250 ohms on Bands 1 and 2. 250 ohms on Bands 3 thru 6. Probably to increase the gain slightly on Bands 1 and 2 to compensate for the single RF Amplifier on these bands. Probably installed by late 1940 to early 1941 but first shown on second version schematic (1941.)

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

1st IF Amplifier Cathode resistance changed to a series combination switched in on Bands 3, 4 and 5. Cathode R of 250 ohms unchanged in Bands 1, 2 and 6. Probably to compensate for gain changes due to single RF amplifier on Bands 1 and 2 and high frequency gain roll-off on Band 6.

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-2105, HA-2126) - this cover is mounted to the condenser box with screws (for comparison, H-172381 has the earlier "screened" condenser box cover)

Front panel texture changed to deeper convolutions, painted "bluish-gray-black" (seen on SX-28s SN H-172381, H-178848, H-181715, HA-2105, HA-2126)

Knobs for MAIN TUNING and BANDSPREAD changed to "webbed" style (SN H-181958, HA-2105, HA-2126)

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

1944 Engineering and Production Changes

SX-28 production stopped (~ 2-44 to 3-44) to introduce SX-28A into production

SX-28A production starts (~4-44) - earliest encountered SX-28A (AN/GRR-2), SN HA-2278, MFP date code April 26, 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

1st IF Amplifier Cathode Resistance - values changed from 1800 ohms to 1200 ohms and 1200 ohms to 1000 ohms. These values are in series with 270 ohm (changed from 250 ohms) Cathode resistor on Bands 3, 4 and 5. Possibly changed to conform with JAN values.

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-25171 ~09/45) HA-31195 rpt'd with inspection tag dated Oct. 8, 1945

1946 Engineering and Production Changes

Selling price listed as $223.00

2nd RF Amplifier tube changed to 6AB7 (seen on HA-25171, HA-53212)

 

SX-28 and SX-28A Variations Seen or Reported

SX-28 Navy Versions:  The rack mount, standard Navy version is from late 1943. It 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.  There was also the RBY-1, a Navy piece of equipment that used an SX-28 combined with a Panadaptor with 3" CRT. The SX-28 has an SO-239 installed on the back apron of the chassis to connect the Panadaptor. The version I saw also had a buffer circuit assembly on the antenna input to allow multiple receivers to use a single antenna. The SX-28 receiver has a military tag where the "SX-28" ID is on the front panel and it identifies the receiver as CHL-46195. The panel is black wrinkle paint - not the standard panel. This receiver did have the standard Hallicrafters serial number tag installed on the rear of the chassis with the number H-181853, indicating late 1943 production. The RBY-1 was an earlier, military version of the S-35. The introduction of the AN/GRR-2 in February 1944 became the Navy SX-28A. The "AN" designates "Army-Navy" so the AN/GRR-2 was used by both U.S.Army Signal Corps and U.S.Navy.

The FCC Variations:  The typical FCC receivers 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 first production run SX-28s were first used by the FCC's RID (Radio Intelligence Division) to listen in or find enemy spy rings in the US. These SX-28s were installed in automobiles with a directional loop antenna on top of the roof. By having roving DF abilities, the RID could triangulate a signal and determine its location. Later versions 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 usually replaced with a single-ended 6V6 audio output. The unused 6V6 tube socket mounting hole has a push-in metal plug installed. (Single 6V6 info thanks to K4ERR.) There are variations of the FCC SX-28 that may have been modified by the FCC to accomplish diversity reception or other special requirements. The FCC diversity set-ups appear to use two SX-28 receivers, one with the S-meter removed and the other with the bandspread removed. This observation is based on a photograph that appeared in CQ magazine in the 1940s. Interestingly, most FCC receivers appear to be from the first production run and will have SNs beginning with H-115xxx. George Sterling, who was head of the FCC during pre-WWII and WWII, wrote a detailed report on the RID's function and successes that included many photographs of banks of SX-28s that were in use by the FCC. Although other receivers were used, it appears the RID was almost exclusively using SX-28s for their intercept and monitoring.

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.

SX-28U and SX-28AU: Usually 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 handle toggles were normally used.

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

Custom Front Panel Colors:  Supposedly, white panels were needed aboard hospital ships however one has to ask - why? Silver panels turn up from time to time but these seem to be panels with the original paint stripped off. "Original" seems to have a different definition to some but claiming a panel finish as "original" doesn't mean it came from the factory that way. Custom finishes were sometimes required in certain installations, sometimes it was personal preference but almost always the end user was responsible for this operation. Though the custom panel finish may be "original" to the end user, it is not "from the factory original." Would Hallicrafters "custom finish" an SX-28 "on request?" It would require procuring special paint that most likely wouldn't have been done for just one receiver order. Besides, the panels were made by Crowe Manufacturing for Hallicrafters and Crowe would have to do the special paint job - unlikely. It's possible that Hallicrafters might have repainted a panel for a special request but it seems like it would have been a difficult to accomplish operation since Hallicrafters production building was essentially as an assembly operation using purchased parts.

White Tuning and Band Spread Dials - I recently received an e-mail photo of a restored SX-28 that had been fitted with modern reproduction "white dials." This is the only SX-28 I've ever seen with white dials installed and they were, of course, repro dials. Were white dials ever installed on the SX-28? On standard production receivers the answer is no. IF any WWII-vintage white dials are authentically installed on a WWII vintage SX-28, they are an anomaly that could probably be attributed to the end-user installing non-OEM parts. Who would have made these dials? The Signal Corps? An unlikely scenario that would then account for the absence of any confirmed vintage examples.

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-28A with a panoramic adaptor 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.

 

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. Real SnPb solder must be used for quality work. 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.

I usually don't re-stuff the paper-wax capacitors in SX-28s or 28As because there are plenty of original examples of these receivers around so preserving the original appearance of the under side of the chassis is not as important as it is with other, more scarce equipment. However, a purist would go ahead and restuff the paper caps to preserve to original under-chassis appearance. If resistors need to be replaced, the purist would use vintage originals that haven't drifted in value. Any other defective components would be replaced with good condition original parts. This makes a restoration difficult and very time consuming but the end results are stunning. As time goes on, and more and more SX-28s are restored with new parts, it will become more important to preserve the authentic appearance of the underneath of the SX-28 chassis. So, why not start now?

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 hocomplish 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, this is a guide - not all SX-28s are exactly like the one described in that procedure. 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. Photo right of 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 dismounting 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. Photo right shows 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: KDWC, Charles Cusick of Va. City, Nevada

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photo left:  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.

photo right:  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.

28a2rf.jpg (16500 bytes)

GEARBOX:  The early SX-28 gearbox will have two separate gear trains that operate the main tuning and the bandspread dials. Later SX-28s and all SX-28A receivers only have a complete gear train on the main tuning and have a dial pulley and cord that operates the bandspread with a minimal number of other parts. The AN/GRR-2 uses a gearbox similar to the early SX-28. In most cases the gearboxes are robust in construction and rarely does anything actually break inside. However, they are victims of too much grease applied in the wrong places and sometimes sticking anti-backlash gears caused by an accumulation of dirt. In most cases, poor operation can be corrected with a thorough cleaning. After removing the gearbox as a unit, flush all of the gears with a constant stream of WD-40 from a spray can. Use the WD-40 liberally (it's cheap) and use a small long handle paint brush to brush off the grease and dirt. This will result in very clean anti-backlash gears that operate correctly. Fortunately, the main shaft and bandspread shaft bearings can be accessed without total disassembly of the gearbox. The main tuning and bandspread shaft bearings that have the greatest influence on the operational feel of the tuning. These bearings have around 13 ball bearings of .0937" diameter on each end of the shaft with the drive gear centrally mounted. On the back of the gearbox are located the tuning shaft weights which need to be removed. Note that the main tuning shaft weight not only has set screws but also is threaded onto the end of the shaft. This is to allow precise setting of the "slip clutch" which prevents damaging the couplers by over driving at the tuning condenser end stops. At the rear of the shaft bearings is a lock nut and the threaded bearing housing. These can be removed and this allows access to the ball bearings and the housing. When the rear bearing cup is removed the shaft can be moved slightly to the rear to allow access to the front bearing for cleaning. It's a good idea to use a small clamp on the the split-gear that engages the main shaft gear to assure that you don't loose the anti-backlash preload. Clean and repack both bearings with wheel bearing grease. Unfortunately, the old "stringy" yellow wheel bearing grease that was used originally is difficult to find (unless you want to import it from Egypt.) Sometimes it can be found listed as "Sodium-based Grease." >>>

>>>  If you can't find the "stringy" grease then use the thickest wheel bearing grease you can find for repacking and then re-fit the threaded bearing cap. Be sure to use the same number of ball bearings in each cup that were removed. Sometimes this total varies by one ball bearing but what was installed is the correct number. Adjust the bearing end thrust until you just begin to not feel any end-play. Replace the tuning shaft weight and the tuning knob and try the operation and feel. Adjust the bearing further if the operation is too loose or too tight. Adjust the main tuning shaft weight so the center shaft operates the gear train but, if the gears are held with your fingers, the shaft slips. If the gearbox is the old style then proceed to the bandspread side which is more or less identical to the main tuning side. The anti-backlash gears all run on a single ball bearing which can be lubed with light grease applied with a syringe or other applicator that can get into tight quarters. The same applies to the ball bearings on the dial drive gears. I have deliberately avoided total disassembly of the gearbox in this description because of the difficultly of reassembly. There are eight anti-backlash gears (early boxes) that have to be held in the "loaded" position with small clamps while the gearbox plates are put back together. This is a difficult operation (yes, I've done it - twice) and should only be attempted if absolutely necessary - like replacing a broken gear or similar operation. Casual disassembly of the gearbox is not recommended. However, if you do need to perform a total disassembly and reassembly the gearbox, Doug Moore KB9TMY has written a thorough article on the rebuilding of the SX-28 Gearbox. It is currently available at Phil's Old Radios' website. Here's a link: SX-28 Gearbox Rebuild

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 manual references the color of the wires for placement of the load but unfortunately most SX-28 wiring has darken considerably and it is difficult to tell where the load is placed except by referencing the schematic. 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. After about 10 seconds 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. Try to find a functional original but in the meantime, the Peter W. Dahl Co. , now a subsidiary of Harbach Electronics LLC, does make a correct replacement audio transformer for the SX-28 (Peter Dahl-Harbach website listed below in references.) 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: Check the Internet for current availability and sources. Harbach has sold some of the transformer business in 2013.

COSMETIC RESTORATION

If ever there is a subject that will produce endless controversy, it will be "what is correct" or "what was original" for the paint color, nomenclature, dials, etc.,...and this is for just about any object that is in need of cosmetic restoration. Of course, I advocate that, whenever possible, only cleaning of the original finishes should be performed. This preserves the originality for future reference. However, sometimes the cosmetics are in such terrible condition a restoration is required for an acceptable appearance. What is presented here is a guide for doing an authentic cosmetic restoration of the SX-28 that conforms to how the receiver appeared when it originally left the Hallicrafters' plant. 

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. Sometimes, if the dirt and grime is stubborn, I'll use a soft brass brush (like a suede brush) to work the dirt out of the convolutions. Don't use excessive brush pressure but the original paint is very tough and can stand up to this aggressive cleaning without any problems. 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. After touch-up, apply either a coat of Armor-All or Boiled Linseed Oil (not both) to the cabinet. It will look like new. Be sure to wipe off the excess of either product. >>>

>>>  If you have cabinet paint that is beyond saving, there are automotive paint shops that specialize in various types of powder coating that can sometimes give good results (but often they don't.) Sometimes their paint jobs can look fairly close to wrinkle finish (at least at a distance.) All of the powder coated SX-28 cabinets I've seen have been painted black, which is incorrect. The one gray wrinkle powder coat job I saw was on a PM-23 speaker cabinet and the color was more blue than it was gray leading me to believe the painter was either careless or color-blind. Definitely have the powder coater provide some samples of his work before having him paint your cabinet. They can do good work but often their schedules have them hurrying through jobs that should require attention to details.

USE ALL OF THE SCREWS - 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 "dark" gray-blue on very early SX-28s. This same color was used on many Hallicrafters' cabinets, like the S-20R. 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 a couple of SX-28s have turned-up with the bluish-gray-black paint on front panels with the light texture (SNs H-161034 and H-173684.) 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 really 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: I cannot stress the following point enough when restoring SX-28/28A front panels. The nomenclature on the panel is NOT white but is correctly MATTE SILVER. Careful examination of a good condition, clean, original panel will show that the fill color is definitely matte-silver. Over the years, the silver fill paint will have discolored with smoke and other "grunge" so that it might look "smoky-white" but - look closely - it is actually silver. Many "hamster-restorers" just don't care whether the panel is correct - as long as it's new paint. The front panels were never jet black with white nomenclature. For authentic restoration of the SX-28/28A the front panel color should be determined by inspection of the back side of the panel and the paint color matched accordingly. Nomenclature is matte-silver with the exception of "Super SKYRIDER" which is filled with dark red (not fire engine red) paint.

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photo above:  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 (although most monitors have a difficult time reproducing silver colors.) Note the red is a very dark shade.

SX-28 Dials - Cleaning, Dial Color Controversy and the White Dial Myth



DIAL CLEANING: 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 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.

 

THE DIAL CHANGING COLOR CONTROVERSY AND THE "WHITE DIAL" MYTH:  Shown to the right are some sample dials. The uppermost photo shows an excellent original dial with no darkening. This is about as close as can be found today of the original dial color, which was "light yellow." The next dial down shows how most dials look today, substantially darker due to exposure to light and some deposits from tobacco smoke and other contaminates. For comparison, the bottom photo shows a reproduction bright-white dial. Although there are some restorers that believe that white plastic was original to the SX-28, this is an erroneous assumption based on the "White Dial" myth.

There are two common versions of the "White Dial" myth. First is the "changing color argument" - that is, the dials "started out" white and darkened to yellow or amber over the years of exposure to light. To debunk this argument one has to only look at various contemporary receivers of the SX-28 that actually had white dials for comparison. For example, the Hammarlund Super-Pro used off-white dials, also the Breting 14 used off-white dials. Neither of these radios are ever found with yellow dials - only off-white, actually kind of a cream color. Then there are the vast numbers of entertainment/consumer radios of the thirties that used white or cream color dials. If anything these dials darken either red or in extreme cases brown. The Collins kilocycle dials from the late-forties found on the 32V series of transmitters are good examples of white dials that turn red with exposure to bright light. Certainly, there is overwhelming evidence showing that the plastic material used for radio dials definitely darkens over time, especially with exposure to bright light. However, the light yellow plastic material turns to a dark orange/amber while the white plastic material turns reddish-brown to dark brown (in severe cases.)

The next "White Dial" myth is that Hallicrafters supplied white dials on some SX-28s during WWII. This is highly unlikely and so far all evidence produced to support the use of "white dials" has been hearsay. If genuine WWII-vintage white dials for the SX-28 existed they were an anomaly that could have been the result of OEM parts shortages causing the military to use non-OEM parts during an echelon rebuild of an SX-28. An unlikely scenario. Who would have made the white non-OEM dials? The Signal Corps? The known evidence - an apparent absence of any authentic examples - leads one to conclude that the use of WWII vintage white dials on an SX-28 is a myth.
 

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The Original Dial Color - Light Yellow

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Moderate darkening of the dial


This photo shows a non-authentic reproduction dial. The production SX-28 never used white dials, despite what you may read elsewhere.

 

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 adapter or oscilloscope - however, the connections are to the last IF amplifier stage where the IF selectivity will severely limit the bandwidth of a panoramic adapter display (use the link below to read about the correct method of connecting a Panadapter to the SX-28.) 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. I have owned one SX-28 that had just the 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 also available in the RME-69LS-1 receiver.) It was the only ANL circuit of the time that worked in CW and AM. As for the Panadapter and Crystal Oscillator mods - they are of dubious value.

>>>  NOTE: I recently (2008) was shown a photo of an SX-28A HA-53212 that had the Hallicrafters' mods installed by a Hallicrafters dealer. The front panel indicators for the mod installation by Hallicrafters are the addition of a toggle switch installed next to the BASS switch and another toggle switch installed next to the SEND-RECEIVE switch. There isn't any nomenclature for the switch functions but their installation appears to match the rest of the receiver's appearance.

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.
 

 

Competition Comparisons

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 material 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 mis-adjustment 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.

What to Expect from the SX-28/28A Today - 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 the minimal 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 CRYSTAL FILTER is actually quite effective in the AM mode also. You have to adjust the PHASING for minimum bandwidth and then tune the AM signal "on the nose." It's really very effective for reducing adjacent frequency interference.

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 control 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.

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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.

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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 triode-connected 6F6 driving a push-pull pair of triode-connected 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 commercial-military 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 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 better than 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. The SX-28 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.

Today's SX-28 Selling Prices - Prices nowadays are skewed due to the popularity of all Hallicrafters and, in particular, the SX-28 and SX-28A. A GOOD condition and COMPLETE but UNRESTORED SX-28 will usually 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. All of these exorbitant prices are found on eBay sales. Ham swap meet prices are at least half of eBay and often much less than that. However, incomplete condition and "hamster restorations" are common with swap meet equipment. Quality, professional-level restorations seldom sell at ham swap meets so that type of receiver is going to be found on eBay.

The Super Pro - 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 much 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.) Any Super Pro without its power supply and cable is "incomplete" and should be priced as a "parts set."

The National HRO - is also dependent on the particular version with early examples (pre-1936) selling for very high prices due to collector interest. First production run versions are rare (< 100 units) and are seldom for sale with "trading" usually required for acquisition. The typical 1936-1941 HRO Sr., in good condition with power supply and four original (matching SN) coils will usually sell in the $400 range. As with the Super Pro, if the HRO is without its power supply or coils, it's a "parts set" and should be priced as such.

There does seem to be a trend for all good condition and complete pre-WWII receivers selling well above $250 and often up to $450+ depending on demand 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 (or less) of "eBay."

 

Using the SX-28 or SX-28A as a Vintage Communications Receiver

Some hams are reluctant to use a pre-war receiver in actual "on-the-air" operations  fearing 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. However, the SX-28 (and almost all other high-end, vintage communication receivers) included a couple of "QRM fighting" devices that seem to be rarely used by AM ham operators.

The easiest device to use is the SX-28's own passband selectivity that is controlled by the Selectivity control. When there's a lot of band activity, you almost always have to switch to SHARP IF. All-to-often, this isn't enough for very near-frequency QRM,...especially if you're in the AM mode and the QRM is from a SSB signal. However, while in SHARP IF, try tuning "off frequency" a couple of kilocycles from your frequency of operation. You can usually greatly reduce interference and still recover enough audio for decent copy of the AM signal. Usually two or three kc, either above or below the operating frequency, will drop the QRM enough for good copy. The SSB operators will always have filters in their rigs to limit their bandwidth to about 2.1kc. A typical AM signal has about 6kc of bandwidth. Also, the typical AM envelope has the audio information in both sidebands. >>>

>>>  By tuning above or below your operating frequency, you will be able to receive one AM sideband and usually "drop" the offending SSB signal out of the receiver's passband. This is the method I use most often to "dodge" SSB QRM and it really works quite well on almost any vintage receiver.

Depending on the type of QRM, for instance, adjacent signals on both sides of the operating frequency, it might be more advantageous to reduce the SX-28 bandwidth even more and bring in the Crystal Filter. Using the Crystal Filter Selectivity and the Phasing Control you can narrow the bandwidth down to less than half a kilocycle. It's amazing how narrow the receiver bandwidth can be and still provide decent copy of an AM signal. However, it's very important that you tune the desired AM signal "on the nose" for good copy. Of course the audio will sound "muffled" but it will still be intelligible. Of course the audio fidelity suffers a lot in this mode. But, the goal is successful copy and a completed AM QSO or net operations,...in other words - communications

 

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. We have added (as of 8/08) an SX-28/28A Serial Number Log. This allows viewing of all SX-28/28A serial numbers that have been sent in thus providing information that can be useful for dating or identification. If you have sent in a serial number in the past and it is not listed, please send it in again and I will make sure that it is added to the log. 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 (or AN/GRR-2) with a four digit serial number earlier than HA-2500 -  Indicates when SX-28A production started.

3. Any SX-28A with serial number later than HA-53,000 - verification of latest SX-28A produced (latest sn reported HA-53445)

4. Any SX-28 with serial number earlier than H-115,000 - verification as to when production began (earliest reported sn H-115251)

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 - this is verification that ALL SX-28s, SX-28As and ALL various military versions were serialized at Hallicrafters. - This is important because we are estimating total production based on the assumption that all receivers had Hallicrafters' serial numbers assigned.

6. Any AN/GRR-2 receivers that were MFP date stamped at any other time period than May, 1944 - confirmation of one production run.

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

8. 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.

Send e-mail information to: WHRM - SX28 INFO

 

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, KDHG, 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 supply 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.   Peter Dahl company is now a subsidiary of Harbach Electronics LLC. Harbach bought the Peter Dahl name along with the transformer designs and design equipment. The orders placed with Harbach Electronics are then forwarded to MAGCAP Engineering for building of the transformer. The completed transformer is shipped to the customer from MAGCAP. NOTE: This information is somewhat dated. Check the Internet for current availability as Harbach has sold some of the transformer business in 2013.

7. Thanks to the literally hundreds of collectors that have sent in their SX-28/28A serial numbers, photos, comments and observations. Also, to those ham user/enthusiasts that have provided information by "on the air" conversations.

Copyright Henry Rogers Feb.2005

New info added:  June 2005, July 2005, Feb 2006, Nov 2006, Mar 2007, Feb 2008, Jul 2008, Aug 2008, Apr 2009, June 2009, Dec 2009, Re-edited layout Apr 2010,

Re-edited layout: Dec 2011

June 2013 Harbach info,  May 2014 minor corrections,  Feb 2017 added photos of SX-28 chassis and AN/GRR-2 chassis,

 

SX-28 PART 1                                                     Return to Home Index

 

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