Western Historic Radio Museum
 

Hallicrafters' 'Super Pro' Receiver

R-274D/FRR Receiver - aka: SX-73

History - Circuit Design - Performance Evaluation
 

Hallicrafters  vs  Hammarlund  

Comparison of Features -

Which 'Super Pro' is Best?  -  Yes, you can vote!

 

by: Henry Rogers WHRM

Hallicrafters built a Super Pro receiver? Well, not exactly. But, the R-274D/FRR has so many similarities to the Hammarlund SP-600 that it is often referred to as "Hallicrafters' Version of the SP-600." So why did Hallicrafters produce a receiver like the R-274D and then later market it as a high-end commercial-amateur receiver dubbed the SX-73? This web-article looks at the history of the R-274D along with its circuitry and construction. The section "Hammarlund versus Hallicrafters" shows the few similarities and many differences between the R-274D and the SP-600 and allows the reader to decide which company built the best version. I thought it would be interesting to see if any readers-owners would want to submit votes as to their opinion of which Super-Pro is best, so I've added an e-mail link for voting. I'll keep the voting tally up-to-date and it should be fun to see what the results will be. - H. Rogers August 2011

Hallicrafters' 'Super-Pro' Receiver

The Hallicrafters Co. -  R-274D/FRR receiver.

Brief History - It is difficult to find any documented historical facts when it comes to the exact reason why Hallicrafters produced a version of the Hammarlund SP-600 for the military and why they would have gone to such an obvious production effort for what appears to be such a small contract. The following is what is generally surmised by most Hallicrafters and Hammarlund collectors and military radio enthusiasts,...

Sometime in 1949, the U.S. Army Signal Corps wanted a manufacturing source of a high-quality receiver that incorporated certain design requirements needed for reliable data reception. The Signal Corps had been modifying WWII Hammarlund Super Pro receivers to have a three-channel crystal oscillator for frequency stability (R-270/FRR.) In fact, the Signal Corps also had a kit, MC-531, that was installed into some of their SP-200 Super Pro receivers that provided the three channel crystal oscillator for improved frequency stability. Certainly, the Signal Corps was looking for this particular feature on their new receivers. General specifications were provided for any companies interested in submitting their samples. Hallicrafters and Hammarlund were the only companies that were accepted and actually produced a significant quantity of receivers for the Signal Corps.   >>>

>>>  Apparently, both companies built some receivers identified as R-274/FRR but this was soon changed. To differentiate the models, Hallicrafters' R-274 was given the "D" suffix while Hammarlund R-274s used the A, B and C suffixes. The Signal Corps apparently favored the Hammarlund SP-600 and over the next decade literally tens of thousands of SP-600s were produced for the military. Hallicrafters' version was produced in probably two contracts in the early 1950s. The typical production quantity of a military contact was for around 5000 receivers, however, the quantity of surviving Hallicrafters examples indicates that perhaps a smaller total number of receivers were actually built for these contracts. Hallicrafters may have expected more contracts but it seems that only one or two contracts account for all R-274D/FRR receivers. How many were used by the Signal Corps is difficult to establish but few (if any) are ever seen in vintage military photographs.

Since the contract only accounted for a relatively small quantity and Hallicrafters had obviously done some production line tooling and had set up suppliers for the required parts, it was not going to be profitable if only the one contract accounted for the expenditures. Like other manufacturers that provided the military with radio equipment, e.g. Hammarlund and their SP-600, Hallicrafters decided to offer the R-274D to the commercial and amateur market. To avoid any confusion as to the intended customers, these receivers were given a new designation of SX-73.

The SX-73

At $975, the sales of SX-73 receivers must have been incredibly slow. A cabinet is mentioned in the Hallicrafters' ads but usually the artwork shows the SX-73 as a rack mount receiver. The R-274 (TM11-897) military manual mentions a CY-699/FRR metal case for table use but it is unlikely that this was what Hallicrafters offered (see below for photos of Hallicrafters' CY-1345/GR cabinet for the R-274D.) There was at least one advertisement artwork that showed an SX-73 that was repackaged to look like a civilian Hallicrafters' product with large dial and meter bezels along with stylized grab handles and cabinet. Whether this version was actually produced is unknown.

Today, the SX-73 version is rarely encountered, however the military version R-274D/FRR does show up from time to time. These receivers have a dedicated following of enthusiasts who actively use their receivers. Many feel that the R-274D was one of Hallicrafters' best efforts in receiver design and performance. From serious SWLs to hams using the R-274D for communications in Vintage Ham Stations, both are users who want a receiver that features impressive sensitivity and great audio reproduction along with a Spartan-like military appearance.

 

 
The Circuit Details - The R-274D is a 19 tube, double conversion, superheterodyne communications receiver that tunes from .54 to 54 mc in six tuning ranges that are selected by a rotating turret-type band switching design. The receiver is single conversion with double preselection below 7.0 mc and double conversion with double preselection above 7.0 mc. The first conversion intermediate frequency is 6.000 mc and the second IF is at 455 kc. Three stages of IF amplification are used along with an IF output stage. A conventional envelope detector is used and two stages of AF amplification are employed. The audio output is to a triple secondary output transformer that provides two 150 ohm Z windings that can be connected in series for 600 ohm Z output or just one winding can be used if 150 ohms Z is desired. The third winding is 50 ohms Z for earphone operation and a 2.2K ohm resistor is in series with the 'phone output to allow either Hi-Z or Lo-Z earphones to be used without changing the 600 ohm output level.

A six-channel crystal oscillator circuit is provided to allow the user the option of six crystal-controlled LO frequencies for better stability. When a channel is selected, the crystal oscillator circuit is capacitive-coupled to the standard LO circuit. The crystal oscillator circuit "swamps" the LO and provides a steady oscillator signal. The crystals installed must take into account where the conversion frequency is in relation to the desired frequency as either 455 kc or 6.000 mc difference will be employed in the calculations. Higher frequencies will require operation on crystal harmonics. Additionally, the receiver must be tuned to the correct frequency for the RF stages and the Mixer to be also in tune. The crystal oscillator allowed for increased frequency stability required for RTTY and other data modes of reception. The six crystals mount in sockets located under a fiber board cover next to V5, the Crystal Oscillator tube.

A six-position selectivity switch provides three bandwidths that are based on IF transformer characteristics and three bandwidths that are determined with a Crystal Filter. Unlike many designs that place the Crystal Filter as part of the First IF amplifier, the R-274 places the Crystal Filter between the Second and Third IF amplifiers. An IF output is provided to drive RTTY converters that operate at 455 kc input. An Audio Input is provided to allow capacitive-coupled access to the Audio Gain control and 1st AF amplifier grid input.

The power supply utilizes a pi-filter with 40uf filter capacitors. A ballast is used in series with a dedicated 12vac power transformer winding that applies a steady heater voltage to the LO and the Mixer to improve receiver stability. There is a series load resistor in the heater supply to the Detector and the First AF Amplifier to reduce hum level. A pair of 120vac 7W lamps are connected in parallel as load resistors for the 0C3 voltage regulator.


photo above
: The top of the R-274D chassis
 

 
R-274D Performance Today - Since the Hallicrafters R-274D was built as a possible replacement for the Hammarlund SP-600, one should expect similar performance from the respective receivers. What follows are my observations of the R-274D operation based on actually using the receiver for two-way communications on the ham bands.

The R-274D is a sensitive receiver and will respond to just about any signal on any of its tuning ranges. For the past few months, I've been using the R-274D receiver on a Vintage Military Radio Net that meets every Sunday morning on 75M. Mode of operation is AM. The R-274D will easily allow copy of all stations in the Net and can cope with any QRN with the ANL provided. As for the R-274D's ability to cope with QRM, the crystal filter has been used on a couple of occasions but, normally, the SHARP non-crystal filter setting provides a narrow-enough bandwidth for QRM relief. The crystal filter operation is very good and works like one would expect, this is, narrows the bandwidth and provides some heterodyne relief. I've found the R-274D sensitivity and selectivity to be at the top of the performance expected from a 1950s vintage receiver.

Most complaints about 1950s vintage receivers, with the obvious exception of those built by Collins, is vague dial accuracy. The R-274D is no exception to this complaint, even though the dial accuracy specification is 0.2%. That type of accuracy was about the best that could be achieved using variable capacitance tuning with wide frequency coverage in each tuning range. Where the R-274D and most other military receivers compensate for the vague dial readout is by providing a very accurate logging scale. The R-274D's logging scale has tremendous resolution and therefore the resetability is very accurate. I use the logging scale for setting Net frequency and it always is quite accurate.

Like all military receivers and most amateur communications receivers of the 1950s, the R-274D uses a standard envelope detector for both AM and CW. To copy SSB signals requires reduction of the RF Gain to a level where the BFO injection has the proper ratio of signal to injection. This is standard operating procedure for all receivers not using a product detector. Unlike most military receivers, the R-274D has pretty good audio response. The use of .01uf coupling capacitors in the audio section allows for some bass response provided a matching transformer is used with a good quality, fairly large speaker. The audio response is still basically "communications grade" but it does have some bass response and sounds better than many other 1950s receivers.

Remote standby is standard in its configuration making the R-274D easy to set up in the ham station environment.

I've also listened to SW BC stations, usually on the 19M and 16M bands since I mostly listen in the afternoons. Sensitivity is up with the best of the 1950s designs for SWLing.

As for problems I've experienced, the only complaints I have are with the main tuning gear box and the tuning condenser bearings. As for the gear box, when tuning across the band at a fairly fast rate of speed the main tuning dial erratically "jitters" but this is not too noticeable if the tuning is done very slowly. This problem is caused by wear in the concentric tuning shaft and main dial drive gear. When the gear box was new, this probably wasn't a problem but correcting for the wear seems a difficult problem to solve nowadays.

The military TM shows that grease should be coated on the split-gears but after years of a grease-coating, some of the grease worked its way between the split-gears and caused them to stick together. This shows up as a backlash problem. I had to remove the gear box and thoroughly clean it with a WD-40 flush and that cleared up the backlash.

There are chronic problems with the bearings on the two "coupled" tuning condensers. The military TM recommends that the tuning condenser bearings be lubricated often with grease. These bearings seem to be unable to retain any lubrication and are frequently "dry." This causes the tuning condenser to "stick" while tuning the receiver. This then extends the bell-crank coupler springs until the tuning condenser is pulled by the force of the springs to "snap" to a new position. If the condenser doesn't "snap" to a new position then when the tuning knob is released, the receiver will "reverse tune" actually turning the dial backwards through the gear box. Frequent lubrication of the tuning condenser bearings will alleviate the problem for awhile but as the TM says, "lubricate frequently." 

As a further note, in August, 2012, I finally had enough of this "sticking" tuning condenser problem and decided to see if it was repairable. This required total removal of both condensers which can be accomplished per the TM manual without too much difficulty. Upon removal I found that both condensers were barely moveable - virtually stuck - and the bearings felt like the balls had several flat spots. The ball bearings seemed to have some grease inside but still the condenser rotor had a particularly "rough" feel to its rotation and tended to "grab and stick" in many places. Adding some light-weight oil to the bearings did seem to loosen the rotation a bit but still the actual rotational feel was rough. I found that only the lightest oil would penetrate the bearing seal so I ended up having to drill small holes in the seal to allow a spray tube to be inserted into the bearing. With this I could then flush out the bearing and then repack with a light-weight grease that could be forced into the ball bearing through the drilled holes. Now, the bearing isn't truly sealed anymore but it wasn't very well sealed to begin with and, because of that, moisture got into the bearings causing corrosion that resulted in the sticking. More on this "fix" in the section "R-274D Tuning Condenser Bearing Fix" below.

 


Hallicrafters' CY-1345/GR Cabinet for the R-274D Receiver
 

K6GLH owned this beautiful R-274D receiver installed in the Hallicrafters' CY-1345/GR cabinet. Note the close-up photograph of the data plate on top of the cabinet indicating that the CY-1345/GR is definitely a Hallicrafters' product and is specifically for the Signal Corps. Though the TM mentions CY-699/FRR as a cabinet, these photos show that the CY-1345/GR was also an option. Also shown in the rear view are the various inputs and outputs available on the rear panel of the receiver.             


photos by Gary Halverson, K6GLH
 

Hallicrafters versus Hammarlund

 

R-274D - Similarities to the SP-600

Hallicrafters built the R-274D to be a viable replacement for the Hammarlund SP-600, not to be an exact copy of that receiver. Consequently, there are some similarities that would be found in any receiver that had to meet certain pre-design specifications, both electronic and mechanical.

1. Both receivers employ a rotating turret band switching assembly that places ANT, RF, MIXER and OSC modules into proper engagement pins that connect the modules to the tuning condenser.

2. Both receivers have a selectable Crystal Oscillator that parallels the standard LO to provide increased stability for data modes of reception. The Hammarlund "X" option is an self-contained unit that mounts on stand-offs and is not used in all versions of the SP-600. Hallicrafters' Crystal Oscillator is chassis mounted and is found on all R-274D/SX-73 receivers. In both receivers, the main tuning has to at set to the intended frequency to assure that the RF and Mixer stages are tuned for the selected crystal channel frequency.

3. The six-position Selectivity switch is similar in both receivers in that three crystal filter positions are provided along with three non-crystal filter positions. Hammarlund specifies the bandwidth in frequency while the Hallicrafters uses general terms like "BROAD," "MEDIUM," "SHARP," etc.

4. The 600 Z ohm, balanced audio output transformer is similar to that used in the SP-600. Impedances are slightly different but the overall operation is the same.

5. An IF output is provided on both receivers. Both receivers were intended to be able to drive IF input type devices for RTTY and other data modes of reception.

6. Both receivers have adjustable BFO injection on the rear panel.

7. During alignment, all adjustments are accessible from the top of the receiver.

8. Both receivers are the same overall physical size. If the ballast tube is counted as a "tube" in the R-274D, then both receivers use 20 tubes.

Differences between the R-274D and the SP-600

There are certainly more differences between the receivers than similarities. Here are some of the more important differences.

1. The R-274D Bandswitch Turret is made up of modules that are built on fiber board chassis. The SP-600 uses ceramic chassis. The fixed contacts are very delicate in the R-274D.

2. The tuning condenser is not shielded in the R-274D unless the top cover is installed. The SP-600 tuning unit is entirely shielded.

3. The R-274D first conversion frequency is 6.000mc. The SP-600 uses 3.955mc for a first conversion frequency.

4. The R-274D switchover to double conversion occurs at 7.0mc. The SP-600 is at 7.4mc. This is interesting in that Hallicrafters obviously deliberately chose 7.0mc to allow double conversion to start at the 40 meter ham band while Hammarlund deliberately chose to place the switchover at 7.4mc, or above the 40 meter ham band.

5. The tuning ranges on the R-274D are set up to allow Range 2 to cover the 160 meter ham band, Range 3 to cover the 80 meter ham band, Range 4 to cover the 40 meter ham band and Range 5 to cover 20, 15 and 10 meters. This set up is much more convenient for ham operation in the 160, 80 and 40 meter ham bands. The SP-600 has Range 3 covering 80 meters at the low end and 40 meters at the high end. This requires spanning the entire Range 3 tuning to move from 80 meters to 40 meters.

6. The gear box of the R-274D uses a very simple approach with anti-backlash gears and an oddball spring-coupled bell-crank condenser coupling. The SP-600 uses a spring-loaded rim drive system that is very smooth. Problems with the R-274D gear box are roughness in the feel and erratic tuning dial movement. Problems with the SP-600 are spring fatigue resulting is dial drive slippage.

7.  Nearly all coupling and decoupling capacitors used in the R-274D are ceramic disk capacitors. The early SP-600s used molded tubular capacitors that developed chronic leakage current that resulted in damage to associated resistors and other circuit problems.

8. The R-274D doesn't have a dual scale meter and circuitry to provide either RF input or AF output measurements. The SP-600 has this capability.

Power Supply Differences

There are so many differences in the receiver power supply it has its own section...

1. The R-274D uses only a Pi-filter for the B+ filtering. The SP-600 uses a dual section filter (two chokes and three filter capacitors.)

2. The R-274D uses a ballast tube to regulate the LO and Mixer filaments for improved stability. The SP-600 does not use any filament regulation.

3. The R-274D uses two 120vac 7W lamps in parallel as series load resistors for the 0C3 voltage regulator. The SP-600 uses a standard resistor series load for the 0A2 voltage regulator.

4. The Detector tube and the First AF Amplifier in the R-274D have a series load resistor in the heater supply to reduce hum - not used on the SP-600.

 

Views on Top of the Chassis

Views of the top of the chassis of the SP-600 JX-21 (left) and the R-274D (right) show the receivers to be similar only in performance and specifications.

The SP-600 has the power supply mounted on the right side of the chassis, the tuner-bandswitching mounted in the center and the IF-DET-AF on the left side of the chassis. Note that the R-274D only utilizes one filter choke. The SP-600 uses two chokes (one is under the "X" option chassis.)

The R-274D has the tuner-bandswitching mounted on the right side of the chassis, the power supply mounted on the far left-rear of the left side and the IF-DET-AF on the right side of the left side chassis. The tuning capacitor is only shielded when the top cover is installed.

Views Under the Chassis

The SP-600 photo (left) shows that under the chassis is wired with the various components mounted at the sockets and components terminals with no use of component boards. The turret is enclosed in the large shielded box and the first conversion crystal oscillator is in the smaller shielded box. Note that all of the original molded capacitors have been replaced (SP-600-25C)

The R-274D photo (right) shows that under the chassis is wired with a combination of some components mounted at the sockets and terminals while most of the components are actually mounted on component boards and connected to the various circuit points via harnesses. While the turret is within its own compartment, full shielding is accomplished only when the bottom cover is installed.

SP-600 & R-274 Turrets


These two photographs show the differences in construction of the band switching turret in the receivers.

The SP-600 (left) uses ceramic bases for each RF module. Each module is secured to the turret frame using clips. The contact studs are stout and the fixed contacts they mate with are very flexible. It is rare to have problems with the SP-600 turret other than those caused by the molded capacitors used in early versions.

The R-274D (right) uses fiber board bases for each RF module. Each module is secured to the turret frame with screws. Note that the contact studs are very short and somewhat smaller than the SP-600 studs. The R-274 fixed contacts are much more delicate than the SP-600 contacts.


photo above: The oddball spring-coupled bell-crank used for coupling the gear box to the tuning condenser shaft. These springs extend whenever the tuning condenser rotor "sticks" due to insufficient lubrication (which occurs often.)

R-274 Tuning Apparatus Problems


photo above
: Close up of the gear box removed. Note that the main shaft to which the tuning knob is mounted drives the gear box and the logging dial while the main tuning dial is mounted to the hub of the dial drive gear. This design results in an erratic main dial movement as the gear box begins to show signs of wear.


photo above: The main and logging dials with gear box mounted. Note that the logging dial is recessed into the main dial when mounted. Though the logging dial usually runs smoothly, the main tuning dial's movement is erratic due to the small support of the hub versus the large size of the dial.

 

Restoration Comparisons

Coupling and Decoupling Capacitors - The rebuilding efforts necessary on early versions of the Hammarlund SP-600 are legend. Over 50 molded tubular capacitors must be replaced and that requires extensive disassembly of the receiver. The RF Platform alone has 20 molded capacitors that must be replaced and you can't access the RF Platform unless it is removed from the receiver. Additionally, it's common to find several burned resistors due to the excessive leakage current of the defective capacitors. No doubt, you wouldn't be operating an all-original early SP-600 without first doing extensive rework.

When the receivers were new, the major reliability problem with the molded capacitors used in the early Hammarlunds wasn't yet apparent. Therefore, our view from 50+ years later gives us the advantage of knowing that all early Hammarlund SP-600 receivers will be fraught with chronic problems because of the molded capacitors used in their construction. Certainly an important factor to consider since the rework involved is so tedious. In 1952, use of ceramic disk capacitors was something that most manufacturers hadn't moved to yet, even though the leakage current problems of paper-wax dielectric capacitors were well-known. The fact that Hallicrafters chose to use ceramic disks in the R-274D is admirable. The fact that Hammarlund changed over to ceramics a few years later does verify that the reliability problem with the molded capacitors was becoming rapidly obvious, even in the mid-to-late 1950s.

Probably one of the most important considerations in a comparison with the SP-600 is that the R-274D uses almost entirely ceramic disk capacitors which do not exhibit leakage current due to their construction. This eliminates the most time consuming and difficult part of the rebuilding process when electronically restoring the R-274D.

Carrier Level Meters - The Carrier Level meter on the R-274D is often found with an open coil and non-operational. This must have been a common problem as the Signal Corps had special replacement meters available for the R-274D. These replacement meters have a black scale with white nomenclature while the originals have the reverse. The SP-600 meter is often found with a moisture stained scale since it is essentially a paper scale mounted on a metal support scale. Additionally, SP-600 meters are often have severely worn bearings that will cause erratic needle behavior. A "sticking" needle is common but it is caused by worn bearings, not the needle rubbing against the scale or the inside of the glass.

IF Transformers - The IF transformers in the SP-600 are physically large and easy to access which is helpful since there is a tubular molded bypass capacitor located inside the can. The adjustments are accomplished using a standard small blade screwdriver. The R-274 uses what may be higher quality IF transformers and no bypass capacitor is located inside the can. The adjustments require a special tool to adjust the secondary of each transformer. The primary can be adjusted with a small blade screwdriver.

Front Panels - The front panel of the R-274D has silk-screened nomenclature which makes any repainting difficult. The nomenclature is very simple with the exception of the arrows for the dial lock operation. A silk-screen job probably wouldn't be too difficult or expensive, if a total repainting of the front panel was necessary. The front panel nomenclature on the SP-600 is engraved which makes the restoration of the panel relatively easy. Repaint the front panel with automotive-quality paint and then refill the engraving with light beige color Artist's Acrylic paint (pure white will look too bright.)

Cabinets - The R-274D is designed so the receiver can be operated without a cabinet either as a table receiver or as a rack mount receiver. Though there was a military cabinet available it is rarely encountered. The normal shielding for the chassis construction essentially becomes a useable cabinet. The SP-600 is rarely found with a cabinet either. The rack mount SP-600 originally had a bottom cover and a top dust cover but these are rarely found installed on any receivers nowadays. The rear pylons of each side plate of the SP-600 give it an odd look when not installed in a rack or a cabinet. Dust ingression is a potential problem for the SP-600 gear box and the chassis top. This would especially be a problem where the humidity is high. Both receivers are dimensionally deep and require special cabinets.

Tuning Gear Boxes and Tuning Condenser Bearings - If you have to work on the gear box, the R-274D gear box is very easy to remove and work on. The SP-600 is a major job to remove but it is so robustly built, it is seldom necessary to do anything other than a general clean and lube. The R-274D gear box is prone to have wear in the concentric shafts that drive the main tuning dial and the logging scale. This wear ends up causing an erratic "jittering" movement of the main tuning dial. Cleaning and lubrication will help smooth the operation. The SP-600 dial drive often will exhibit some slippage. This is almost always due to spring fatigue on the "S" spring that applies the force for the drive wheel against the logging dial rim. The R-274D tuning condenser has bearings that require constant lubrication, even the TM mentions this. If lubrication is not sufficient, the tuning condenser rotor will "stick" and then extend the spring couplers on the bell-crank which then forces the rotor to jump to a new position. Sometimes the tuning will actually "reverse tune" when the knob is released. Needless to say, this makes tuning in any signals problematic. R-274D Tuning Condenser Bearing Fix - The only way to fix the tuning condenser bearing problem is to remove both of the condensers from the receiver and modify the bearings for better lubrication. The problem comes from the larger condenser use of two sealed ball bearings to support the rotor and the smaller condenser use of a sealed ball bearing in the front and a thrust ball bearing in the rear. These bearings were manufactured using steel with no protection from corrosion and, consequently, if the receiver is in a humid environment for several decades, these bearings will rust. Apparently the rust is able to work its way inside the sealed bearing. Since the bearing seems to be incapable of remaining sealed, it leaks out its lubricant over time and then the rust sets in causing roughness in rotation and eventually grabbing and sticking. To fix this problem, first remove the two tuning condensers. This can be accomplished from the top of the chassis with only removing the top and rear panels. When the condensers are out of the receiver, drill two small 3/32" holes in the metal seal between all three of the ball bearings. This allows easy application of a spray tube (the little red pipette) from a can of WD-40. Flush out the bearing until the WD-40 runs out clean. Next, I use a fairly light-weight, modern grease and work this grease into each of the bearings through the two access holes. Stop working the grease into the bearing when you see it coming out around the shaft at the rear of the bearing. Now, pack some more grease to seal the holes and remount the bearing cover. This fix seems to have lasted for quite a long time and is about as close as you can come to installing new bearings, which by the way, is literally impossible without destroying the tuning condenser. Use the procedure shown in TM11-897 to remove and reinstall the tuning condensers.
 

Pro & Con Comparisons - Which Super Pro is Better?

SP-600 - Pros

Domination of the Ham Station Landscape - There's no doubt that the SP-600 will fill up the desk top if it's installed in an original cabinet. The receiver is huge. The knobs are massive. The twin viewing ports that surround the tuning dial and logging scale are reminiscent of ship's "port holes." The receiver does become a focal point due to its impressive appearance.

Super Smooth Tuning - The action of a brass wheel driving the brass rim of a large dial imparts a certain ultra-smooth feel to the tuning of the SP-600. This effect is magnified by the very large knobs used for tuning and band switching functions. Every user is impressed with the tuning "feel" of the SP-600.

Sensitivity - No doubt the SP-600 is a very sensitive receiver. It will respond to everything that can be received with band conditions being the only limitation.

Ease of Alignment - The entire alignment procedure can be accomplished with the receiver setting on the bench. No alignment adjustments are under the chassis. One caveat though is the engagement of the alignment tool through the holes in the RF Platform. This operation requires actually looking into the holes with a flashlight to assure the proper engagement of the alignment tool into the adjustment slot.

Front Panel Nomenclature - The nomenclature is engraved making any repaint of the front panel easy to accomplish.

High Production Levels - This means that since so many SP-600s were built over the years, replacement parts (or even entire "parts sets") are very easy to find which will shorten the total restoration time.

SP-600 - Cons

Molded Capacitors - The early SP-600s require extensive molded capacitor replacement before they can be operated safely. Burned resistors are common due to excessive leakage current. Later SP-600 receivers have ceramic disk capacitors installed that eliminated this problem. Molded caps were used from 1950 up to about 1956.

Communications Grade Audio - The SP-600 audio response is rolled off 3db at 125 hz, resulting in a very obvious lack of bass response. This was to assure that weak signal copy was possible and that CW and RTTY were not affected by an over-emphasis of bass response.

600 ohm Z Audio Output - A 600 ohm Z to 8 ohm Z matching transformer is generally required for matching the stock audio output to a standard 8 ohm Z speaker.

Fixed Antenna Input Impedance - The alignment of the Antenna Input stage usually will set the input impedance to around 100 ohms. There is no other compensation in the receiver for various antenna loads.

Crystal Filter - The SP-600 Crystal Filter is very subtle in its operation. Many operators find that it doesn't do much at all. It will narrow the bandwidth but it is not very effective at eliminating heterodynes.

Double Conversion & Tuning Ranges - The changeover to double conversion is set at 7.4mc, just above the 40M ham band. Tuning Range 3 has 80M at the low end and 40M at the high end requiring spanning the entire band to switch from 80M to 40M.

Dial Accuracy - All 1950s vintage receivers (not built by Collins) have vague dial accuracy. The accuracy is in the logging scale on the SP-600.

SP-600 JX-17 - This diversity version of the SP-600 does not have the Remote Standby option. All other versions have a standard Remote Standby function.

Cabinet - The stock Hammarlund cabinet is usually not found with the receivers as most receivers were rack mounted. Most rack mounted versions will be missing their dust cover and most of the time the bottom cover is also missing. The Hammarlund cabinet is relatively easy to find but it can also be fairly expensive.

 

R-274D - Pros

Unique Appearance - The R-274D always is an "attention getter" because of its very Spartan-like military appearance.

Sensitivity - The R-274D is very sensitive and will respond to any signals on the band. Limitations would be due to band conditions.

Audio Response - The R-274D uses .01uf coupling capacitors in the audio section that allows a better lower-end to the audio response. It's still communications grade audio though.

Double Conversion & Tuning Ranges - The changeover to double conversion is set to 7.0mc allowing double conversion to be used on 40M. The tuning ranges allow 160M, 80M and 40M to be covered on separate bands.

Antenna Trimmer - The R-274D has an Antenna Trimmer control to allow adjustment of the antenna input circuit impedance to compensate for various antenna loads.

Ceramic Disk Capacitors - The majority of coupling and decoupling capacitors used are ceramic disk capacitors which eliminates leakage current problems and eases the rebuilding process.

Ease of Alignment - All alignment adjustments are on top of the chassis. The exception to "ease of alignment" might be that the IF transformers requires a special tool for adjusting the secondary.

Remote Standby - This function is easy to access and use.

R-274D - Cons

First Conversion Frequency - 6.000mc is used for the first conversion frequency. It is possible that strong SW BC stations could "leak" into the IF of the receiver causing heterodynes or even dominating the first IF. Radio Havana, a particularly strong SW BC station in the Eastern USA, operates on 6.000mc. A 6.000mc wave trap could be installed ahead of the antenna input to reduce the unwanted signal level if this problem is encountered.

Gear Box and Tuning Condenser - The R-274D gear box is not very robust and is subject to a lot of wear in the concentric shafts that operate the tuning dial and the logging scale. The tuning condenser itself requires a lot of attention to proper lubrication if smooth tuning is to result (even the TM mentions that the tuning condenser must be lubricated "often.") If there is an "Achilles Heel" to the R-274D, it is the tuning condenser gear box wear and the tuning condenser bearings' problems that are caused by poor design and the inferior material used in their construction.

Dial Illumination - The dial illumination is borderline feeble. It's difficult to tell if the receiver is on in a well-lighted room since the only indication of "power on" is the illuminated dial.

Dial Accuracy - The dial accuracy is vague which is typical of 1950s design limitations of "variable C" tuning receivers. The Logging Scale has great resolution and allows accurate resetability.

600 ohm Z Audio Output - A 600 ohm Z to 8 ohm Z matching transformer is generally required for matching the stock audio output to a standard 8 ohm Z speaker.

Front Panel Nomenclature - The nomenclature is silk-screened making any repainting of the front panel difficult.

Knobs - The Hallicrafters knobs are very ease to chip around the edges. The good news is that they are stock Hallicrafters knobs and easy to find replacements for.

Cabinets - The CY-699/FRR cabinet is next to impossible to find. The Hallicrafters CY-1345/GR is also a rare item.

Low Production Level - The R-274D is not a common receiver. Therefore replacement parts can be difficult or impossible to find. Many parts are either standard components or stock Hallicrafters' parts but some items are in the "unobtainium" category. It's unlikely that any R-274D is going to be scraped for parts.

 
Current Selling Prices - The comparisons above don't address the current selling prices of the respective receivers - certainly an important consideration. Just looking at the asking prices ignores the fact that most equipment sells for somewhat less than the "asking price." However, all things being equal, "asking price" versus "selling price" will generally affect either type of receiver when it comes to the actual transaction. It appears that if you are looking specifically for the R-274D, you'll end up paying about twice what you would pay for a comparable condition SP-600. There will be exceptions to this, of course. But, be prepared to pay substantially money more for the R-274D receiver.
 
Decision Time - Hopefully I've provided enough information for you to make a decision as to which company built the best version of the "SP-600." Since Hallicrafters only built a couple of military contracts and the R-274D/SX73 production lasted from late-1951 up to early-1954 while Hammarlund built over 40 different versions of the SP-600 over a period lasting two decades, it seems that the military-commercial market favored Hammarlund.

Since the actual performance of the SP-600 and the R-274D are so similar, it is mostly the non-performance related issues that will ultimately be the deciding factor. These would include the tuning range set up that is advantageous in the R-274. Or the fabulous velvet-smooth tuning of the SP-600. The restoration work required for each receiver must be heavily weighed in the decision. Ultimately, it might come down to the actual selling prices today that will affect your decision. It's all subjective so, let's vote on it.

 

You can place your vote here - send me an e-mail that indicates which Super-Pro is the best, the Hammarlund SP-600 or the Hallicrafters R-274D aka: SX-73 and briefly why you chose your particular favorite. Please let me know in the e-mail if you own one or the other receiver or if you own (or have owned) examples of both. To vote, just "Click" on "Super Pro Vote" and send your choice:  SUPER PRO VOTE
 

VOTING RESULTS FOR THE BEST SUPER PRO

HAMMARLUND SP-600

HALLICRAFTERS R-274D/SX73

X - Easy to Find Parts For - WA7YBS

O - Tuning Condenser Bearing Problems - WA7YBS

O - NC

X - Better Operational Characteristics - VE7XF

X - Smooth tuning, Better Looking  - A.L.

O - NC

O - NC

X - More Reliable, Better Parts  -  KØMZ

X - NC

O  -  6.00MC Radio Havana leaks into IF of 274D - WA1HRJ

X - Docs and parts easy to find

X - Better design and better performance - KM1H

O - Ugly

X - Better performance, reliable

                              O - NC

X- Better construction and performance - W1FU

X - Sen., Tuning, Dial, Parts Av., Mech Design

O - Lack of sensitivity above 28 mc - P. Silitch

X - Tuning, Dial, Parts, Looks

O - Lack of parts, knobs, dial, - KB7VT

O - NC

X - Hands Down Winner - K8UV

 X - Better AGC, weak sig performance - K5LRX

X - Better Audio, appearance, disk caps - N3IBX

X - Reliability, excellent performance - K7EWE

O - NC

 X - Perf, Looks, Mech Design, owns four - W2VMC

O - Appearance, construction, has used - W2VMC

X = A positive vote                 O = A negative vote                 NC = No Comment

 

                          Come on,...we need more votes for the best Super Pro.

 +9 & -5 votes SP-600,...   +7 & -5 votes R-274D,...      SP-600 ahead by 2 - Jan 2017

Henry Rogers - Western Historic Radio Museum © 2011
 

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Website Navigation Index

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National Co. - HRO Receiver
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Hallicrafters SX-28
"A Pre-war Masterpiece"

Comprehensive History, Serial Number Analysis, Restoration Details & More

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   PR-10 Receiver & Pre-selector              

Comprehensive History, Los Angeles Radio Mfgs History, Circuit Details, Serial Numbers, Restoration Details & More

 RCA's Amazing AR-88 Receivers
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RCA's Legendary AR-60 Receiver
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Hammarlund Mfg.Co.,Inc
The Incredible Pre-War 'Super-Pro'
C
omprehensive History, Serial Number Analysis, Restoration Details. Includes info on the Hammarlund Comet Pro

Hallicrafters DD-1 "Skyrider Diversity"
Comprehensive History, Serial Numbers & Restoration Details

Hallicrafters' "Super-Pro" R-274 Receiver
Comparison of the SP-600 to the R-274(SX-73) in detail, best features of each. VOTE for your favorite Super Pro

 

-  Rebuilding Communications Equipment  ~  Full Length Articles with Photos -

Rebuilding the R-390A Receiver
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Rebuilding the ART-13 Transmitter
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Rebuilding the Hammarlund SP-600
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NEW!       T-368 Military Transmitter                    
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Rebuilding and Operating the AN/GRC-19
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 Detailed Information with Lots of Photos

Successfully Operating the BC-375 on the Ham Bands Today
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Rebuilding the Collins 51J Series Receivers
Detailed Restoration Information with Lots of Photos - Includes R-388 Receiver

Rebuilding the BC-348 Receiver
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Building an Authentic 1937 Ham Station
Utah Radio Products - UAT-1 Transmitter

 

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Radio Boulevard
Western Historic Radio Museum

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