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' 'Super-Pro' Receiver

Brief History - It is difficult to find any documented historical facts when it comes to the reason why Hallicrafters offered their version of the Hammarlund SP-600 to the military and why they would have gone to such an obvious production effort for such a small contract. Here is what is generally surmised by most Hallicrafters collectors and military radio enthusiasts. Sometime in 1951, the U.S. Army Signal Corps wanted to assure itself that if it became necessary to stop purchasing Hammarlund's SP-600 Super Pro receivers they would have another source of a similar design, high-performance receiver. General specifications were provided for any companies interested in submitting their samples. Hallicrafters is the only company that was accepted and actually produced a significant quantity of receivers for the Signal Corps.   >>>

>>> The Hallicrafters receiver was given the same designation as earlier Hammarlund SP-600 receivers, R-274. To differentiate the models, Hallicrafters' R-274 was given the "D" suffix while Hammarlund R-274s used the A, B and C suffixes. The contract was "3464 PHILA 52" and apparently only that one contract accounts for all of the R-274D examples that are found today. The typical production quantity of a military contact was for around 5000 receivers, however, the quantity of surviving examples indicates that perhaps a smaller total number of receivers were actually built for the contract. Hallicrafters may have expected more contracts but it seems that only one contract accounts 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.

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.

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. This problem shows up as severe backlash. Frequent lubrication of the tuning condenser bearings will alleviate the problem for awhile but as the TM says, "lubricate frequently."

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.

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 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 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.

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.

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.)

Restoration Comparisons

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.

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

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. Additionally, an oddball spring-coupled bell-crank is used to couple the gear box to the tuning condenser. 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.")

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 dial lamps.

Dial Accuracy - The dial accuracy is vague which is typical of 1950s design limitations. 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.

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