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Hallicrafters'  Super Pro  Receiver

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

History - Circuit Design - Performance - Problems

Hallicrafters  vs  Hammarlund,...Super Pros, that is,...  

Comparison of Features on Both Receivers

Which Super Pro is Best?  -  Yes! You can vote!

by: Henry Rogers WA7YBS-WHRM

Hallicrafters' Super-Pro Receiver - R-274/FRR

In the late-1940s, the U.S. Army Signal Corps was searching for a manufacturing source for a high-quality receiver that incorporated certain design requirements needed for reliable data (RTTY) reception. Immediately after WWII ended, the Signal Corps started working on increasing the accuracy of  RTTY communications by modifying BC-342 receivers, adding voltage regulation and other improvements to the stability of that receiver. By 1947, the Signal Corps was modifying WWII Hammarlund Super Pro receivers (SP-200 Series) to have a three-channel crystal oscillator for increased frequency stability along with a crystal-controlled BFO with the receivers working in diversity to further reduce both signal and frequency drift for reliable RTTY copy. These receivers were all Hammarlund BC-794 receivers that were professionally modified by Wickes Engineering Company and designated as the R-270/FRR to be used with CV-31 Diversity RTTY Converters. Additionally, the Signal Corps also had a modification kit, designated as the MC-531, that could be installed into any of their SP-200 Super Pro receivers that provided the three-channel crystal oscillator for improved frequency stability.

Besides frequency stability, the Signal Corps was looking for  several other modernization requirements in their new receiver design. For example, turret band switching was another feature that was required because this allowed for relatively simple repair of the receiver's front-end since each RF coil assembly was easy to remove and replace (or repair.)

Double conversion above around 7.0mc was going to be necessary since much of the post-war communications were now using much higher frequencies than during WWII. Due to the higher frequencies being used, these new receivers had to have a tuning range from that started at the bottom of the AM BC band, .54mc, and went up to the VHF region at 54.0mc. Audio output impedance had to be 600Z ohms. It's likely that the Signal Corps had done most of the initial design work on what they thought their new receiver circuits should include and how it should be built. Whether the Signal Corps had a complete receiver design prepared and prototyped or whether the Signal Corps' receiver consisted of a list of specifications is unknown.   >>>

photo left: Hallicrafters CV31D Diversity RTTY Converter from 1951 used with R-270/FRR receivers

The SP-600, The R-274, The Contacts and Later Production

Generally, it's believed that Hammarlund's SP-600 was introduced in 1950 but that may have been an "available to the public" announcement. Hammarlund was often slow to release official announcements. As an example, fifteen years earlier, Hammarlund had produced the SPA receiver, a receiver identical to the SP-10 Super Pro, on a Signal Corps contract in June 1935. However, they didn't officially announce the "new" Super Pro SP-10 receiver to the public until nine months later, in March 1936. It's probable that Hammarlund was supplying some SP-600-type receivers to the Signal Corps for testing and evaluation (and getting feedback for what was needed) long before the official announcement in 1950. However, there are no official contracts for SP-600 receivers prior to 1950 and no contract build dates earlier than September, 1951. Since Hammarlund was also closely working with the Signal Corps on better stability in their receivers, just how much of the SP-600 design was influenced by the Signal Corps is  unknown (other than the obvious.)

There appear to be two contracts for Hallicrafters R274 and R-274D receivers spanning 1949 up to 1952. On any of these contracts ordering and building could take place over several orders using the same contract number and fulfilling the order might take more than one production run and could take many months to finish (maybe even years.) 

Suffixes - When the Signal Corps and the USN began ordering the Hammarlund SP-600 version of the receiver, the R-274 designation had already been assigned to Hallicrafters' production. So, rather than assign an entirely different number to the Hammarlund SP-600, these receivers were given suffix letters of A (Signal Corps,) B (USN) and C (Signal Corps.) Even though the non-suffix designation had been assigned to Hallicrafters, their next new contract receivers would be designated R-274D/FRR. The next known Hallicrafters' contract for the R-274D was Order No. 3464-PHILA-52 from 1952.

The Hallicrafters R-274 "D" Version - Probably the biggest change in the new Hallicrafters R-274D was how the remote standby worked. The original R-274 remote standby worked in parallel with the front panel Send-Receive switch requiring a NC contact on the remote standby line for receive and a NO for standby. The NO on the remote line (with the panel Send-Receive switch in Send) would disconnect the screen voltage to the two RF amplifiers and the first conversion mixer stage. To disconnect the antenna from the receiver in the Send mode would have to be accomplished with an external T-R relay. The R-274D changed the remote standby to an antenna input protection circuit. Remote standby required that 12 volts DC be connected to the rear terminal strip to operate an antenna relay that was inside the receiver. The relay shorts the antenna input to ground when in the Send position (12vdc applied to remote terminals.) Also, the receiver antenna coil selected becomes an open circuit to protect it from any RF radiation energy. A neon bulb was also added to protect the receiver antenna input circuit from static discharges. An external T/R relay would still be required. Appearance-wise, some R-274D receivers will have gray-color dial escutcheons.  >>>

Judging by the scant quantity of Hallicrafters R-274 receivers encountered today, the contract quantity was probably fairly low, maybe 1000 receivers per contract. Hallicrafters may have expected more contracts from Signal Corps but it seems that only a couple of contracts account for all R-274/FRR and R-274D/FRR receivers. How popular the R-274 receivers were with the Signal Corps is difficult to establish but few (if any) are ever seen in vintage military photographs.

Since the contracts only accounted for a relatively small quantity of receivers and Hallicrafters had obviously done substantial production line tooling and had set up suppliers for the required parts, it was not going to be profitable if only a couple of contracts resulted from these 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 Hallicrafters 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. Note the B&W artwork in the header photo and see that the data plate was changed to "SX-73" and the "CAUTION" tag deleted. It's entirely possible that the SX-73 receivers that were sold were surplus builds of the R-274D that Hallicrafters had produced in anticipation of further orders from the Army that didn't materialize. A SX-73 cabinet is mentioned in the Hallicrafters' ads but usually the artwork shows the SX-73 as a rack mount receiver. 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. It seems unlikely that Hallicrafters would have dumped even more money into a design revamp that would have probably ended up with the SX-73 being even more expensive.

Today, the SX-73 version is rarely encountered, however the military versions R-274 and R-274D do show up from time to time. These receivers have a dedicated following of enthusiasts who actively use their receivers. From serious SWLs to hams using the R-274 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.

R-274 and SP-600 versus Collins for Signal Corps RTTY Use - Radio Teletype (RTTY) was the initial reason for the Signal Corps' interest in a stable, crystal-controlled receiver that was capable of dependable data reception. By 1950, Collins Radio Company had started to supply the Signal Corps with 51J receivers that had a frequency readout that was accurate "to the kilocycle" and frequency stability that was essentially drift-free. The next year, the Collins R-390 was released and it was also accurate and drift-free (and expensive.) Ultimately, these Collins receivers were what the Signal Corps used for RTTY, installing both the 51J (R-388) and the R-390/R-390A in portable communication huts specifically for RTTY communications. The fate of the Hallicrafters R-274 was no more contracts after 1952. The fate of the Hammarlund SP-600 was greatly reduced numbers produced after the mid-to-late-1950s. Although Hallicrafters tried to market the R-274D as the SX-73, very few were actually sold. It was the end of the design for Hallicrafters and they really never produced a similar receiver afterwards. Hallicrafters must have lost a substantial amount of money with the R-274/SX73 production. However, most enthusiasts maintain that the R-274 was Hallicrafters' best-effort in receiver performance and design (acknowledging the Signal Corps influence, of course.)

NOTE: Throughout the remainder of this article, the Hallicrafters' receiver designation R-274 is used almost exclusively to avoid repetitious listing of both designations. Just remember that the earliest version was the R-274 receiver and the later version was designated as the R-274D receiver. Also, Hammarlund R-274A, B or C receivers are referred to as "SP-600" for the same reason.

R-274 Circuit Details

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.

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.
 

R-274 Tubes Used and Their Functions 1st RF Amp - 6AG5,        2nd RF Amp - 6BA6,       1st Mixer - 6BE6,       VFO - 6C4, Xtal Osc - 6AG5,         6mc IF Amp - 6BA6,        2nd Mixer - 6BE6,   6.455mc Xtal Osc. - 6BA6,         1st IF Amp - 6BA6,    2nd IF Amp- 6BA6,        3rd IF Amp - 6BA6,        455kc IF Output - 6BA6 Detector - 6AL5,       1st Audio Amp - 6AT6,        AVC/Noise Limiter - 6AL5   BFO - 6BA6,         Audio Output - 6Y6 Voltage Regulator - 0C3,        Rectifier - 5U4G,      Ballast Regulator for tube heaters on 1st Mixer & VFO  -   Amperite 6-4

Rework - This CY-699/FRR cabinet was complete and in fairly good condition although it did have a few minor problems. The partial rear cover was reverse mounted. There were a few bends that needed straightening. All of the pieces needed cleaning. All four bumpers were missing and several screws, washers and nuts were also missing. I disassembled the cabinet into individual pieces. Each piece was cleaned with Glass Plus and 0000 steel wool. Used with lots of Glass Plus and using a "light touch" the 0000 steel wool does a good job of cleaning heavy dirt and contaminates without damaging the paint. I installed all new hardware for reassembly. A search through the "used rubber foot box" turned up four matching rubber bumpers (rubber feet.) Reassembly completed the rework and cleaning process on this cabinet.
 

The photo to the right shows R-274 SN: 762 installed in the CY-699/FRR cabinet.

Estimating the Build-date for a R-274 Receiver

R-274 Performance Today

The R-274 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-274 receiver on a Vintage Military Radio Net that meets every Sunday morning on 75M. Mode of operation is AM. The R-274 will easily allow copy of all stations in the Net and can cope with any QRN with the ANL provided. As for the R-274'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-274 sensitivity and selectivity to be at the top of the performance expected from an early-1950s vintage receiver.

A common complaint about 1950s vintage receivers, with the obvious exception of those built by Collins, is vague dial accuracy due to the limited resolution of an analog readout that covers wide frequency ranges within each band. The R-274 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-274 and most other military receivers compensate for the vague dial readout is by providing a very accurate logging scale. The R-274'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. Heterodyne frequency meters were always employed by military and commercial users when an accurate receive frequency setup was necessary.

Like all military receivers and most amateur communications receivers of the 1950s, the R-274 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 BFO level. This is standard operating procedure for all receivers not using a product detector. Even though the receiver has a BFO Injection Level control on the rear panel, the maximum injection level is not high enough for SSB demodulation (it wasn't designed for that anyway.) You'll still have to "ride the RF Gain control."

Unlike most military receivers, the R-274 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 notable bass response and sounds better than many other 1950s receivers. Of course, I'm using a floor-mount, bass-reflex speaker with matching transformer for the best bass response and over-all great sound (especially on strong SW-BC stations playing music.) NOTE: I've switched to a table speaker and still have nice audio response.

Remote standby is standard in its configuration that parallels the SEND-RECEIVE panel switch making the R-274 easy to set up in the ham station environment.  NOTE: This easy setup is only true for the R-274/FRR. The later R-274D is similar to the Collins R-388 in that it requires a 12vdc input on the remote terminals to actuate an internal relay that grounds the antenna input while disconnecting the receiver antenna coils.

I've also listened to SW BC stations, usually on the 19M and 16M bands, since I mostly listen in the afternoons. I'm able to easily copy the Maritime Coastal Beacons from China XSG and XSQ operating on several different frequencies from 12.6mc up to 16.8mc. Also, Marine Beacon TAH in Turkey is easy copy in the late afternoons on 8.4mc. These maritime beacons "chirp" digital information consisting of four 1 second bursts and then send their ID in CW. Trenton Military Aviation Weather for Canada on 15.035mc is easy copy out of Trenton, Ontario. The R-274 sensitivity is up with the best of the 1950s designs for SWLing. Audio on some SW BC stations is impressive on the R-274 - it isn't "high fidelity" but for a military receiver it sounds pretty good. The Chinese SW-BC have particularly nice sounding audio. I have no idea what they're saying but it sounds like high-quality AM audio. On the other hand, Radio Havana can put in a very, very strong signal here but their audio is pretty rough-sounding and their announcers all seem to have irritating voices.

R-274 Problems Encountered

The military TM shows that grease should be coated on the split-gears but, after years of a grease-coating, some of the grease may work its way between the split-gears and cause 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. Only light oil should be used on the split-gears, 10W machine oil is fine. 

Tuning Condensers - Bearings -  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 may be found to be "dry" which can cause 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." You might find that the ball bearings have rusted due to poor storage conditions. The bearing corrosion can be removed but it does require removal of the tuning condensers from the chassis. There is a procedure for alleviating this condition in the "Restoration Comparisons" section further down this page. 
 

Turret Pins and Spring Contacts - This potential problem is relatively easy to fix. The rotating band switch turret contact pins work in conjunction with fixed-position, flexible spring contacts that are mounted under the chassis and under the turret. These spring contacts are made from fairly light-weight material. Sometimes the material seems to get fatigued and barely contacts some of the turret pins. Upon close inspection, I've found that maybe the spring contacts work fine on four of the band positions but bands five and six turret contact pins are barely touching or not touching at all. The fix is to first clean the contact pins on the turret. These can become highly oxidized and should be cleaned with De-Oxit and a small brass brush wiping off the excess with a paper towel. The spring contact cleaning will require the receiver to be upside-down on the bench. Rotate the turret to a position where no turret pins are contacting and you can see the spring contacts easily. Use a small paint brush and De-Oxit to clean these contacts. Test their ability to make contact by visual inspection while rotating the band switching turret and checking that each turret pin slightly depresses the spring contact. You might have to adjust one or two of the spring contacts very slightly. Use a small, right-angle bent dental pick to slightly adjust the spring contact upwards. Just a slight pressure is enough. Check the contact by rotating the turret to see if the spring contact now moves down as the turret pins slide by. There are seventeen contacts to check so this is somewhat time-consuming. Fortunately, usually only one or two spring contacts will need adjusting. Be careful, these contacts are delicate so go slowly and only make slight adjustments.

Dual Conversion Relay K1 Contacts - The 6.000mc conversion circuit uses a relay to make the connection changes from single conversion to dual conversion. Sometimes the contacts of the relay (K1) can become oxidized or just dirty which will have the receiver working okay on bands one thru three but not on bands four thru six. The relay contacts can be cleaned with De-Oxit sprayed on a small piece of heavy paper. This paper can be pulled thru the contacts in both NO and NC to clean. This will usually clear up any problems with K1. K1 is mounted near the power transformer under the chassis.

Cleaning the Turret Pins - These pins should be cleaned gently with a small brass brush and De-Oxit. Don't be too aggressive, you only want to remove dirt and oxidation not the plating. Be sure to only use a BRASS bristle brush. Polish with a paper towel to remove residual De-Oxit.

Getting the most from the Dial Lamps - The R-274 dial illumination is not dazzling. It's difficult to tell visually if the receiver is turned on in a brightly lighted room. To obtain the most from the two dial lamps requires some adjustment that can only be accomplished with the front dial escutcheon removed. Only four screws mount the escutcheon, so its removal is easy. Each dial lamp socket is mounted with a single screw that can be accessed after the escutcheon is dismounted. Check the bulbs that are installed. Many times, #47s are installed because they draw less current and therefore run cooler. The manual specifies the lamps should be type LM27 6v-8v @ 0.25A which describes a #44 bulb. Since we aren't running the receiver 24 hours a day-7 days a week (like the military did) we really don't have to worry about the slight increase in heat from using the specified #44. #44 lamps have a tendency of darkening the inside of the bulb as they are used and age, so, first install a new pair of #44 lamps. The easiest method to replace the dial lamps is to dismount the socket, install the bulb and then remount the socket. With full visual access to the dial, you can now see the lamp filaments and how their alignment affects the illumination of the dial. Most of the time one or the other of the socket mounts will need a slight bend to have the filament of the bulb exactly align with the dial and provide the greatest illumination. Originally, the lamps had a directional shield over the bulb to eliminate stray light and help direct the light towards the dial. If your R-274 has these shields adjust them for the proper direction of the illumination. Re-install the escutcheon. With the dial lamps aligned with the dial and with #44 lamps installed, your R-274's dial will be about as brightly illuminated as it gets.

Hallicrafters versus Hammarlund,...Super Pros, that is,...

R-274 - Similarities to the SP-600

Hallicrafters' R-274 receiver and Hammarlund's SP-600 receiver were designed to meet the requirements of the Signal Corps. Consequently, there are some specific similarities that would be found in any receiver that had to meet certain pre-design specifications, both electronic and mechanical.

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-274/D/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 kilocycles 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-274, then both receivers use 20 tubes.

Differences between the R-274 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-274 unless the top cover is installed. The SP-600 tuning unit is entirely shielded.

3. The R-274 first conversion oscillator frequency is 6.455mc (6.0mc conversion frequency.) The SP-600 uses 3.955mc for a first conversion oscillator frequency (3.5mc conversion frequency.)

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

5. The tuning ranges on the R-274 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-274 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-274 gear box are roughness in the feel and erratic tuning dial movement. Problems with the SP-600 are drive-wheel spring-load fatigue and dirt contamination resulting in dial drive slippage.

7.  Nearly all coupling and decoupling capacitors used in the R-274 are ceramic disk capacitors. The early SP-600s used molded tubular capacitors that over time developed chronic leakage current that resulted in damage to associated resistors and other circuit problems. Later SP-600s were built with almost all ceramic disk capacitors - started about 1955.

8. The R-274 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-274 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-274 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-274 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-274 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-274 (right) show that the receivers are similar only in performance and specifications.

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

The R-274 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-274 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.

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 paper-dielectric capacitors used in their construction. Certainly an important factor to consider since the rework involved is so tedious. In 1950, use of ceramic disk capacitors was something that most manufacturers hadn't moved to yet, even though the leakage current problem of paper-dielectric capacitors was well-known. The fact that Hallicrafters (well,...the Signal Corps) chose to use ceramic disks in the R-274 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 rapidly becoming obvious to all manufacturers by the mid-to-late 1950s.

Probably one of the most important considerations in a comparison with the SP-600 is that the R-274 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-274.

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 each can. The adjustments are easily 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-274 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 absolutely 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.)

If "body work" is necessary, like straightening a bent front panel, the R-274 panel is fairly soft aluminum that is easy to straighten. The SP-600 aluminum panels are heat-treated and, if bent, are difficult to straighten.

Cabinets - The R-274 is designed so the receiver can be operated without a cabinet either as a table receiver or as a rack mount receiver. Though there were a couple of military cabinets available they are 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.

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.) Keep the bearings lubed.

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 tactile interface 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 - Every early SP-600 will require replacement of all of the molded capacitors 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 1955.

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. The SP-600's stock audio really is communications grade.

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.

No Antenna Trimmer Control -  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 thus requiring the use of a "tuned" antenna.

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

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 popular 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 sometimes be fairly expensive.

R-274, R-274D - Pros

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

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

Audio Response - The R-274 uses .01uf coupling capacitors in the audio section that allows a better lower-end to the audio response. Some might consider it communications grade audio but communications grade audio never sounded so good.

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.

Crystal Filter - The R-274 Crystal Filter is located between the second and third IF amplifier. It functions quite nicely with good Phasing action that narrows bandwidth dramatically and can reduce heterodynes.

Antenna Trimmer - The R-274 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 turret adjustments are easy to see (no flashlight required.) The exception to "ease of alignment" might be that the IF transformers requires a sort-of-special tool for adjusting the secondary. A broad but thin blade screw driver will work (but it's easy to either make a tool or modify an existing tool.)

Remote Standby - This function is easy to access and use on the early version, the R-274/FRR. 12vdc required for remote standby in the later version R-274D.

R-274, R-274D - Cons

First Conversion Frequency IF Leakage - 6.000mc is used for the dual conversion frequency. It's possible that an extremely strong SW BC station on 6.000mc could "leak" into the IF of the receiver causing heterodynes or even dominating the first IF on Bands 4, 5 or 6. Radio Havana is a particularly strong SW BC station that "beams its signal" into the Eastern USA. Radio Havana operates on 6.000mc (along with several other frequencies) and can present a formidable signal if conditions are favorable possibly resulting in signal leakage into the IF of the R-274. More information below including testing results.*

Gear Box and Tuning Condenser - The R-274 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 which causes "jittering" when "fast tuned." 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 illuminated dial. (see "Getting the Most from the Dial Lamps" in the "R-274 Problems Encountered" section above.)

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 to a known, logged frequency.

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 easy 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. Luckily, unlike the SP-600 which looks pretty bad without a cabinet, the R-274 looks nice since it's a fully shielded box when the covers are installed.

Low Production Level - The R-274 or R-274D are not a common receivers. 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 examples of either of these receivers are going to be scraped for parts.   NOTE: Some parts and parts sets have turned up on eBay from time to time (May 2017.)

Since the actual performance of the SP-600 and the R-274 are so similar, maybe it will be the non-performance related issues that will ultimately be the deciding factor. These might 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 should be heavily weighed in your decision. Ultimately, it might come down to the actual selling prices today. 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-274 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 Comments - May 2017 - Wow! I can't believe that the Hallicrafters R-274 actually has more negative votes than positive votes! It appears that most of the positive comments on the SP-600 have to do with its ease of restoration while the positives for the R-274 are mostly performance and construction related. The only SP-600 negative comment is that it's ugly,...well,...okay. The negatives for the R-274 are lack of parts, three mechanical issues and two performance issues. Keep the votes coming and thanks to those who have voted so far. 

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

Read voting results across from left to right, SP-600 then R-274 then name or ham call of voter

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

 +10 & -7 votes SP-600,...   +8 & -9 votes R-274,...      SP-600 ahead by 2 positive and has 2 less negative votes - May 2017

1. Military manual, TM11-897 - Technical data, circuit description, schematic, parts list. Version May 1952.

2. Shortwave Receivers Past & Present  by Fred Osterman - details on SP-600 contracts based on Les Locklear's data. A very complete reference book on a wide variety of different shortwave receivers. Especially good for more modern types and foreign-made examples.

3. Thanks to all of the R-274 owners and SP-600 owners that have supplied reports, information and votes. Much of this information has been incorporated into this updated and expanded version of this article - May 2017